API

This part of the documentation covers all the interfaces of Flask. For parts where Flask depends on external libraries, we document the most important right here and provide links to the canonical documentation.

Application Object

class flask.Flask(import_name, static_url_path=None, static_folder=’static’, static_host=None, host_matching=False, subdomain_matching=False, template_folder=’templates’, instance_path=None, instance_relative_config=False, root_path=None)

The flask object implements a WSGI application and acts as the central object. It is passed the name of the module or package of the application. Once it is created it will act as a central registry for the view functions, the URL rules, template configuration and much more.

The name of the package is used to resolve resources from inside the package or the folder the module is contained in depending on if the package parameter resolves to an actual python package (a folder with an __init__.py file inside) or a standard module (just a .py file).

For more information about resource loading, see open_resource().

Usually you create a Flask instance in your main module or in the __init__.py file of your package like this:

  1. from flask import Flask
  2. app = Flask(__name__)

About the First Parameter

The idea of the first parameter is to give Flask an idea of what belongs to your application. This name is used to find resources on the filesystem, can be used by extensions to improve debugging information and a lot more.

So it’s important what you provide there. If you are using a single module, __name__ is always the correct value. If you however are using a package, it’s usually recommended to hardcode the name of your package there.

For example if your application is defined in yourapplication/app.py you should create it with one of the two versions below:

  1. app = Flask('yourapplication')
  2. app = Flask(__name__.split('.')[0])

Why is that? The application will work even with __name__, thanks to how resources are looked up. However it will make debugging more painful. Certain extensions can make assumptions based on the import name of your application. For example the Flask-SQLAlchemy extension will look for the code in your application that triggered an SQL query in debug mode. If the import name is not properly set up, that debugging information is lost. (For example it would only pick up SQL queries in yourapplication.app and not yourapplication.views.frontend)

Changelog

New in version 1.0: The host_matching and static_host parameters were added.

New in version 1.0: The subdomain_matching parameter was added. Subdomain matching needs to be enabled manually now. Setting SERVER_NAME does not implicitly enable it.

New in version 0.11: The root_path parameter was added.

New in version 0.8: The instance_path and instance_relative_config parameters were added.

New in version 0.7: The static_url_path, static_folder, and template_folder parameters were added.

  • Parameters

    • import_name (str) – the name of the application package

    • static_url_path (Optional[str]) – can be used to specify a different path for the static files on the web. Defaults to the name of the static_folder folder.

    • static_folder (Optional[Union[str, os.PathLike]**]) – The folder with static files that is served at static_url_path. Relative to the application root_path or an absolute path. Defaults to 'static'.

    • static_host (Optional[str]) – the host to use when adding the static route. Defaults to None. Required when using host_matching=True with a static_folder configured.

    • host_matching (bool) – set url_map.host_matching attribute. Defaults to False.

    • subdomain_matching (bool) – consider the subdomain relative to SERVER_NAME when matching routes. Defaults to False.

    • template_folder (Optional[str]) – the folder that contains the templates that should be used by the application. Defaults to 'templates' folder in the root path of the application.

    • instance_path (Optional[str]) – An alternative instance path for the application. By default the folder 'instance' next to the package or module is assumed to be the instance path.

    • instance_relative_config (bool) – if set to True relative filenames for loading the config are assumed to be relative to the instance path instead of the application root.

    • root_path (Optional[str]) – The path to the root of the application files. This should only be set manually when it can’t be detected automatically, such as for namespace packages.

  • add_template_filter(f, name=None)

    Register a custom template filter. Works exactly like the template_filter() decorator.

    • Parameters

      • name (Optional[str]) – the optional name of the filter, otherwise the function name will be used.

      • f (Callable[[…], Any]) –

      Return type

      None

  • add_template_global(f, name=None)

    Register a custom template global function. Works exactly like the template_global() decorator.

    Changelog

    New in version 0.10.

    • Parameters

      • name (Optional[str]) – the optional name of the global function, otherwise the function name will be used.

      • f (Callable[[…], Any]) –

      Return type

      None

  • add_template_test(f, name=None)

    Register a custom template test. Works exactly like the template_test() decorator.

    Changelog

    New in version 0.10.

    • Parameters

      • name (Optional[str]) – the optional name of the test, otherwise the function name will be used.

      • f (Callable[[…], bool]) –

      Return type

      None

  • add_url_rule(rule, endpoint=None, view_func=None, provide_automatic_options=None, \*options*)

    Register a rule for routing incoming requests and building URLs. The route() decorator is a shortcut to call this with the view_func argument. These are equivalent:

    1. @app.route("/")
    2. def index():
    3.     ...
    1. def index():
    2.     ...
    4. app.add_url_rule("/", view_func=index)

    See URL Route Registrations.

    The endpoint name for the route defaults to the name of the view function if the endpoint parameter isn’t passed. An error will be raised if a function has already been registered for the endpoint.

    The methods parameter defaults to ["GET"]. HEAD is always added automatically, and OPTIONS is added automatically by default.

    view_func does not necessarily need to be passed, but if the rule should participate in routing an endpoint name must be associated with a view function at some point with the endpoint() decorator.

    1. app.add_url_rule("/", endpoint="index")
    3. @app.endpoint("index")
    4. def index():
    5.     ...

    If view_func has a required_methods attribute, those methods are added to the passed and automatic methods. If it has a provide_automatic_methods attribute, it is used as the default if the parameter is not passed.

  • after_request(f)

    Register a function to run after each request to this object.

    The function is called with the response object, and must return a response object. This allows the functions to modify or replace the response before it is sent.

    If a function raises an exception, any remaining after_request functions will not be called. Therefore, this should not be used for actions that must execute, such as to close resources. Use teardown_request() for that.

    • Parameters

      f (Callable[[flask.typing.ResponseClass], flask.typing.ResponseClass**]) –

      Return type

      Callable[[flask.typing.ResponseClass], flask.typing.ResponseClass]

  • after_request_funcs: t.Dict[ft.AppOrBlueprintKey, t.List[ft.AfterRequestCallable]]

    A data structure of functions to call at the end of each request, in the format {scope: [functions]}. The scope key is the name of a blueprint the functions are active for, or None for all requests.

    To register a function, use the after_request() decorator.

    This data structure is internal. It should not be modified directly and its format may change at any time.

  • app_context()

    Create an AppContext. Use as a with block to push the context, which will make current_app point at this application.

    An application context is automatically pushed by RequestContext.push() when handling a request, and when running a CLI command. Use this to manually create a context outside of these situations.

    1. with app.app_context():
    2.     init_db()

    See The Application Context.

    Changelog

    New in version 0.9.

  • app_ctx_globals_class

    alias of flask.ctx._AppCtxGlobals

  • async_to_sync(func)

    Return a sync function that will run the coroutine function.

    1. result = app.async_to_sync(func)(*args, **kwargs)

    Override this method to change how the app converts async code to be synchronously callable.

    Changelog

    New in version 2.0.

  • auto_find_instance_path()

    Tries to locate the instance path if it was not provided to the constructor of the application class. It will basically calculate the path to a folder named instance next to your main file or the package.

    Changelog

    New in version 0.8.

    • Return type

      str

  • before_first_request(f)

    Registers a function to be run before the first request to this instance of the application.

    The function will be called without any arguments and its return value is ignored.

    Changelog

    New in version 0.8.

  • before_first_request_funcs: t.List[ft.BeforeFirstRequestCallable]

    A list of functions that will be called at the beginning of the first request to this instance. To register a function, use the before_first_request() decorator.

    Changelog

    New in version 0.8.

  • before_request(f)

    Register a function to run before each request.

    For example, this can be used to open a database connection, or to load the logged in user from the session.

    1. @app.before_request
    2. def load_user():
    3.     if "user_id" in session:
    4.         g.user = db.session.get(session["user_id"])

    The function will be called without any arguments. If it returns a non-None value, the value is handled as if it was the return value from the view, and further request handling is stopped.

  • before_request_funcs: t.Dict[ft.AppOrBlueprintKey, t.List[ft.BeforeRequestCallable]]

    A data structure of functions to call at the beginning of each request, in the format {scope: [functions]}. The scope key is the name of a blueprint the functions are active for, or None for all requests.

    To register a function, use the before_request() decorator.

    This data structure is internal. It should not be modified directly and its format may change at any time.

  • blueprints: t.Dict[str, ‘Blueprint’]

    Maps registered blueprint names to blueprint objects. The dict retains the order the blueprints were registered in. Blueprints can be registered multiple times, this dict does not track how often they were attached.

    Changelog

    New in version 0.7.

  • cli

    The Click command group for registering CLI commands for this object. The commands are available from the flask command once the application has been discovered and blueprints have been registered.

  • config

    The configuration dictionary as Config. This behaves exactly like a regular dictionary but supports additional methods to load a config from files.

  • config_class

    alias of flask.config.Config

  • context_processor(f)

    Registers a template context processor function.

  • create_global_jinja_loader()

    Creates the loader for the Jinja2 environment. Can be used to override just the loader and keeping the rest unchanged. It’s discouraged to override this function. Instead one should override the jinja_loader() function instead.

    The global loader dispatches between the loaders of the application and the individual blueprints.

    Changelog

    New in version 0.7.

    • Return type

      flask.templating.DispatchingJinjaLoader

  • create_jinja_environment()

    Create the Jinja environment based on jinja_options and the various Jinja-related methods of the app. Changing jinja_options after this will have no effect. Also adds Flask-related globals and filters to the environment.

    Changelog

    Changed in version 0.11: Environment.auto_reload set in accordance with TEMPLATES_AUTO_RELOAD configuration option.

    New in version 0.5.

    • Return type

      flask.templating.Environment

  • create_url_adapter(request)

    Creates a URL adapter for the given request. The URL adapter is created at a point where the request context is not yet set up so the request is passed explicitly.

    Changelog

    Changed in version 1.0: SERVER_NAME no longer implicitly enables subdomain matching. Use subdomain_matching instead.

    Changed in version 0.9: This can now also be called without a request object when the URL adapter is created for the application context.

    New in version 0.6.

  • property debug: bool

    Whether debug mode is enabled. When using flask run to start the development server, an interactive debugger will be shown for unhandled exceptions, and the server will be reloaded when code changes. This maps to the DEBUG config key. This is enabled when env is 'development' and is overridden by the FLASK_DEBUG environment variable. It may not behave as expected if set in code.

    Do not enable debug mode when deploying in production.

    Default: True if env is 'development', or False otherwise.

  • default_config = {‘APPLICATION_ROOT’: ‘/‘, ‘DEBUG’: None, ‘ENV’: None, ‘EXPLAIN_TEMPLATE_LOADING’: False, ‘JSONIFY_MIMETYPE’: ‘application/json’, ‘JSONIFY_PRETTYPRINT_REGULAR’: False, ‘JSON_AS_ASCII’: True, ‘JSON_SORT_KEYS’: True, ‘MAX_CONTENT_LENGTH’: None, ‘MAX_COOKIE_SIZE’: 4093, ‘PERMANENT_SESSION_LIFETIME’: datetime.timedelta(days=31), ‘PREFERRED_URL_SCHEME’: ‘http’, ‘PRESERVE_CONTEXT_ON_EXCEPTION’: None, ‘PROPAGATE_EXCEPTIONS’: None, ‘SECRET_KEY’: None, ‘SEND_FILE_MAX_AGE_DEFAULT’: None, ‘SERVER_NAME’: None, ‘SESSION_COOKIE_DOMAIN’: None, ‘SESSION_COOKIE_HTTPONLY’: True, ‘SESSION_COOKIE_NAME’: ‘session’, ‘SESSION_COOKIE_PATH’: None, ‘SESSION_COOKIE_SAMESITE’: None, ‘SESSION_COOKIE_SECURE’: False, ‘SESSION_REFRESH_EACH_REQUEST’: True, ‘TEMPLATES_AUTO_RELOAD’: None, ‘TESTING’: False, ‘TRAP_BAD_REQUEST_ERRORS’: None, ‘TRAP_HTTP_EXCEPTIONS’: False, ‘USE_X_SENDFILE’: False}

    Default configuration parameters.

  • delete(rule, \*options*)

    Shortcut for route() with methods=["DELETE"].

    Changelog

    New in version 2.0.

    • Parameters

      • rule (str) –

      • options (Any) –

      Return type

      Callable[[flask.typing.RouteDecorator], flask.typing.RouteDecorator]

  • dispatch_request()

    Does the request dispatching. Matches the URL and returns the return value of the view or error handler. This does not have to be a response object. In order to convert the return value to a proper response object, call make_response().

    Changelog

    Changed in version 0.7: This no longer does the exception handling, this code was moved to the new full_dispatch_request().

  • do_teardown_appcontext(exc=<object object>)

    Called right before the application context is popped.

    When handling a request, the application context is popped after the request context. See do_teardown_request().

    This calls all functions decorated with teardown_appcontext(). Then the appcontext_tearing_down signal is sent.

    This is called by AppContext.pop().

    Changelog

    New in version 0.9.

  • do_teardown_request(exc=<object object>)

    Called after the request is dispatched and the response is returned, right before the request context is popped.

    This calls all functions decorated with teardown_request(), and Blueprint.teardown_request() if a blueprint handled the request. Finally, the request_tearing_down signal is sent.

    This is called by RequestContext.pop(), which may be delayed during testing to maintain access to resources.

    • Parameters

      exc (Optional[BaseException]) – An unhandled exception raised while dispatching the request. Detected from the current exception information if not passed. Passed to each teardown function.

      Return type

      None

    Changelog

    Changed in version 0.9: Added the exc argument.

  • endpoint(endpoint)

    Decorate a view function to register it for the given endpoint. Used if a rule is added without a view_func with add_url_rule().

    1. app.add_url_rule("/ex", endpoint="example")
    3. @app.endpoint("example")
    4. def example():
    5.     ...

    • Parameters

      endpoint (str) – The endpoint name to associate with the view function.

      Return type

      Callable

  • ensure_sync(func)

    Ensure that the function is synchronous for WSGI workers. Plain def functions are returned as-is. async def functions are wrapped to run and wait for the response.

    Override this method to change how the app runs async views.

    Changelog

    New in version 2.0.

  • env

    What environment the app is running in. Flask and extensions may enable behaviors based on the environment, such as enabling debug mode. This maps to the ENV config key. This is set by the FLASK_ENV environment variable and may not behave as expected if set in code.

    Do not enable development when deploying in production.

    Default: 'production'

  • error_handler_spec: t.Dict[ft.AppOrBlueprintKey, t.Dict[t.Optional[int], t.Dict[t.Type[Exception], ft.ErrorHandlerCallable]]]

    A data structure of registered error handlers, in the format {scope: {code: {class: handler}}}. The scope key is the name of a blueprint the handlers are active for, or None for all requests. The code key is the HTTP status code for HTTPException, or None for other exceptions. The innermost dictionary maps exception classes to handler functions.

    To register an error handler, use the errorhandler() decorator.

    This data structure is internal. It should not be modified directly and its format may change at any time.

  • errorhandler(code_or_exception)

    Register a function to handle errors by code or exception class.

    A decorator that is used to register a function given an error code. Example:

    1. @app.errorhandler(404)
    2. def page_not_found(error):
    3.     return 'This page does not exist', 404

    You can also register handlers for arbitrary exceptions:

    1. @app.errorhandler(DatabaseError)
    2. def special_exception_handler(error):
    3.     return 'Database connection failed', 500

    Changelog

    New in version 0.7: Use register_error_handler() instead of modifying error_handler_spec directly, for application wide error handlers.

    New in version 0.7: One can now additionally also register custom exception types that do not necessarily have to be a subclass of the HTTPException class.

    • Parameters

      code_or_exception (Union[Type[Exception]**, int]) – the code as integer for the handler, or an arbitrary exception

      Return type

      Callable[[flask.typing.ErrorHandlerDecorator], flask.typing.ErrorHandlerDecorator]

  • extensions: dict

    a place where extensions can store application specific state. For example this is where an extension could store database engines and similar things.

    The key must match the name of the extension module. For example in case of a “Flask-Foo” extension in flask_foo, the key would be 'foo'.

    Changelog

    New in version 0.7.

  • full_dispatch_request()

    Dispatches the request and on top of that performs request pre and postprocessing as well as HTTP exception catching and error handling.

    Changelog

    New in version 0.7.

  • get(rule, \*options*)

    Shortcut for route() with methods=["GET"].

    Changelog

    New in version 2.0.

    • Parameters

      • rule (str) –

      • options (Any) –

      Return type

      Callable[[flask.typing.RouteDecorator], flask.typing.RouteDecorator]

  • get_send_file_max_age(filename)

    Used by send_file() to determine the max_age cache value for a given file path if it wasn’t passed.

    By default, this returns SEND_FILE_MAX_AGE_DEFAULT from the configuration of current_app. This defaults to None, which tells the browser to use conditional requests instead of a timed cache, which is usually preferable.

    Changelog

    Changed in version 2.0: The default configuration is None instead of 12 hours.

    New in version 0.9.

  • property got_first_request: bool

    This attribute is set to True if the application started handling the first request.

    Changelog

    New in version 0.8.

  • handle_exception(e)

    Handle an exception that did not have an error handler associated with it, or that was raised from an error handler. This always causes a 500 InternalServerError.

    Always sends the got_request_exception signal.

    If propagate_exceptions is True, such as in debug mode, the error will be re-raised so that the debugger can display it. Otherwise, the original exception is logged, and an InternalServerError is returned.

    If an error handler is registered for InternalServerError or 500, it will be used. For consistency, the handler will always receive the InternalServerError. The original unhandled exception is available as e.original_exception.

    Changelog

    Changed in version 1.1.0: Always passes the InternalServerError instance to the handler, setting original_exception to the unhandled error.

    Changed in version 1.1.0: after_request functions and other finalization is done even for the default 500 response when there is no handler.

    New in version 0.3.

  • handle_http_exception(e)

    Handles an HTTP exception. By default this will invoke the registered error handlers and fall back to returning the exception as response.

    Changelog

    Changed in version 1.0.3: RoutingException, used internally for actions such as slash redirects during routing, is not passed to error handlers.

    Changed in version 1.0: Exceptions are looked up by code and by MRO, so HTTPException subclasses can be handled with a catch-all handler for the base HTTPException.

    New in version 0.3.

  • handle_url_build_error(error, endpoint, values)

    Handle BuildError on url_for().

  • handle_user_exception(e)

    This method is called whenever an exception occurs that should be handled. A special case is HTTPException which is forwarded to the handle_http_exception() method. This function will either return a response value or reraise the exception with the same traceback.

    Changelog

    Changed in version 1.0: Key errors raised from request data like form show the bad key in debug mode rather than a generic bad request message.

    New in version 0.7.

  • property has_static_folder: bool

    True if static_folder is set.

    Changelog

    New in version 0.5.

  • import_name

    The name of the package or module that this object belongs to. Do not change this once it is set by the constructor.

  • inject_url_defaults(endpoint, values)

    Injects the URL defaults for the given endpoint directly into the values dictionary passed. This is used internally and automatically called on URL building.

    Changelog

    New in version 0.7.

    • Parameters

      • endpoint (str) –

      • values (dict) –

      Return type

      None

  • instance_path

    Holds the path to the instance folder.

    Changelog

    New in version 0.8.

  • iter_blueprints()

    Iterates over all blueprints by the order they were registered.

    Changelog

    New in version 0.11.

  • property jinja_env: flask.templating.Environment

    The Jinja environment used to load templates.

    The environment is created the first time this property is accessed. Changing jinja_options after that will have no effect.

  • jinja_environment

    alias of flask.templating.Environment

  • property jinja_loader: Optional[jinja2.loaders.FileSystemLoader]

    The Jinja loader for this object’s templates. By default this is a class jinja2.loaders.FileSystemLoader to template_folder if it is set.

    Changelog

    New in version 0.5.

  • jinja_options: dict = {}

    Options that are passed to the Jinja environment in create_jinja_environment(). Changing these options after the environment is created (accessing jinja_env) will have no effect.

    Changelog

    Changed in version 1.1.0: This is a dict instead of an ImmutableDict to allow easier configuration.

  • json_decoder

    alias of flask.json.JSONDecoder

  • json_encoder

    alias of flask.json.JSONEncoder

  • log_exception(exc_info)

    Logs an exception. This is called by handle_exception() if debugging is disabled and right before the handler is called. The default implementation logs the exception as error on the logger.

    Changelog

    New in version 0.8.

  • property logger: logging.Logger

    A standard Python Logger for the app, with the same name as name.

    In debug mode, the logger’s level will be set to DEBUG.

    If there are no handlers configured, a default handler will be added. See Logging for more information.

    Changelog

    Changed in version 1.1.0: The logger takes the same name as name rather than hard-coding "flask.app".

    Changed in version 1.0.0: Behavior was simplified. The logger is always named "flask.app". The level is only set during configuration, it doesn’t check app.debug each time. Only one format is used, not different ones depending on app.debug. No handlers are removed, and a handler is only added if no handlers are already configured.

    New in version 0.3.

  • make_config(instance_relative=False)

    Used to create the config attribute by the Flask constructor. The instance_relative parameter is passed in from the constructor of Flask (there named instance_relative_config) and indicates if the config should be relative to the instance path or the root path of the application.

    Changelog

    New in version 0.8.

  • make_default_options_response()

    This method is called to create the default OPTIONS response. This can be changed through subclassing to change the default behavior of OPTIONS responses.

    Changelog

    New in version 0.7.

  • make_response(rv)

    Convert the return value from a view function to an instance of response_class.

    Changelog

    Changed in version 0.9: Previously a tuple was interpreted as the arguments for the response object.

  • make_shell_context()

    Returns the shell context for an interactive shell for this application. This runs all the registered shell context processors.

    Changelog

    New in version 0.11.

  • property name: str

    The name of the application. This is usually the import name with the difference that it’s guessed from the run file if the import name is main. This name is used as a display name when Flask needs the name of the application. It can be set and overridden to change the value.

    Changelog

    New in version 0.8.

  • open_instance_resource(resource, mode=’rb’)

    Opens a resource from the application’s instance folder (instance_path). Otherwise works like open_resource(). Instance resources can also be opened for writing.

    • Parameters

      • resource (str) – the name of the resource. To access resources within subfolders use forward slashes as separator.

      • mode (str) – resource file opening mode, default is ‘rb’.

      Return type

      IO

  • open_resource(resource, mode=’rb’)

    Open a resource file relative to root_path for reading.

    For example, if the file schema.sql is next to the file app.py where the Flask app is defined, it can be opened with:

    1. with app.open_resource("schema.sql") as f:
    2.     conn.executescript(f.read())

    • Parameters

      • resource (str) – Path to the resource relative to root_path.

      • mode (str) – Open the file in this mode. Only reading is supported, valid values are “r” (or “rt”) and “rb”.

      Return type

      IO

  • patch(rule, \*options*)

    Shortcut for route() with methods=["PATCH"].

    Changelog

    New in version 2.0.

    • Parameters

      • rule (str) –

      • options (Any) –

      Return type

      Callable[[flask.typing.RouteDecorator], flask.typing.RouteDecorator]

  • permanent_session_lifetime

    A timedelta which is used to set the expiration date of a permanent session. The default is 31 days which makes a permanent session survive for roughly one month.

    This attribute can also be configured from the config with the PERMANENT_SESSION_LIFETIME configuration key. Defaults to timedelta(days=31)

  • post(rule, \*options*)

    Shortcut for route() with methods=["POST"].

    Changelog

    New in version 2.0.

    • Parameters

      • rule (str) –

      • options (Any) –

      Return type

      Callable[[flask.typing.RouteDecorator], flask.typing.RouteDecorator]

  • preprocess_request()

    Called before the request is dispatched. Calls url_value_preprocessors registered with the app and the current blueprint (if any). Then calls before_request_funcs registered with the app and the blueprint.

    If any before_request() handler returns a non-None value, the value is handled as if it was the return value from the view, and further request handling is stopped.

  • property preserve_context_on_exception: bool

    Returns the value of the PRESERVE_CONTEXT_ON_EXCEPTION configuration value in case it’s set, otherwise a sensible default is returned.

    Changelog

    New in version 0.7.

  • process_response(response)

    Can be overridden in order to modify the response object before it’s sent to the WSGI server. By default this will call all the after_request() decorated functions.

    Changelog

    Changed in version 0.5: As of Flask 0.5 the functions registered for after request execution are called in reverse order of registration.

  • property propagate_exceptions: bool

    Returns the value of the PROPAGATE_EXCEPTIONS configuration value in case it’s set, otherwise a sensible default is returned.

    Changelog

    New in version 0.7.

  • put(rule, \*options*)

    Shortcut for route() with methods=["PUT"].

    Changelog

    New in version 2.0.

    • Parameters

      • rule (str) –

      • options (Any) –

      Return type

      Callable[[flask.typing.RouteDecorator], flask.typing.RouteDecorator]

  • register_blueprint(blueprint, \*options*)

    Register a Blueprint on the application. Keyword arguments passed to this method will override the defaults set on the blueprint.

    Calls the blueprint’s register() method after recording the blueprint in the application’s blueprints.

    • Parameters

      • blueprint (Blueprint) – The blueprint to register.

      • url_prefix – Blueprint routes will be prefixed with this.

      • subdomain – Blueprint routes will match on this subdomain.

      • url_defaults – Blueprint routes will use these default values for view arguments.

      • options (Any) – Additional keyword arguments are passed to BlueprintSetupState. They can be accessed in record() callbacks.

      Return type

      None

    Changelog

    Changed in version 2.0.1: The name option can be used to change the (pre-dotted) name the blueprint is registered with. This allows the same blueprint to be registered multiple times with unique names for url_for.

    New in version 0.7.

  • register_error_handler(code_or_exception, f)

    Alternative error attach function to the errorhandler() decorator that is more straightforward to use for non decorator usage.

    Changelog

    New in version 0.7.

  • request_class

    alias of flask.wrappers.Request

  • request_context(environ)

    Create a RequestContext representing a WSGI environment. Use a with block to push the context, which will make request point at this request.

    See The Request Context.

    Typically you should not call this from your own code. A request context is automatically pushed by the wsgi_app() when handling a request. Use test_request_context() to create an environment and context instead of this method.

  • response_class

    alias of flask.wrappers.Response

  • root_path

    Absolute path to the package on the filesystem. Used to look up resources contained in the package.

  • route(rule, \*options*)

    Decorate a view function to register it with the given URL rule and options. Calls add_url_rule(), which has more details about the implementation.

    1. @app.route("/")
    2. def index():
    3.     return "Hello, World!"

    See URL Route Registrations.

    The endpoint name for the route defaults to the name of the view function if the endpoint parameter isn’t passed.

    The methods parameter defaults to ["GET"]. HEAD and OPTIONS are added automatically.

    • Parameters

      • rule (str) – The URL rule string.

      • options (Any) – Extra options passed to the Rule object.

      Return type

      Callable[[flask.typing.RouteDecorator], flask.typing.RouteDecorator]

  • run(host=None, port=None, debug=None, load_dotenv=True, \*options*)

    Runs the application on a local development server.

    Do not use run() in a production setting. It is not intended to meet security and performance requirements for a production server. Instead, see Deploying to Production for WSGI server recommendations.

    If the debug flag is set the server will automatically reload for code changes and show a debugger in case an exception happened.

    If you want to run the application in debug mode, but disable the code execution on the interactive debugger, you can pass use_evalex=False as parameter. This will keep the debugger’s traceback screen active, but disable code execution.

    It is not recommended to use this function for development with automatic reloading as this is badly supported. Instead you should be using the flask command line script’s run support.

    Keep in Mind

    Flask will suppress any server error with a generic error page unless it is in debug mode. As such to enable just the interactive debugger without the code reloading, you have to invoke run() with debug=True and use_reloader=False. Setting use_debugger to True without being in debug mode won’t catch any exceptions because there won’t be any to catch.

    • Parameters

      • host (Optional[str]) – the hostname to listen on. Set this to '0.0.0.0' to have the server available externally as well. Defaults to '127.0.0.1' or the host in the SERVER_NAME config variable if present.

      • port (Optional[int]) – the port of the webserver. Defaults to 5000 or the port defined in the SERVER_NAME config variable if present.

      • debug (Optional[bool]) – if given, enable or disable debug mode. See debug.

      • load_dotenv (bool) – Load the nearest .env and .flaskenv files to set environment variables. Will also change the working directory to the directory containing the first file found.

      • options (Any) – the options to be forwarded to the underlying Werkzeug server. See werkzeug.serving.run_simple() for more information.

      Return type

      None

    Changelog

    Changed in version 1.0: If installed, python-dotenv will be used to load environment variables from .env and .flaskenv files.

    If set, the FLASK_ENV and FLASK_DEBUG environment variables will override env and debug.

    Threaded mode is enabled by default.

    Changed in version 0.10: The default port is now picked from the SERVER_NAME variable.

  • secret_key

    If a secret key is set, cryptographic components can use this to sign cookies and other things. Set this to a complex random value when you want to use the secure cookie for instance.

    This attribute can also be configured from the config with the SECRET_KEY configuration key. Defaults to None.

  • select_jinja_autoescape(filename)

    Returns True if autoescaping should be active for the given template name. If no template name is given, returns True.

    Changelog

    New in version 0.5.

    • Parameters

      filename (str) –

      Return type

      bool

  • send_file_max_age_default

    A timedelta or number of seconds which is used as the default max_age for send_file(). The default is None, which tells the browser to use conditional requests instead of a timed cache.

    Configured with the SEND_FILE_MAX_AGE_DEFAULT configuration key.

    Changelog

    Changed in version 2.0: Defaults to None instead of 12 hours.

  • send_static_file(filename)

    The view function used to serve files from static_folder. A route is automatically registered for this view at static_url_path if static_folder is set.

    Changelog

    New in version 0.5.

  • session_cookie_name

    The secure cookie uses this for the name of the session cookie.

    This attribute can also be configured from the config with the SESSION_COOKIE_NAME configuration key. Defaults to 'session'

  • session_interface: flask.sessions.SessionInterface = <flask.sessions.SecureCookieSessionInterface object>

    the session interface to use. By default an instance of SecureCookieSessionInterface is used here.

    Changelog

    New in version 0.8.

  • shell_context_processor(f)

    Registers a shell context processor function.

    Changelog

    New in version 0.11.

  • shell_context_processors: t.List[t.Callable[[], t.Dict[str, t.Any]]]

    A list of shell context processor functions that should be run when a shell context is created.

    Changelog

    New in version 0.11.

  • should_ignore_error(error)

    This is called to figure out if an error should be ignored or not as far as the teardown system is concerned. If this function returns True then the teardown handlers will not be passed the error.

    Changelog

    New in version 0.10.

  • property static_folder: Optional[str]

    The absolute path to the configured static folder. None if no static folder is set.

  • property static_url_path: Optional[str]

    The URL prefix that the static route will be accessible from.

    If it was not configured during init, it is derived from static_folder.

  • teardown_appcontext(f)

    Registers a function to be called when the application context ends. These functions are typically also called when the request context is popped.

    Example:

    1. ctx = app.app_context()
    2. ctx.push()
    3. ...
    4. ctx.pop()

    When ctx.pop() is executed in the above example, the teardown functions are called just before the app context moves from the stack of active contexts. This becomes relevant if you are using such constructs in tests.

    Since a request context typically also manages an application context it would also be called when you pop a request context.

    When a teardown function was called because of an unhandled exception it will be passed an error object. If an errorhandler() is registered, it will handle the exception and the teardown will not receive it.

    The return values of teardown functions are ignored.

    Changelog

    New in version 0.9.

  • teardown_appcontext_funcs: t.List[ft.TeardownCallable]

    A list of functions that are called when the application context is destroyed. Since the application context is also torn down if the request ends this is the place to store code that disconnects from databases.

    Changelog

    New in version 0.9.

  • teardown_request(f)

    Register a function to be run at the end of each request, regardless of whether there was an exception or not. These functions are executed when the request context is popped, even if not an actual request was performed.

    Example:

    1. ctx = app.test_request_context()
    2. ctx.push()
    3. ...
    4. ctx.pop()

    When ctx.pop() is executed in the above example, the teardown functions are called just before the request context moves from the stack of active contexts. This becomes relevant if you are using such constructs in tests.

    Teardown functions must avoid raising exceptions. If they execute code that might fail they will have to surround the execution of that code with try/except statements and log any errors.

    When a teardown function was called because of an exception it will be passed an error object.

    The return values of teardown functions are ignored.

    Debug Note

    In debug mode Flask will not tear down a request on an exception immediately. Instead it will keep it alive so that the interactive debugger can still access it. This behavior can be controlled by the PRESERVE_CONTEXT_ON_EXCEPTION configuration variable.

  • teardown_request_funcs: t.Dict[ft.AppOrBlueprintKey, t.List[ft.TeardownCallable]]

    A data structure of functions to call at the end of each request even if an exception is raised, in the format {scope: [functions]}. The scope key is the name of a blueprint the functions are active for, or None for all requests.

    To register a function, use the teardown_request() decorator.

    This data structure is internal. It should not be modified directly and its format may change at any time.

  • template_context_processors: t.Dict[ft.AppOrBlueprintKey, t.List[ft.TemplateContextProcessorCallable]]

    A data structure of functions to call to pass extra context values when rendering templates, in the format {scope: [functions]}. The scope key is the name of a blueprint the functions are active for, or None for all requests.

    To register a function, use the context_processor() decorator.

    This data structure is internal. It should not be modified directly and its format may change at any time.

  • template_filter(name=None)

    A decorator that is used to register custom template filter. You can specify a name for the filter, otherwise the function name will be used. Example:

    1. @app.template_filter()
    2. def reverse(s):
    3.     return s[::-1]

  • template_folder

    The path to the templates folder, relative to root_path, to add to the template loader. None if templates should not be added.

  • template_global(name=None)

    A decorator that is used to register a custom template global function. You can specify a name for the global function, otherwise the function name will be used. Example:

    1. @app.template_global()
    2. def double(n):
    3.     return 2 * n

    Changelog

    New in version 0.10.

  • template_test(name=None)

    A decorator that is used to register custom template test. You can specify a name for the test, otherwise the function name will be used. Example:

    1. @app.template_test()
    2. def is_prime(n):
    3.     if n == 2:
    4.         return True
    5.     for i in range(2, int(math.ceil(math.sqrt(n))) + 1):
    6.         if n % i == 0:
    7.             return False
    8.     return True

    Changelog

    New in version 0.10.

  • property templates_auto_reload: bool

    Reload templates when they are changed. Used by create_jinja_environment().

    This attribute can be configured with TEMPLATES_AUTO_RELOAD. If not set, it will be enabled in debug mode.

    Changelog

    New in version 1.0: This property was added but the underlying config and behavior already existed.

  • test_cli_runner(\*kwargs*)

    Create a CLI runner for testing CLI commands. See Running Commands with the CLI Runner.

    Returns an instance of test_cli_runner_class, by default FlaskCliRunner. The Flask app object is passed as the first argument.

    Changelog

    New in version 1.0.

  • test_cli_runner_class: Optional[Type[FlaskCliRunner]] = None

    The CliRunner subclass, by default FlaskCliRunner that is used by test_cli_runner(). Its __init__ method should take a Flask app object as the first argument.

    Changelog

    New in version 1.0.

  • test_client(use_cookies=True, \*kwargs*)

    Creates a test client for this application. For information about unit testing head over to Testing Flask Applications.

    Note that if you are testing for assertions or exceptions in your application code, you must set app.testing = True in order for the exceptions to propagate to the test client. Otherwise, the exception will be handled by the application (not visible to the test client) and the only indication of an AssertionError or other exception will be a 500 status code response to the test client. See the testing attribute. For example:

    1. app.testing = True
    2. client = app.test_client()

    The test client can be used in a with block to defer the closing down of the context until the end of the with block. This is useful if you want to access the context locals for testing:

    1. with app.test_client() as c:
    2.     rv = c.get('/?vodka=42')
    3.     assert request.args['vodka'] == '42'

    Additionally, you may pass optional keyword arguments that will then be passed to the application’s test_client_class constructor. For example:

    1. from flask.testing import FlaskClient
    3. class CustomClient(FlaskClient):
    4.     def __init__(self, *args, **kwargs):
    5.         self._authentication = kwargs.pop("authentication")
    6.         super(CustomClient,self).__init__( *args, **kwargs)
    8. app.test_client_class = CustomClient
    9. client = app.test_client(authentication='Basic ....')

    See FlaskClient for more information.

    Changelog

    Changed in version 0.11: Added **kwargs to support passing additional keyword arguments to the constructor of test_client_class.

    New in version 0.7: The use_cookies parameter was added as well as the ability to override the client to be used by setting the test_client_class attribute.

    Changed in version 0.4: added support for with block usage for the client.

  • test_client_class: Optional[Type[FlaskClient]] = None

    The test_client() method creates an instance of this test client class. Defaults to FlaskClient.

    Changelog

    New in version 0.7.

  • test_request_context(\args, **kwargs*)

    Create a RequestContext for a WSGI environment created from the given values. This is mostly useful during testing, where you may want to run a function that uses request data without dispatching a full request.

    See The Request Context.

    Use a with block to push the context, which will make request point at the request for the created environment.

    1. with test_request_context(...):
    2.     generate_report()

    When using the shell, it may be easier to push and pop the context manually to avoid indentation.

    1. ctx = app.test_request_context(...)
    2. ctx.push()
    3. ...
    4. ctx.pop()

    Takes the same arguments as Werkzeug’s EnvironBuilder, with some defaults from the application. See the linked Werkzeug docs for most of the available arguments. Flask-specific behavior is listed here.

    • Parameters

      • path – URL path being requested.

      • base_url – Base URL where the app is being served, which path is relative to. If not given, built from PREFERRED_URL_SCHEME, subdomain, SERVER_NAME, and APPLICATION_ROOT.

      • subdomain – Subdomain name to append to SERVER_NAME.

      • url_scheme – Scheme to use instead of PREFERRED_URL_SCHEME.

      • data – The request body, either as a string or a dict of form keys and values.

      • json – If given, this is serialized as JSON and passed as data. Also defaults content_type to application/json.

      • args (Any) – other positional arguments passed to EnvironBuilder.

      • kwargs (Any) – other keyword arguments passed to EnvironBuilder.

      Return type

      flask.ctx.RequestContext

  • testing

    The testing flag. Set this to True to enable the test mode of Flask extensions (and in the future probably also Flask itself). For example this might activate test helpers that have an additional runtime cost which should not be enabled by default.

    If this is enabled and PROPAGATE_EXCEPTIONS is not changed from the default it’s implicitly enabled.

    This attribute can also be configured from the config with the TESTING configuration key. Defaults to False.

  • trap_http_exception(e)

    Checks if an HTTP exception should be trapped or not. By default this will return False for all exceptions except for a bad request key error if TRAP_BAD_REQUEST_ERRORS is set to True. It also returns True if TRAP_HTTP_EXCEPTIONS is set to True.

    This is called for all HTTP exceptions raised by a view function. If it returns True for any exception the error handler for this exception is not called and it shows up as regular exception in the traceback. This is helpful for debugging implicitly raised HTTP exceptions.

    Changelog

    Changed in version 1.0: Bad request errors are not trapped by default in debug mode.

    New in version 0.8.

  • update_template_context(context)

    Update the template context with some commonly used variables. This injects request, session, config and g into the template context as well as everything template context processors want to inject. Note that the as of Flask 0.6, the original values in the context will not be overridden if a context processor decides to return a value with the same key.

    • Parameters

      context (dict) – the context as a dictionary that is updated in place to add extra variables.

      Return type

      None

  • url_build_error_handlers: t.List[t.Callable[[Exception, str, dict], str]]

    A list of functions that are called when url_for() raises a BuildError. Each function registered here is called with error, endpoint and values. If a function returns None or raises a BuildError the next function is tried.

    Changelog

    New in version 0.9.

  • url_default_functions: t.Dict[ft.AppOrBlueprintKey, t.List[ft.URLDefaultCallable]]

    A data structure of functions to call to modify the keyword arguments when generating URLs, in the format {scope: [functions]}. The scope key is the name of a blueprint the functions are active for, or None for all requests.

    To register a function, use the url_defaults() decorator.

    This data structure is internal. It should not be modified directly and its format may change at any time.

  • url_defaults(f)

    Callback function for URL defaults for all view functions of the application. It’s called with the endpoint and values and should update the values passed in place.

  • url_map

    The Map for this instance. You can use this to change the routing converters after the class was created but before any routes are connected. Example:

    1. from werkzeug.routing import BaseConverter
    3. class ListConverter(BaseConverter):
    4.     def to_python(self, value):
    5.         return value.split(',')
    6.     def to_url(self, values):
    7.         return ','.join(super(ListConverter, self).to_url(value)
    8.                         for value in values)
    10. app = Flask(__name__)
    11. app.url_map.converters['list'] = ListConverter

  • url_map_class

    alias of werkzeug.routing.map.Map

  • url_rule_class

    alias of werkzeug.routing.rules.Rule

  • url_value_preprocessor(f)

    Register a URL value preprocessor function for all view functions in the application. These functions will be called before the before_request() functions.

    The function can modify the values captured from the matched url before they are passed to the view. For example, this can be used to pop a common language code value and place it in g rather than pass it to every view.

    The function is passed the endpoint name and values dict. The return value is ignored.

  • url_value_preprocessors: t.Dict[ft.AppOrBlueprintKey, t.List[ft.URLValuePreprocessorCallable]]

    A data structure of functions to call to modify the keyword arguments passed to the view function, in the format {scope: [functions]}. The scope key is the name of a blueprint the functions are active for, or None for all requests.

    To register a function, use the url_value_preprocessor() decorator.

    This data structure is internal. It should not be modified directly and its format may change at any time.

  • use_x_sendfile

    Enable this if you want to use the X-Sendfile feature. Keep in mind that the server has to support this. This only affects files sent with the send_file() method.

    Changelog

    New in version 0.2.

    This attribute can also be configured from the config with the USE_X_SENDFILE configuration key. Defaults to False.

  • view_functions: t.Dict[str, t.Callable]

    A dictionary mapping endpoint names to view functions.

    To register a view function, use the route() decorator.

    This data structure is internal. It should not be modified directly and its format may change at any time.

  • wsgi_app(environ, start_response)

    The actual WSGI application. This is not implemented in __call__() so that middlewares can be applied without losing a reference to the app object. Instead of doing this:

    1. app = MyMiddleware(app)

    It’s a better idea to do this instead:

    1. app.wsgi_app = MyMiddleware(app.wsgi_app)

    Then you still have the original application object around and can continue to call methods on it.

    Changelog

    Changed in version 0.7: Teardown events for the request and app contexts are called even if an unhandled error occurs. Other events may not be called depending on when an error occurs during dispatch. See Callbacks and Errors.

    • Parameters

      • environ (dict) – A WSGI environment.

      • start_response (Callable) – A callable accepting a status code, a list of headers, and an optional exception context to start the response.

      Return type

      Any

Blueprint Objects

class flask.Blueprint(name, import_name, static_folder=None, static_url_path=None, template_folder=None, url_prefix=None, subdomain=None, url_defaults=None, root_path=None, cli_group=<object object>)

Represents a blueprint, a collection of routes and other app-related functions that can be registered on a real application later.

A blueprint is an object that allows defining application functions without requiring an application object ahead of time. It uses the same decorators as Flask, but defers the need for an application by recording them for later registration.

Decorating a function with a blueprint creates a deferred function that is called with BlueprintSetupState when the blueprint is registered on an application.

See Modular Applications with Blueprints for more information.

  • Parameters

    • name (str) – The name of the blueprint. Will be prepended to each endpoint name.

    • import_name (str) – The name of the blueprint package, usually __name__. This helps locate the root_path for the blueprint.

    • static_folder (Optional[Union[str, os.PathLike]**]) – A folder with static files that should be served by the blueprint’s static route. The path is relative to the blueprint’s root path. Blueprint static files are disabled by default.

    • static_url_path (Optional[str]) – The url to serve static files from. Defaults to static_folder. If the blueprint does not have a url_prefix, the app’s static route will take precedence, and the blueprint’s static files won’t be accessible.

    • template_folder (Optional[str]) – A folder with templates that should be added to the app’s template search path. The path is relative to the blueprint’s root path. Blueprint templates are disabled by default. Blueprint templates have a lower precedence than those in the app’s templates folder.

    • url_prefix (Optional[str]) – A path to prepend to all of the blueprint’s URLs, to make them distinct from the rest of the app’s routes.

    • subdomain (Optional[str]) – A subdomain that blueprint routes will match on by default.

    • url_defaults (Optional[dict]) – A dict of default values that blueprint routes will receive by default.

    • root_path (Optional[str]) – By default, the blueprint will automatically set this based on import_name. In certain situations this automatic detection can fail, so the path can be specified manually instead.

    • cli_group (Optional[str]) –

Changelog

Changed in version 1.1.0: Blueprints have a cli group to register nested CLI commands. The cli_group parameter controls the name of the group under the flask command.

New in version 0.7.

  • add_app_template_filter(f, name=None)

    Register a custom template filter, available application wide. Like Flask.add_template_filter() but for a blueprint. Works exactly like the app_template_filter() decorator.

    • Parameters

      • name (Optional[str]) – the optional name of the filter, otherwise the function name will be used.

      • f (Callable[[…], Any]) –

      Return type

      None

  • add_app_template_global(f, name=None)

    Register a custom template global, available application wide. Like Flask.add_template_global() but for a blueprint. Works exactly like the app_template_global() decorator.

    Changelog

    New in version 0.10.

    • Parameters

      • name (Optional[str]) – the optional name of the global, otherwise the function name will be used.

      • f (Callable[[…], Any]) –

      Return type

      None

  • add_app_template_test(f, name=None)

    Register a custom template test, available application wide. Like Flask.add_template_test() but for a blueprint. Works exactly like the app_template_test() decorator.

    Changelog

    New in version 0.10.

    • Parameters

      • name (Optional[str]) – the optional name of the test, otherwise the function name will be used.

      • f (Callable[[…], bool]) –

      Return type

      None

  • add_url_rule(rule, endpoint=None, view_func=None, provide_automatic_options=None, \*options*)

    Like Flask.add_url_rule() but for a blueprint. The endpoint for the url_for() function is prefixed with the name of the blueprint.

  • after_app_request(f)

    Like Flask.after_request() but for a blueprint. Such a function is executed after each request, even if outside of the blueprint.

    • Parameters

      f (Callable[[flask.typing.ResponseClass], flask.typing.ResponseClass**]) –

      Return type

      Callable[[flask.typing.ResponseClass], flask.typing.ResponseClass]

  • after_request(f)

    Register a function to run after each request to this object.

    The function is called with the response object, and must return a response object. This allows the functions to modify or replace the response before it is sent.

    If a function raises an exception, any remaining after_request functions will not be called. Therefore, this should not be used for actions that must execute, such as to close resources. Use teardown_request() for that.

    • Parameters

      f (Callable[[flask.typing.ResponseClass], flask.typing.ResponseClass**]) –

      Return type

      Callable[[flask.typing.ResponseClass], flask.typing.ResponseClass]

  • after_request_funcs: t.Dict[ft.AppOrBlueprintKey, t.List[ft.AfterRequestCallable]]

    A data structure of functions to call at the end of each request, in the format {scope: [functions]}. The scope key is the name of a blueprint the functions are active for, or None for all requests.

    To register a function, use the after_request() decorator.

    This data structure is internal. It should not be modified directly and its format may change at any time.

  • app_context_processor(f)

    Like Flask.context_processor() but for a blueprint. Such a function is executed each request, even if outside of the blueprint.

  • app_errorhandler(code)

    Like Flask.errorhandler() but for a blueprint. This handler is used for all requests, even if outside of the blueprint.

    • Parameters

      code (Union[Type[Exception]**, int]) –

      Return type

      Callable[[flask.typing.ErrorHandlerDecorator], flask.typing.ErrorHandlerDecorator]

  • app_template_filter(name=None)

    Register a custom template filter, available application wide. Like Flask.template_filter() but for a blueprint.

  • app_template_global(name=None)

    Register a custom template global, available application wide. Like Flask.template_global() but for a blueprint.

    Changelog

    New in version 0.10.

  • app_template_test(name=None)

    Register a custom template test, available application wide. Like Flask.template_test() but for a blueprint.

    Changelog

    New in version 0.10.

  • app_url_defaults(f)

    Same as url_defaults() but application wide.

  • app_url_value_preprocessor(f)

    Same as url_value_preprocessor() but application wide.

  • before_app_first_request(f)

    Like Flask.before_first_request(). Such a function is executed before the first request to the application.

  • before_app_request(f)

    Like Flask.before_request(). Such a function is executed before each request, even if outside of a blueprint.

  • before_request(f)

    Register a function to run before each request.

    For example, this can be used to open a database connection, or to load the logged in user from the session.

    1. @app.before_request
    2. def load_user():
    3.     if "user_id" in session:
    4.         g.user = db.session.get(session["user_id"])

    The function will be called without any arguments. If it returns a non-None value, the value is handled as if it was the return value from the view, and further request handling is stopped.

  • before_request_funcs: t.Dict[ft.AppOrBlueprintKey, t.List[ft.BeforeRequestCallable]]

    A data structure of functions to call at the beginning of each request, in the format {scope: [functions]}. The scope key is the name of a blueprint the functions are active for, or None for all requests.

    To register a function, use the before_request() decorator.

    This data structure is internal. It should not be modified directly and its format may change at any time.

  • cli

    The Click command group for registering CLI commands for this object. The commands are available from the flask command once the application has been discovered and blueprints have been registered.

  • context_processor(f)

    Registers a template context processor function.

  • delete(rule, \*options*)

    Shortcut for route() with methods=["DELETE"].

    Changelog

    New in version 2.0.

    • Parameters

      • rule (str) –

      • options (Any) –

      Return type

      Callable[[flask.typing.RouteDecorator], flask.typing.RouteDecorator]

  • endpoint(endpoint)

    Decorate a view function to register it for the given endpoint. Used if a rule is added without a view_func with add_url_rule().

    1. app.add_url_rule("/ex", endpoint="example")
    3. @app.endpoint("example")
    4. def example():
    5.     ...

    • Parameters

      endpoint (str) – The endpoint name to associate with the view function.

      Return type

      Callable

  • error_handler_spec: t.Dict[ft.AppOrBlueprintKey, t.Dict[t.Optional[int], t.Dict[t.Type[Exception], ft.ErrorHandlerCallable]]]

    A data structure of registered error handlers, in the format {scope: {code: {class: handler}}}. The scope key is the name of a blueprint the handlers are active for, or None for all requests. The code key is the HTTP status code for HTTPException, or None for other exceptions. The innermost dictionary maps exception classes to handler functions.

    To register an error handler, use the errorhandler() decorator.

    This data structure is internal. It should not be modified directly and its format may change at any time.

  • errorhandler(code_or_exception)

    Register a function to handle errors by code or exception class.

    A decorator that is used to register a function given an error code. Example:

    1. @app.errorhandler(404)
    2. def page_not_found(error):
    3.     return 'This page does not exist', 404

    You can also register handlers for arbitrary exceptions:

    1. @app.errorhandler(DatabaseError)
    2. def special_exception_handler(error):
    3.     return 'Database connection failed', 500

    Changelog

    New in version 0.7: Use register_error_handler() instead of modifying error_handler_spec directly, for application wide error handlers.

    New in version 0.7: One can now additionally also register custom exception types that do not necessarily have to be a subclass of the HTTPException class.

    • Parameters

      code_or_exception (Union[Type[Exception]**, int]) – the code as integer for the handler, or an arbitrary exception

      Return type

      Callable[[flask.typing.ErrorHandlerDecorator], flask.typing.ErrorHandlerDecorator]

  • get(rule, \*options*)

    Shortcut for route() with methods=["GET"].

    Changelog

    New in version 2.0.

    • Parameters

      • rule (str) –

      • options (Any) –

      Return type

      Callable[[flask.typing.RouteDecorator], flask.typing.RouteDecorator]

  • get_send_file_max_age(filename)

    Used by send_file() to determine the max_age cache value for a given file path if it wasn’t passed.

    By default, this returns SEND_FILE_MAX_AGE_DEFAULT from the configuration of current_app. This defaults to None, which tells the browser to use conditional requests instead of a timed cache, which is usually preferable.

    Changelog

    Changed in version 2.0: The default configuration is None instead of 12 hours.

    New in version 0.9.

  • property has_static_folder: bool

    True if static_folder is set.

    Changelog

    New in version 0.5.

  • import_name

    The name of the package or module that this object belongs to. Do not change this once it is set by the constructor.

  • property jinja_loader: Optional[jinja2.loaders.FileSystemLoader]

    The Jinja loader for this object’s templates. By default this is a class jinja2.loaders.FileSystemLoader to template_folder if it is set.

    Changelog

    New in version 0.5.

  • json_decoder: t.Optional[t.Type[JSONDecoder]] = None

    Blueprint local JSON decoder class to use. Set to None to use the app’s json_decoder.

  • json_encoder: t.Optional[t.Type[JSONEncoder]] = None

    Blueprint local JSON encoder class to use. Set to None to use the app’s json_encoder.

  • make_setup_state(app, options, first_registration=False)

    Creates an instance of BlueprintSetupState() object that is later passed to the register callback functions. Subclasses can override this to return a subclass of the setup state.

  • open_resource(resource, mode=’rb’)

    Open a resource file relative to root_path for reading.

    For example, if the file schema.sql is next to the file app.py where the Flask app is defined, it can be opened with:

    1. with app.open_resource("schema.sql") as f:
    2.     conn.executescript(f.read())

    • Parameters

      • resource (str) – Path to the resource relative to root_path.

      • mode (str) – Open the file in this mode. Only reading is supported, valid values are “r” (or “rt”) and “rb”.

      Return type

      IO

  • patch(rule, \*options*)

    Shortcut for route() with methods=["PATCH"].

    Changelog

    New in version 2.0.

    • Parameters

      • rule (str) –

      • options (Any) –

      Return type

      Callable[[flask.typing.RouteDecorator], flask.typing.RouteDecorator]

  • post(rule, \*options*)

    Shortcut for route() with methods=["POST"].

    Changelog

    New in version 2.0.

    • Parameters

      • rule (str) –

      • options (Any) –

      Return type

      Callable[[flask.typing.RouteDecorator], flask.typing.RouteDecorator]

  • put(rule, \*options*)

    Shortcut for route() with methods=["PUT"].

    Changelog

    New in version 2.0.

    • Parameters

      • rule (str) –

      • options (Any) –

      Return type

      Callable[[flask.typing.RouteDecorator], flask.typing.RouteDecorator]

  • record(func)

    Registers a function that is called when the blueprint is registered on the application. This function is called with the state as argument as returned by the make_setup_state() method.

    • Parameters

      func (Callable) –

      Return type

      None

  • record_once(func)

    Works like record() but wraps the function in another function that will ensure the function is only called once. If the blueprint is registered a second time on the application, the function passed is not called.

    • Parameters

      func (Callable) –

      Return type

      None

  • register(app, options)

    Called by Flask.register_blueprint() to register all views and callbacks registered on the blueprint with the application. Creates a BlueprintSetupState and calls each record() callback with it.

    • Parameters

      • app (Flask) – The application this blueprint is being registered with.

      • options (dict) – Keyword arguments forwarded from register_blueprint().

      Return type

      None

    Changelog

    Changed in version 2.0.1: Nested blueprints are registered with their dotted name. This allows different blueprints with the same name to be nested at different locations.

    Changed in version 2.0.1: The name option can be used to change the (pre-dotted) name the blueprint is registered with. This allows the same blueprint to be registered multiple times with unique names for url_for.

    Changed in version 2.0.1: Registering the same blueprint with the same name multiple times is deprecated and will become an error in Flask 2.1.

  • register_blueprint(blueprint, \*options*)

    Register a Blueprint on this blueprint. Keyword arguments passed to this method will override the defaults set on the blueprint.

    Changelog

    Changed in version 2.0.1: The name option can be used to change the (pre-dotted) name the blueprint is registered with. This allows the same blueprint to be registered multiple times with unique names for url_for.

    New in version 2.0.

  • register_error_handler(code_or_exception, f)

    Alternative error attach function to the errorhandler() decorator that is more straightforward to use for non decorator usage.

    Changelog

    New in version 0.7.

  • root_path

    Absolute path to the package on the filesystem. Used to look up resources contained in the package.

  • route(rule, \*options*)

    Decorate a view function to register it with the given URL rule and options. Calls add_url_rule(), which has more details about the implementation.

    1. @app.route("/")
    2. def index():
    3.     return "Hello, World!"

    See URL Route Registrations.

    The endpoint name for the route defaults to the name of the view function if the endpoint parameter isn’t passed.

    The methods parameter defaults to ["GET"]. HEAD and OPTIONS are added automatically.

    • Parameters

      • rule (str) – The URL rule string.

      • options (Any) – Extra options passed to the Rule object.

      Return type

      Callable[[flask.typing.RouteDecorator], flask.typing.RouteDecorator]

  • send_static_file(filename)

    The view function used to serve files from static_folder. A route is automatically registered for this view at static_url_path if static_folder is set.

    Changelog

    New in version 0.5.

  • property static_folder: Optional[str]

    The absolute path to the configured static folder. None if no static folder is set.

  • property static_url_path: Optional[str]

    The URL prefix that the static route will be accessible from.

    If it was not configured during init, it is derived from static_folder.

  • teardown_app_request(f)

    Like Flask.teardown_request() but for a blueprint. Such a function is executed when tearing down each request, even if outside of the blueprint.

  • teardown_request(f)

    Register a function to be run at the end of each request, regardless of whether there was an exception or not. These functions are executed when the request context is popped, even if not an actual request was performed.

    Example:

    1. ctx = app.test_request_context()
    2. ctx.push()
    3. ...
    4. ctx.pop()

    When ctx.pop() is executed in the above example, the teardown functions are called just before the request context moves from the stack of active contexts. This becomes relevant if you are using such constructs in tests.

    Teardown functions must avoid raising exceptions. If they execute code that might fail they will have to surround the execution of that code with try/except statements and log any errors.

    When a teardown function was called because of an exception it will be passed an error object.

    The return values of teardown functions are ignored.

    Debug Note

    In debug mode Flask will not tear down a request on an exception immediately. Instead it will keep it alive so that the interactive debugger can still access it. This behavior can be controlled by the PRESERVE_CONTEXT_ON_EXCEPTION configuration variable.

  • teardown_request_funcs: t.Dict[ft.AppOrBlueprintKey, t.List[ft.TeardownCallable]]

    A data structure of functions to call at the end of each request even if an exception is raised, in the format {scope: [functions]}. The scope key is the name of a blueprint the functions are active for, or None for all requests.

    To register a function, use the teardown_request() decorator.

    This data structure is internal. It should not be modified directly and its format may change at any time.

  • template_context_processors: t.Dict[ft.AppOrBlueprintKey, t.List[ft.TemplateContextProcessorCallable]]

    A data structure of functions to call to pass extra context values when rendering templates, in the format {scope: [functions]}. The scope key is the name of a blueprint the functions are active for, or None for all requests.

    To register a function, use the context_processor() decorator.

    This data structure is internal. It should not be modified directly and its format may change at any time.

  • template_folder

    The path to the templates folder, relative to root_path, to add to the template loader. None if templates should not be added.

  • url_default_functions: t.Dict[ft.AppOrBlueprintKey, t.List[ft.URLDefaultCallable]]

    A data structure of functions to call to modify the keyword arguments when generating URLs, in the format {scope: [functions]}. The scope key is the name of a blueprint the functions are active for, or None for all requests.

    To register a function, use the url_defaults() decorator.

    This data structure is internal. It should not be modified directly and its format may change at any time.

  • url_defaults(f)

    Callback function for URL defaults for all view functions of the application. It’s called with the endpoint and values and should update the values passed in place.

  • url_value_preprocessor(f)

    Register a URL value preprocessor function for all view functions in the application. These functions will be called before the before_request() functions.

    The function can modify the values captured from the matched url before they are passed to the view. For example, this can be used to pop a common language code value and place it in g rather than pass it to every view.

    The function is passed the endpoint name and values dict. The return value is ignored.

  • url_value_preprocessors: t.Dict[ft.AppOrBlueprintKey, t.List[ft.URLValuePreprocessorCallable]]

    A data structure of functions to call to modify the keyword arguments passed to the view function, in the format {scope: [functions]}. The scope key is the name of a blueprint the functions are active for, or None for all requests.

    To register a function, use the url_value_preprocessor() decorator.

    This data structure is internal. It should not be modified directly and its format may change at any time.

  • view_functions: t.Dict[str, t.Callable]

    A dictionary mapping endpoint names to view functions.

    To register a view function, use the route() decorator.

    This data structure is internal. It should not be modified directly and its format may change at any time.

Incoming Request Data

class flask.Request(environ, populate_request=True, shallow=False)

The request object used by default in Flask. Remembers the matched endpoint and view arguments.

It is what ends up as request. If you want to replace the request object used you can subclass this and set request_class to your subclass.

The request object is a Request subclass and provides all of the attributes Werkzeug defines plus a few Flask specific ones.

  • Parameters

    • environ (WSGIEnvironment) –

    • populate_request (bool) –

    • shallow (bool) –

    Return type

    None

  • property accept_charsets: werkzeug.datastructures.CharsetAccept

    List of charsets this client supports as CharsetAccept object.

  • property accept_encodings: werkzeug.datastructures.Accept

    List of encodings this client accepts. Encodings in a HTTP term are compression encodings such as gzip. For charsets have a look at accept_charset.

  • property accept_languages: werkzeug.datastructures.LanguageAccept

    List of languages this client accepts as LanguageAccept object.

  • property accept_mimetypes: werkzeug.datastructures.MIMEAccept

    List of mimetypes this client supports as MIMEAccept object.

  • access_control_request_headers

    Sent with a preflight request to indicate which headers will be sent with the cross origin request. Set access_control_allow_headers on the response to indicate which headers are allowed.

  • access_control_request_method

    Sent with a preflight request to indicate which method will be used for the cross origin request. Set access_control_allow_methods on the response to indicate which methods are allowed.

  • property access_route: List[str]

    If a forwarded header exists this is a list of all ip addresses from the client ip to the last proxy server.

  • classmethod application(f)

    Decorate a function as responder that accepts the request as the last argument. This works like the responder() decorator but the function is passed the request object as the last argument and the request object will be closed automatically:

    1. @Request.application
    2. def my_wsgi_app(request):
    3.     return Response('Hello World!')

    As of Werkzeug 0.14 HTTP exceptions are automatically caught and converted to responses instead of failing.

    • Parameters

      f (Callable[**[Request]**, WSGIApplication**]) – the WSGI callable to decorate

      Returns

      a new WSGI callable

      Return type

      WSGIApplication

  • property args: werkzeug.datastructures.MultiDict[str, str]

    The parsed URL parameters (the part in the URL after the question mark).

    By default an ImmutableMultiDict is returned from this function. This can be changed by setting parameter_storage_class to a different type. This might be necessary if the order of the form data is important.

  • property authorization: Optional[werkzeug.datastructures.Authorization]

    The Authorization object in parsed form.

  • property base_url: str

    Like url but without the query string.

  • property blueprint: Optional[str]

    The registered name of the current blueprint.

    This will be None if the endpoint is not part of a blueprint, or if URL matching failed or has not been performed yet.

    This does not necessarily match the name the blueprint was created with. It may have been nested, or registered with a different name.

  • property blueprints: List[str]

    The registered names of the current blueprint upwards through parent blueprints.

    This will be an empty list if there is no current blueprint, or if URL matching failed.

    Changelog

    New in version 2.0.1.

  • property cache_control: werkzeug.datastructures.RequestCacheControl

    A RequestCacheControl object for the incoming cache control headers.

  • close()

    Closes associated resources of this request object. This closes all file handles explicitly. You can also use the request object in a with statement which will automatically close it.

    Changelog

    New in version 0.9.

    • Return type

      None

  • content_encoding

    The Content-Encoding entity-header field is used as a modifier to the media-type. When present, its value indicates what additional content codings have been applied to the entity-body, and thus what decoding mechanisms must be applied in order to obtain the media-type referenced by the Content-Type header field.

    Changelog

    New in version 0.9.

  • property content_length: Optional[int]

    The Content-Length entity-header field indicates the size of the entity-body in bytes or, in the case of the HEAD method, the size of the entity-body that would have been sent had the request been a GET.

  • content_md5

    The Content-MD5 entity-header field, as defined in RFC 1864, is an MD5 digest of the entity-body for the purpose of providing an end-to-end message integrity check (MIC) of the entity-body. (Note: a MIC is good for detecting accidental modification of the entity-body in transit, but is not proof against malicious attacks.)

    Changelog

    New in version 0.9.

  • content_type

    The Content-Type entity-header field indicates the media type of the entity-body sent to the recipient or, in the case of the HEAD method, the media type that would have been sent had the request been a GET.

  • property cookies: werkzeug.datastructures.ImmutableMultiDict[str, str]

    A dict with the contents of all cookies transmitted with the request.

  • property data: bytes

    Contains the incoming request data as string in case it came with a mimetype Werkzeug does not handle.

  • date

    The Date general-header field represents the date and time at which the message was originated, having the same semantics as orig-date in RFC 822.

    Changelog

    Changed in version 2.0: The datetime object is timezone-aware.

  • dict_storage_class

    alias of werkzeug.datastructures.ImmutableMultiDict

  • property endpoint: Optional[str]

    The endpoint that matched the request URL.

    This will be None if matching failed or has not been performed yet.

    This in combination with view_args can be used to reconstruct the same URL or a modified URL.

  • environ: WSGIEnvironment

    The WSGI environment containing HTTP headers and information from the WSGI server.

  • property files: werkzeug.datastructures.ImmutableMultiDict[str, werkzeug.datastructures.FileStorage]

    MultiDict object containing all uploaded files. Each key in files is the name from the <input type="file" name="">. Each value in files is a Werkzeug FileStorage object.

    It basically behaves like a standard file object you know from Python, with the difference that it also has a save() function that can store the file on the filesystem.

    Note that files will only contain data if the request method was POST, PUT or PATCH and the <form> that posted to the request had enctype="multipart/form-data". It will be empty otherwise.

    See the MultiDict / FileStorage documentation for more details about the used data structure.

  • property form: werkzeug.datastructures.ImmutableMultiDict[str, str]

    The form parameters. By default an ImmutableMultiDict is returned from this function. This can be changed by setting parameter_storage_class to a different type. This might be necessary if the order of the form data is important.

    Please keep in mind that file uploads will not end up here, but instead in the files attribute.

    Changelog

    Changed in version 0.9: Previous to Werkzeug 0.9 this would only contain form data for POST and PUT requests.

  • form_data_parser_class

    alias of werkzeug.formparser.FormDataParser

  • classmethod from_values(\args, **kwargs*)

    Create a new request object based on the values provided. If environ is given missing values are filled from there. This method is useful for small scripts when you need to simulate a request from an URL. Do not use this method for unittesting, there is a full featured client object (Client) that allows to create multipart requests, support for cookies etc.

    This accepts the same options as the EnvironBuilder.

    Changelog

    Changed in version 0.5: This method now accepts the same arguments as EnvironBuilder. Because of this the environ parameter is now called environ_overrides.

  • property full_path: str

    Requested path, including the query string.

  • get_data(cache=True, as_text=False, parse_form_data=False)

    This reads the buffered incoming data from the client into one bytes object. By default this is cached but that behavior can be changed by setting cache to False.

    Usually it’s a bad idea to call this method without checking the content length first as a client could send dozens of megabytes or more to cause memory problems on the server.

    Note that if the form data was already parsed this method will not return anything as form data parsing does not cache the data like this method does. To implicitly invoke form data parsing function set parse_form_data to True. When this is done the return value of this method will be an empty string if the form parser handles the data. This generally is not necessary as if the whole data is cached (which is the default) the form parser will used the cached data to parse the form data. Please be generally aware of checking the content length first in any case before calling this method to avoid exhausting server memory.

    If as_text is set to True the return value will be a decoded string.

    Changelog

    New in version 0.9.

  • get_json(force=False, silent=False, cache=True)

    Parse data as JSON.

    If the mimetype does not indicate JSON (application/json, see is_json), or parsing fails, on_json_loading_failed() is called and its return value is used as the return value. By default this raises a 400 Bad Request error.

    • Parameters

      • force (bool) – Ignore the mimetype and always try to parse JSON.

      • silent (bool) – Silence mimetype and parsing errors, and return None instead.

      • cache (bool) – Store the parsed JSON to return for subsequent calls.

      Return type

      Optional[Any]

    Changed in version 2.1: Raise a 400 error if the content type is incorrect.

  • headers

    The headers received with the request.

  • property host: str

    The host name the request was made to, including the port if it’s non-standard. Validated with trusted_hosts.

  • property host_url: str

    The request URL scheme and host only.

  • property if_match: werkzeug.datastructures.ETags

    An object containing all the etags in the If-Match header.

  • property if_modified_since: Optional[datetime.datetime]

    The parsed If-Modified-Since header as a datetime object.

    Changelog

    Changed in version 2.0: The datetime object is timezone-aware.

  • property if_none_match: werkzeug.datastructures.ETags

    An object containing all the etags in the If-None-Match header.

  • property if_range: werkzeug.datastructures.IfRange

    The parsed If-Range header.

    Changelog

    Changed in version 2.0: IfRange.date is timezone-aware.

    New in version 0.7.

  • property if_unmodified_since: Optional[datetime.datetime]

    The parsed If-Unmodified-Since header as a datetime object.

    Changelog

    Changed in version 2.0: The datetime object is timezone-aware.

  • input_stream

    The WSGI input stream.

    In general it’s a bad idea to use this one because you can easily read past the boundary. Use the stream instead.

  • property is_json: bool

    Check if the mimetype indicates JSON data, either application/json or application/\+json*.

  • is_multiprocess

    boolean that is True if the application is served by a WSGI server that spawns multiple processes.

  • is_multithread

    boolean that is True if the application is served by a multithreaded WSGI server.

  • is_run_once

    boolean that is True if the application will be executed only once in a process lifetime. This is the case for CGI for example, but it’s not guaranteed that the execution only happens one time.

  • property is_secure: bool

    True if the request was made with a secure protocol (HTTPS or WSS).

  • property json: Optional[Any]

    The parsed JSON data if mimetype indicates JSON (application/json, see is_json).

    Calls get_json() with default arguments.

    If the request content type is not application/json, this will raise a 400 Bad Request error.

    Changed in version 2.1: Raise a 400 error if the content type is incorrect.

  • list_storage_class

    alias of werkzeug.datastructures.ImmutableList

  • make_form_data_parser()

    Creates the form data parser. Instantiates the form_data_parser_class with some parameters.

    Changelog

    New in version 0.8.

  • property max_content_length: Optional[int]

    Read-only view of the MAX_CONTENT_LENGTH config key.

  • max_forwards

    The Max-Forwards request-header field provides a mechanism with the TRACE and OPTIONS methods to limit the number of proxies or gateways that can forward the request to the next inbound server.

  • method

    The method the request was made with, such as GET.

  • property mimetype: str

    Like content_type, but without parameters (eg, without charset, type etc.) and always lowercase. For example if the content type is text/HTML; charset=utf-8 the mimetype would be 'text/html'.

  • property mimetype_params: Dict[str, str]

    The mimetype parameters as dict. For example if the content type is text/html; charset=utf-8 the params would be {'charset': 'utf-8'}.

  • on_json_loading_failed(e)

    Called if get_json() fails and isn’t silenced.

    If this method returns a value, it is used as the return value for get_json(). The default implementation raises BadRequest.

    • Parameters

      e (Optional[ValueError]) – If parsing failed, this is the exception. It will be None if the content type wasn’t application/json.

      Return type

      Any

  • origin

    The host that the request originated from. Set access_control_allow_origin on the response to indicate which origins are allowed.

  • parameter_storage_class

    alias of werkzeug.datastructures.ImmutableMultiDict

  • path

    The path part of the URL after root_path. This is the path used for routing within the application.

  • property pragma: werkzeug.datastructures.HeaderSet

    The Pragma general-header field is used to include implementation-specific directives that might apply to any recipient along the request/response chain. All pragma directives specify optional behavior from the viewpoint of the protocol; however, some systems MAY require that behavior be consistent with the directives.

  • query_string

    The part of the URL after the “?”. This is the raw value, use args for the parsed values.

  • property range: Optional[werkzeug.datastructures.Range]

    The parsed Range header.

    Changelog

    New in version 0.7.

  • referrer

    The Referer[sic] request-header field allows the client to specify, for the server’s benefit, the address (URI) of the resource from which the Request-URI was obtained (the “referrer”, although the header field is misspelled).

  • remote_addr

    The address of the client sending the request.

  • remote_user

    If the server supports user authentication, and the script is protected, this attribute contains the username the user has authenticated as.

  • root_path

    The prefix that the application is mounted under, without a trailing slash. path comes after this.

  • property root_url: str

    The request URL scheme, host, and root path. This is the root that the application is accessed from.

  • routing_exception: Optional[Exception] = None

    If matching the URL failed, this is the exception that will be raised / was raised as part of the request handling. This is usually a NotFound exception or something similar.

  • scheme

    The URL scheme of the protocol the request used, such as https or wss.

  • property script_root: str

    Alias for self.root_path. environ["SCRIPT_ROOT"] without a trailing slash.

  • server

    The address of the server. (host, port), (path, None) for unix sockets, or None if not known.

  • shallow: bool

    Set when creating the request object. If True, reading from the request body will cause a RuntimeException. Useful to prevent modifying the stream from middleware.

  • property stream: IO[bytes]

    If the incoming form data was not encoded with a known mimetype the data is stored unmodified in this stream for consumption. Most of the time it is a better idea to use data which will give you that data as a string. The stream only returns the data once.

    Unlike input_stream this stream is properly guarded that you can’t accidentally read past the length of the input. Werkzeug will internally always refer to this stream to read data which makes it possible to wrap this object with a stream that does filtering.

    Changelog

    Changed in version 0.9: This stream is now always available but might be consumed by the form parser later on. Previously the stream was only set if no parsing happened.

  • property url: str

    The full request URL with the scheme, host, root path, path, and query string.

  • property url_charset: str

    The charset that is assumed for URLs. Defaults to the value of charset.

    Changelog

    New in version 0.6.

  • property url_root: str

    Alias for root_url. The URL with scheme, host, and root path. For example, https://example.com/app/.

  • url_rule: Optional[Rule] = None

    The internal URL rule that matched the request. This can be useful to inspect which methods are allowed for the URL from a before/after handler (request.url_rule.methods) etc. Though if the request’s method was invalid for the URL rule, the valid list is available in routing_exception.valid_methods instead (an attribute of the Werkzeug exception MethodNotAllowed) because the request was never internally bound.

    Changelog

    New in version 0.6.

  • property user_agent: werkzeug.user_agent.UserAgent

    The user agent. Use user_agent.string to get the header value. Set user_agent_class to a subclass of UserAgent to provide parsing for the other properties or other extended data.

    Changelog

    Changed in version 2.0: The built in parser is deprecated and will be removed in Werkzeug 2.1. A UserAgent subclass must be set to parse data from the string.

  • user_agent_class

    alias of werkzeug.user_agent.UserAgent

  • property values: werkzeug.datastructures.CombinedMultiDict[str, str]

    A werkzeug.datastructures.CombinedMultiDict that combines args and form.

    For GET requests, only args are present, not form.

    Changelog

    Changed in version 2.0: For GET requests, only args are present, not form.

  • view_args: Optional[Dict[str, Any]] = None

    A dict of view arguments that matched the request. If an exception happened when matching, this will be None.

  • property want_form_data_parsed: bool

    True if the request method carries content. By default this is true if a Content-Type is sent.

    Changelog

    New in version 0.8.

flask.request

To access incoming request data, you can use the global request object. Flask parses incoming request data for you and gives you access to it through that global object. Internally Flask makes sure that you always get the correct data for the active thread if you are in a multithreaded environment.

This is a proxy. See Notes On Proxies for more information.

The request object is an instance of a Request.

Response Objects

class flask.Response(response=None, status=None, headers=None, mimetype=None, content_type=None, direct_passthrough=False)

The response object that is used by default in Flask. Works like the response object from Werkzeug but is set to have an HTML mimetype by default. Quite often you don’t have to create this object yourself because make_response() will take care of that for you.

If you want to replace the response object used you can subclass this and set response_class to your subclass.

Changelog

Changed in version 1.0: JSON support is added to the response, like the request. This is useful when testing to get the test client response data as JSON.

Changed in version 1.0: Added max_cookie_size.

  • Parameters

    Return type

    None

  • accept_ranges

    The Accept-Ranges header. Even though the name would indicate that multiple values are supported, it must be one string token only.

    The values 'bytes' and 'none' are common.

    Changelog

    New in version 0.7.

  • property access_control_allow_credentials: bool

    Whether credentials can be shared by the browser to JavaScript code. As part of the preflight request it indicates whether credentials can be used on the cross origin request.

  • access_control_allow_headers

    Which headers can be sent with the cross origin request.

  • access_control_allow_methods

    Which methods can be used for the cross origin request.

  • access_control_allow_origin

    The origin or ‘*’ for any origin that may make cross origin requests.

  • access_control_expose_headers

    Which headers can be shared by the browser to JavaScript code.

  • access_control_max_age

    The maximum age in seconds the access control settings can be cached for.

  • add_etag(overwrite=False, weak=False)

    Add an etag for the current response if there is none yet.

    Changelog

    Changed in version 2.0: SHA-1 is used to generate the value. MD5 may not be available in some environments.

    • Parameters

      Return type

      None

  • age

    The Age response-header field conveys the sender’s estimate of the amount of time since the response (or its revalidation) was generated at the origin server.

    Age values are non-negative decimal integers, representing time in seconds.

  • property allow: werkzeug.datastructures.HeaderSet

    The Allow entity-header field lists the set of methods supported by the resource identified by the Request-URI. The purpose of this field is strictly to inform the recipient of valid methods associated with the resource. An Allow header field MUST be present in a 405 (Method Not Allowed) response.

  • property cache_control: werkzeug.datastructures.ResponseCacheControl

    The Cache-Control general-header field is used to specify directives that MUST be obeyed by all caching mechanisms along the request/response chain.

  • calculate_content_length()

    Returns the content length if available or None otherwise.

  • call_on_close(func)

    Adds a function to the internal list of functions that should be called as part of closing down the response. Since 0.7 this function also returns the function that was passed so that this can be used as a decorator.

    Changelog

    New in version 0.6.

  • close()

    Close the wrapped response if possible. You can also use the object in a with statement which will automatically close it.

    Changelog

    New in version 0.9: Can now be used in a with statement.

    • Return type

      None

  • content_encoding

    The Content-Encoding entity-header field is used as a modifier to the media-type. When present, its value indicates what additional content codings have been applied to the entity-body, and thus what decoding mechanisms must be applied in order to obtain the media-type referenced by the Content-Type header field.

  • property content_language: werkzeug.datastructures.HeaderSet

    The Content-Language entity-header field describes the natural language(s) of the intended audience for the enclosed entity. Note that this might not be equivalent to all the languages used within the entity-body.

  • content_length

    The Content-Length entity-header field indicates the size of the entity-body, in decimal number of OCTETs, sent to the recipient or, in the case of the HEAD method, the size of the entity-body that would have been sent had the request been a GET.

  • content_location

    The Content-Location entity-header field MAY be used to supply the resource location for the entity enclosed in the message when that entity is accessible from a location separate from the requested resource’s URI.

  • content_md5

    The Content-MD5 entity-header field, as defined in RFC 1864, is an MD5 digest of the entity-body for the purpose of providing an end-to-end message integrity check (MIC) of the entity-body. (Note: a MIC is good for detecting accidental modification of the entity-body in transit, but is not proof against malicious attacks.)

  • property content_range: werkzeug.datastructures.ContentRange

    The Content-Range header as a ContentRange object. Available even if the header is not set.

    Changelog

    New in version 0.7.

  • property content_security_policy: werkzeug.datastructures.ContentSecurityPolicy

    The Content-Security-Policy header as a ContentSecurityPolicy object. Available even if the header is not set.

    The Content-Security-Policy header adds an additional layer of security to help detect and mitigate certain types of attacks.

  • property content_security_policy_report_only: werkzeug.datastructures.ContentSecurityPolicy

    The Content-Security-policy-report-only header as a ContentSecurityPolicy object. Available even if the header is not set.

    The Content-Security-Policy-Report-Only header adds a csp policy that is not enforced but is reported thereby helping detect certain types of attacks.

  • content_type

    The Content-Type entity-header field indicates the media type of the entity-body sent to the recipient or, in the case of the HEAD method, the media type that would have been sent had the request been a GET.

  • cross_origin_embedder_policy

    Prevents a document from loading any cross-origin resources that do not explicitly grant the document permission. Values must be a member of the werkzeug.http.COEP enum.

  • cross_origin_opener_policy

    Allows control over sharing of browsing context group with cross-origin documents. Values must be a member of the werkzeug.http.COOP enum.

  • property data: Union[bytes, str]

    A descriptor that calls get_data() and set_data().

  • date

    The Date general-header field represents the date and time at which the message was originated, having the same semantics as orig-date in RFC 822.

    Changelog

    Changed in version 2.0: The datetime object is timezone-aware.

  • delete_cookie(key, path=’/‘, domain=None, secure=False, httponly=False, samesite=None)

    Delete a cookie. Fails silently if key doesn’t exist.

    • Parameters

      • key (str) – the key (name) of the cookie to be deleted.

      • path (str) – if the cookie that should be deleted was limited to a path, the path has to be defined here.

      • domain (Optional[str]) – if the cookie that should be deleted was limited to a domain, that domain has to be defined here.

      • secure (bool) – If True, the cookie will only be available via HTTPS.

      • httponly (bool) – Disallow JavaScript access to the cookie.

      • samesite (Optional[str]) – Limit the scope of the cookie to only be attached to requests that are “same-site”.

      Return type

      None

  • direct_passthrough

    Pass the response body directly through as the WSGI iterable. This can be used when the body is a binary file or other iterator of bytes, to skip some unnecessary checks. Use send_file() instead of setting this manually.

  • expires

    The Expires entity-header field gives the date/time after which the response is considered stale. A stale cache entry may not normally be returned by a cache.

    Changelog

    Changed in version 2.0: The datetime object is timezone-aware.

  • classmethod force_type(response, environ=None)

    Enforce that the WSGI response is a response object of the current type. Werkzeug will use the Response internally in many situations like the exceptions. If you call get_response() on an exception you will get back a regular Response object, even if you are using a custom subclass.

    This method can enforce a given response type, and it will also convert arbitrary WSGI callables into response objects if an environ is provided:

    1. # convert a Werkzeug response object into an instance of the
    2. # MyResponseClass subclass.
    3. response = MyResponseClass.force_type(response)
    5. # convert any WSGI application into a response object
    6. response = MyResponseClass.force_type(response, environ)

    This is especially useful if you want to post-process responses in the main dispatcher and use functionality provided by your subclass.

    Keep in mind that this will modify response objects in place if possible!

    • Parameters

      • response (Response) – a response object or wsgi application.

      • environ (Optional[WSGIEnvironment]) – a WSGI environment object.

      Returns

      a response object.

      Return type

      Response

  • freeze()

    Make the response object ready to be pickled. Does the following:

    • Buffer the response into a list, ignoring implicity_sequence_conversion and direct_passthrough.

    • Set the Content-Length header.

    • Generate an ETag header if one is not already set.

    Changed in version 2.1: Removed the no_etag parameter.

    Changelog

    Changed in version 2.0: An ETag header is added, the no_etag parameter is deprecated and will be removed in Werkzeug 2.1.

    Changed in version 0.6: The Content-Length header is set.

    • Return type

      None

  • classmethod from_app(app, environ, buffered=False)

    Create a new response object from an application output. This works best if you pass it an application that returns a generator all the time. Sometimes applications may use the write() callable returned by the start_response function. This tries to resolve such edge cases automatically. But if you don’t get the expected output you should set buffered to True which enforces buffering.

    • Parameters

      • app (WSGIApplication) – the WSGI application to execute.

      • environ (WSGIEnvironment) – the WSGI environment to execute against.

      • buffered (bool) – set to True to enforce buffering.

      Returns

      a response object.

      Return type

      Response

  • get_app_iter(environ)

    Returns the application iterator for the given environ. Depending on the request method and the current status code the return value might be an empty response rather than the one from the response.

    If the request method is HEAD or the status code is in a range where the HTTP specification requires an empty response, an empty iterable is returned.

    Changelog

    New in version 0.6.

    • Parameters

      environ (WSGIEnvironment) – the WSGI environment of the request.

      Returns

      a response iterable.

      Return type

      Iterable[bytes]

  • get_data(as_text=False)

    The string representation of the response body. Whenever you call this property the response iterable is encoded and flattened. This can lead to unwanted behavior if you stream big data.

    This behavior can be disabled by setting implicit_sequence_conversion to False.

    If as_text is set to True the return value will be a decoded string.

    Changelog

    New in version 0.9.

  • get_etag()

    Return a tuple in the form (etag, is_weak). If there is no ETag the return value is (None, None).

  • get_json(force=False, silent=False)

    Parse data as JSON. Useful during testing.

    If the mimetype does not indicate JSON (application/json, see is_json), this returns None.

    Unlike Request.get_json(), the result is not cached.

    • Parameters

      • force (bool) – Ignore the mimetype and always try to parse JSON.

      • silent (bool) – Silence parsing errors and return None instead.

      Return type

      Optional[Any]

  • get_wsgi_headers(environ)

    This is automatically called right before the response is started and returns headers modified for the given environment. It returns a copy of the headers from the response with some modifications applied if necessary.

    For example the location header (if present) is joined with the root URL of the environment. Also the content length is automatically set to zero here for certain status codes.

    Changelog

    Changed in version 0.6: Previously that function was called fix_headers and modified the response object in place. Also since 0.6, IRIs in location and content-location headers are handled properly.

    Also starting with 0.6, Werkzeug will attempt to set the content length if it is able to figure it out on its own. This is the case if all the strings in the response iterable are already encoded and the iterable is buffered.

  • get_wsgi_response(environ)

    Returns the final WSGI response as tuple. The first item in the tuple is the application iterator, the second the status and the third the list of headers. The response returned is created specially for the given environment. For example if the request method in the WSGI environment is 'HEAD' the response will be empty and only the headers and status code will be present.

    Changelog

    New in version 0.6.

    • Parameters

      environ (WSGIEnvironment) – the WSGI environment of the request.

      Returns

      an (app_iter, status, headers) tuple.

      Return type

      Tuple[Iterable[bytes], str, List[Tuple[str, str]]]

  • property is_json: bool

    Check if the mimetype indicates JSON data, either application/json or application/\+json*.

  • property is_sequence: bool

    If the iterator is buffered, this property will be True. A response object will consider an iterator to be buffered if the response attribute is a list or tuple.

    Changelog

    New in version 0.6.

  • property is_streamed: bool

    If the response is streamed (the response is not an iterable with a length information) this property is True. In this case streamed means that there is no information about the number of iterations. This is usually True if a generator is passed to the response object.

    This is useful for checking before applying some sort of post filtering that should not take place for streamed responses.

  • iter_encoded()

    Iter the response encoded with the encoding of the response. If the response object is invoked as WSGI application the return value of this method is used as application iterator unless direct_passthrough was activated.

  • property json: Optional[Any]

    The parsed JSON data if mimetype indicates JSON (application/json, see is_json).

    Calls get_json() with default arguments.

  • last_modified

    The Last-Modified entity-header field indicates the date and time at which the origin server believes the variant was last modified.

    Changelog

    Changed in version 2.0: The datetime object is timezone-aware.

  • location

    The Location response-header field is used to redirect the recipient to a location other than the Request-URI for completion of the request or identification of a new resource.

  • make_conditional(request_or_environ, accept_ranges=False, complete_length=None)

    Make the response conditional to the request. This method works best if an etag was defined for the response already. The add_etag method can be used to do that. If called without etag just the date header is set.

    This does nothing if the request method in the request or environ is anything but GET or HEAD.

    For optimal performance when handling range requests, it’s recommended that your response data object implements seekable, seek and tell methods as described by io.IOBase. Objects returned by wrap_file() automatically implement those methods.

    It does not remove the body of the response because that’s something the __call__() function does for us automatically.

    Returns self so that you can do return resp.make_conditional(req) but modifies the object in-place.

    • Parameters

      • request_or_environ (Union[WSGIEnvironment, Request]) – a request object or WSGI environment to be used to make the response conditional against.

      • accept_ranges (Union[bool, str]) – This parameter dictates the value of Accept-Ranges header. If False (default), the header is not set. If True, it will be set to "bytes". If None, it will be set to "none". If it’s a string, it will use this value.

      • complete_length (Optional[int]) – Will be used only in valid Range Requests. It will set Content-Range complete length value and compute Content-Length real value. This parameter is mandatory for successful Range Requests completion.

      Raises

      RequestedRangeNotSatisfiable if Range header could not be parsed or satisfied.

      Return type

      Response

    Changelog

    Changed in version 2.0: Range processing is skipped if length is 0 instead of raising a 416 Range Not Satisfiable error.

  • make_sequence()

    Converts the response iterator in a list. By default this happens automatically if required. If implicit_sequence_conversion is disabled, this method is not automatically called and some properties might raise exceptions. This also encodes all the items.

    Changelog

    New in version 0.6.

    • Return type

      None

  • property max_cookie_size: int

    Read-only view of the MAX_COOKIE_SIZE config key.

    See max_cookie_size in Werkzeug’s docs.

  • property mimetype: Optional[str]

    The mimetype (content type without charset etc.)

  • property mimetype_params: Dict[str, str]

    The mimetype parameters as dict. For example if the content type is text/html; charset=utf-8 the params would be {'charset': 'utf-8'}.

    Changelog

    New in version 0.5.

  • response: t.Union[t.Iterable[str], t.Iterable[bytes]]

    The response body to send as the WSGI iterable. A list of strings or bytes represents a fixed-length response, any other iterable is a streaming response. Strings are encoded to bytes as UTF-8.

    Do not set to a plain string or bytes, that will cause sending the response to be very inefficient as it will iterate one byte at a time.

  • property retry_after: Optional[datetime.datetime]

    The Retry-After response-header field can be used with a 503 (Service Unavailable) response to indicate how long the service is expected to be unavailable to the requesting client.

    Time in seconds until expiration or date.

    Changelog

    Changed in version 2.0: The datetime object is timezone-aware.

  • set_cookie(key, value=’’, max_age=None, expires=None, path=’/‘, domain=None, secure=False, httponly=False, samesite=None)

    Sets a cookie.

    A warning is raised if the size of the cookie header exceeds max_cookie_size, but the header will still be set.

    • Parameters

      • key (str) – the key (name) of the cookie to be set.

      • value (str) – the value of the cookie.

      • max_age (Optional[Union[datetime.timedelta, int]**]) – should be a number of seconds, or None (default) if the cookie should last only as long as the client’s browser session.

      • expires (Optional[Union[str, datetime.datetime, int, float]**]) – should be a datetime object or UNIX timestamp.

      • path (Optional[str]) – limits the cookie to a given path, per default it will span the whole domain.

      • domain (Optional[str]) – if you want to set a cross-domain cookie. For example, domain=".example.com" will set a cookie that is readable by the domain www.example.com, foo.example.com etc. Otherwise, a cookie will only be readable by the domain that set it.

      • secure (bool) – If True, the cookie will only be available via HTTPS.

      • httponly (bool) – Disallow JavaScript access to the cookie.

      • samesite (Optional[str]) – Limit the scope of the cookie to only be attached to requests that are “same-site”.

      Return type

      None

  • set_data(value)

    Sets a new string as response. The value must be a string or bytes. If a string is set it’s encoded to the charset of the response (utf-8 by default).

    Changelog

    New in version 0.9.

  • set_etag(etag, weak=False)

    Set the etag, and override the old one if there was one.

    • Parameters

      Return type

      None

  • property status: str

    The HTTP status code as a string.

  • property status_code: int

    The HTTP status code as a number.

  • property stream: werkzeug.wrappers.response.ResponseStream

    The response iterable as write-only stream.

  • property vary: werkzeug.datastructures.HeaderSet

    The Vary field value indicates the set of request-header fields that fully determines, while the response is fresh, whether a cache is permitted to use the response to reply to a subsequent request without revalidation.

  • property www_authenticate: werkzeug.datastructures.WWWAuthenticate

    The WWW-Authenticate header in a parsed form.

Sessions

If you have set Flask.secret_key (or configured it from SECRET_KEY) you can use sessions in Flask applications. A session makes it possible to remember information from one request to another. The way Flask does this is by using a signed cookie. The user can look at the session contents, but can’t modify it unless they know the secret key, so make sure to set that to something complex and unguessable.

To access the current session you can use the session object:

class flask.session

The session object works pretty much like an ordinary dict, with the difference that it keeps track of modifications.

This is a proxy. See Notes On Proxies for more information.

The following attributes are interesting:

  • new

    True if the session is new, False otherwise.

  • modified

    True if the session object detected a modification. Be advised that modifications on mutable structures are not picked up automatically, in that situation you have to explicitly set the attribute to True yourself. Here an example:

    1. # this change is not picked up because a mutable object (here
    2. # a list) is changed.
    3. session['objects'].append(42)
    4. # so mark it as modified yourself
    5. session.modified = True

  • permanent

    If set to True the session lives for permanent_session_lifetime seconds. The default is 31 days. If set to False (which is the default) the session will be deleted when the user closes the browser.

Session Interface

Changelog

New in version 0.8.

The session interface provides a simple way to replace the session implementation that Flask is using.

class flask.sessions.SessionInterface

The basic interface you have to implement in order to replace the default session interface which uses werkzeug’s securecookie implementation. The only methods you have to implement are open_session() and save_session(), the others have useful defaults which you don’t need to change.

The session object returned by the open_session() method has to provide a dictionary like interface plus the properties and methods from the SessionMixin. We recommend just subclassing a dict and adding that mixin:

  1. class Session(dict, SessionMixin):
  2.     pass

If open_session() returns None Flask will call into make_null_session() to create a session that acts as replacement if the session support cannot work because some requirement is not fulfilled. The default NullSession class that is created will complain that the secret key was not set.

To replace the session interface on an application all you have to do is to assign flask.Flask.session_interface:

  1. app = Flask(__name__)
  2. app.session_interface = MySessionInterface()

Multiple requests with the same session may be sent and handled concurrently. When implementing a new session interface, consider whether reads or writes to the backing store must be synchronized. There is no guarantee on the order in which the session for each request is opened or saved, it will occur in the order that requests begin and end processing.

Changelog

New in version 0.8.

  • get_cookie_domain(app)

    Returns the domain that should be set for the session cookie.

    Uses SESSION_COOKIE_DOMAIN if it is configured, otherwise falls back to detecting the domain based on SERVER_NAME.

    Once detected (or if not set at all), SESSION_COOKIE_DOMAIN is updated to avoid re-running the logic.

  • get_cookie_httponly(app)

    Returns True if the session cookie should be httponly. This currently just returns the value of the SESSION_COOKIE_HTTPONLY config var.

    • Parameters

      app (Flask) –

      Return type

      bool

  • get_cookie_name(app)

    Returns the name of the session cookie.

    Uses app.session_cookie_name which is set to SESSION_COOKIE_NAME

    • Parameters

      app (Flask) –

      Return type

      str

  • get_cookie_path(app)

    Returns the path for which the cookie should be valid. The default implementation uses the value from the SESSION_COOKIE_PATH config var if it’s set, and falls back to APPLICATION_ROOT or uses / if it’s None.

    • Parameters

      app (Flask) –

      Return type

      str

  • get_cookie_samesite(app)

    Return 'Strict' or 'Lax' if the cookie should use the SameSite attribute. This currently just returns the value of the SESSION_COOKIE_SAMESITE setting.

    • Parameters

      app (Flask) –

      Return type

      str

  • get_cookie_secure(app)

    Returns True if the cookie should be secure. This currently just returns the value of the SESSION_COOKIE_SECURE setting.

    • Parameters

      app (Flask) –

      Return type

      bool

  • get_expiration_time(app, session)

    A helper method that returns an expiration date for the session or None if the session is linked to the browser session. The default implementation returns now + the permanent session lifetime configured on the application.

  • is_null_session(obj)

    Checks if a given object is a null session. Null sessions are not asked to be saved.

    This checks if the object is an instance of null_session_class by default.

  • make_null_session(app)

    Creates a null session which acts as a replacement object if the real session support could not be loaded due to a configuration error. This mainly aids the user experience because the job of the null session is to still support lookup without complaining but modifications are answered with a helpful error message of what failed.

    This creates an instance of null_session_class by default.

  • null_session_class

    make_null_session() will look here for the class that should be created when a null session is requested. Likewise the is_null_session() method will perform a typecheck against this type.

    alias of flask.sessions.NullSession

  • open_session(app, request)

    This is called at the beginning of each request, after pushing the request context, before matching the URL.

    This must return an object which implements a dictionary-like interface as well as the SessionMixin interface.

    This will return None to indicate that loading failed in some way that is not immediately an error. The request context will fall back to using make_null_session() in this case.

  • pickle_based = False

    A flag that indicates if the session interface is pickle based. This can be used by Flask extensions to make a decision in regards to how to deal with the session object.

    Changelog

    New in version 0.10.

  • save_session(app, session, response)

    This is called at the end of each request, after generating a response, before removing the request context. It is skipped if is_null_session() returns True.

  • should_set_cookie(app, session)

    Used by session backends to determine if a Set-Cookie header should be set for this session cookie for this response. If the session has been modified, the cookie is set. If the session is permanent and the SESSION_REFRESH_EACH_REQUEST config is true, the cookie is always set.

    This check is usually skipped if the session was deleted.

    Changelog

    New in version 0.11.

class flask.sessions.SecureCookieSessionInterface

The default session interface that stores sessions in signed cookies through the itsdangerous module.

  • static digest_method(string=b’’, **, usedforsecurity=True*)

    the hash function to use for the signature. The default is sha1

  • key_derivation = ‘hmac’

    the name of the itsdangerous supported key derivation. The default is hmac.

  • open_session(app, request)

    This is called at the beginning of each request, after pushing the request context, before matching the URL.

    This must return an object which implements a dictionary-like interface as well as the SessionMixin interface.

    This will return None to indicate that loading failed in some way that is not immediately an error. The request context will fall back to using make_null_session() in this case.

  • salt = ‘cookie-session’

    the salt that should be applied on top of the secret key for the signing of cookie based sessions.

  • save_session(app, session, response)

    This is called at the end of each request, after generating a response, before removing the request context. It is skipped if is_null_session() returns True.

  • serializer = <flask.json.tag.TaggedJSONSerializer object>

    A python serializer for the payload. The default is a compact JSON derived serializer with support for some extra Python types such as datetime objects or tuples.

  • session_class

    alias of flask.sessions.SecureCookieSession

class flask.sessions.SecureCookieSession(initial=None)

Base class for sessions based on signed cookies.

This session backend will set the modified and accessed attributes. It cannot reliably track whether a session is new (vs. empty), so new remains hard coded to False.

  • Parameters

    initial (Any) –

    Return type

    None

  • accessed = False

    header, which allows caching proxies to cache different pages for different users.

  • get(key, default=None)

    Return the value for key if key is in the dictionary, else default.

  • modified = False

    When data is changed, this is set to True. Only the session dictionary itself is tracked; if the session contains mutable data (for example a nested dict) then this must be set to True manually when modifying that data. The session cookie will only be written to the response if this is True.

  • setdefault(key, default=None)

    Insert key with a value of default if key is not in the dictionary.

    Return the value for key if key is in the dictionary, else default.

class flask.sessions.NullSession(initial=None)

Class used to generate nicer error messages if sessions are not available. Will still allow read-only access to the empty session but fail on setting.

  • Parameters

    initial (Any) –

    Return type

    None

  • clear() → None. Remove all items from D.

    • Parameters

      • args (Any) –

      • kwargs (Any) –

      Return type

      te.NoReturn

  • pop(k[, d]) → v, remove specified key and return the corresponding value.

    If the key is not found, return the default if given; otherwise, raise a KeyError.

    • Parameters

      • args (Any) –

      • kwargs (Any) –

      Return type

      te.NoReturn

  • popitem(\args, **kwargs*)

    Remove and return a (key, value) pair as a 2-tuple.

    Pairs are returned in LIFO (last-in, first-out) order. Raises KeyError if the dict is empty.

    • Parameters

      • args (Any) –

      • kwargs (Any) –

      Return type

      te.NoReturn

  • setdefault(\args, **kwargs*)

    Insert key with a value of default if key is not in the dictionary.

    Return the value for key if key is in the dictionary, else default.

    • Parameters

      • args (Any) –

      • kwargs (Any) –

      Return type

      te.NoReturn

  • update([E, ]\*F*) → None. Update D from dict/iterable E and F.

    If E is present and has a .keys() method, then does: for k in E: D[k] = E[k] If E is present and lacks a .keys() method, then does: for k, v in E: D[k] = v In either case, this is followed by: for k in F: D[k] = F[k]

    • Parameters

      • args (Any) –

      • kwargs (Any) –

      Return type

      te.NoReturn

class flask.sessions.SessionMixin

Expands a basic dictionary with session attributes.

  • accessed = True

    Some implementations can detect when session data is read or written and set this when that happens. The mixin default is hard coded to True.

  • modified = True

    Some implementations can detect changes to the session and set this when that happens. The mixin default is hard coded to True.

  • property permanent: bool

    This reflects the '_permanent' key in the dict.

Notice

The PERMANENT_SESSION_LIFETIME config key can also be an integer starting with Flask 0.8. Either catch this down yourself or use the permanent_session_lifetime attribute on the app which converts the result to an integer automatically.

Test Client

class flask.testing.FlaskClient(\args, **kwargs*)

Works like a regular Werkzeug test client but has some knowledge about how Flask works to defer the cleanup of the request context stack to the end of a with body when used in a with statement. For general information about how to use this class refer to werkzeug.test.Client.

Changelog

Changed in version 0.12: app.test_client() includes preset default environment, which can be set after instantiation of the app.test_client() object in client.environ_base.

Basic usage is outlined in the Testing Flask Applications chapter.

  • Parameters

    • args (Any) –

    • kwargs (Any) –

    Return type

    None

  • open(\args, buffered=False, follow_redirects=False, **kwargs*)

    Generate an environ dict from the given arguments, make a request to the application using it, and return the response.

    • Parameters

      • args (Any) – Passed to EnvironBuilder to create the environ for the request. If a single arg is passed, it can be an existing EnvironBuilder or an environ dict.

      • buffered (bool) – Convert the iterator returned by the app into a list. If the iterator has a close() method, it is called automatically.

      • follow_redirects (bool) – Make additional requests to follow HTTP redirects until a non-redirect status is returned. TestResponse.history lists the intermediate responses.

      • kwargs (Any) –

      Return type

      TestResponse

    Changed in version 2.1: Removed the as_tuple parameter.

    Changelog

    Changed in version 2.0: as_tuple is deprecated and will be removed in Werkzeug 2.1. Use TestResponse.request and request.environ instead.

    Changed in version 2.0: The request input stream is closed when calling response.close(). Input streams for redirects are automatically closed.

    Changed in version 0.5: If a dict is provided as file in the dict for the data parameter the content type has to be called content_type instead of mimetype. This change was made for consistency with werkzeug.FileWrapper.

    Changed in version 0.5: Added the follow_redirects parameter.

  • session_transaction(\args, **kwargs*)

    When used in combination with a with statement this opens a session transaction. This can be used to modify the session that the test client uses. Once the with block is left the session is stored back.

    1. with client.session_transaction() as session:
    2.     session['value'] = 42

    Internally this is implemented by going through a temporary test request context and since session handling could depend on request variables this function accepts the same arguments as test_request_context() which are directly passed through.

Test CLI Runner

class flask.testing.FlaskCliRunner(app, \*kwargs*)

A CliRunner for testing a Flask app’s CLI commands. Typically created using test_cli_runner(). See Running Commands with the CLI Runner.

  • Parameters

    Return type

    None

  • invoke(cli=None, args=None, \*kwargs*)

    Invokes a CLI command in an isolated environment. See CliRunner.invoke for full method documentation. See Running Commands with the CLI Runner for examples.

    If the obj argument is not given, passes an instance of ScriptInfo that knows how to load the Flask app being tested.

    • Parameters

      • cli (Optional[Any]) – Command object to invoke. Default is the app’s cli group.

      • args (Optional[Any]) – List of strings to invoke the command with.

      • kwargs (Any) –

      Returns

      a Result object.

      Return type

      Any

Application Globals

To share data that is valid for one request only from one function to another, a global variable is not good enough because it would break in threaded environments. Flask provides you with a special object that ensures it is only valid for the active request and that will return different values for each request. In a nutshell: it does the right thing, like it does for request and session.

flask.g

A namespace object that can store data during an application context. This is an instance of Flask.app_ctx_globals_class, which defaults to ctx._AppCtxGlobals.

This is a good place to store resources during a request. For example, a before_request function could load a user object from a session id, then set g.user to be used in the view function.

This is a proxy. See Notes On Proxies for more information.

Changelog

Changed in version 0.10: Bound to the application context instead of the request context.

class flask.ctx._AppCtxGlobals

A plain object. Used as a namespace for storing data during an application context.

Creating an app context automatically creates this object, which is made available as the g proxy.

  • ‘key’ in g

    Check whether an attribute is present.

    Changelog

    New in version 0.10.

  • iter(g)

    Return an iterator over the attribute names.

    Changelog

    New in version 0.10.

  • get(name, default=None)

    Get an attribute by name, or a default value. Like dict.get().

    • Parameters

      • name (str) – Name of attribute to get.

      • default (Optional[Any]) – Value to return if the attribute is not present.

      Return type

      Any

    Changelog

    New in version 0.10.

  • pop(name, default=<object object>)

    Get and remove an attribute by name. Like dict.pop().

    • Parameters

      • name (str) – Name of attribute to pop.

      • default (Any) – Value to return if the attribute is not present, instead of raising a KeyError.

      Return type

      Any

    Changelog

    New in version 0.11.

  • setdefault(name, default=None)

    Get the value of an attribute if it is present, otherwise set and return a default value. Like dict.setdefault().

    • Parameters

      • name (str) – Name of attribute to get.

      • default (Optional[Any]) – Value to set and return if the attribute is not present.

      Return type

      Any

    Changelog

    New in version 0.11.

Useful Functions and Classes

flask.current_app

A proxy to the application handling the current request. This is useful to access the application without needing to import it, or if it can’t be imported, such as when using the application factory pattern or in blueprints and extensions.

This is only available when an application context is pushed. This happens automatically during requests and CLI commands. It can be controlled manually with app_context().

This is a proxy. See Notes On Proxies for more information.

flask.has_request_context()

If you have code that wants to test if a request context is there or not this function can be used. For instance, you may want to take advantage of request information if the request object is available, but fail silently if it is unavailable.

  1. class User(db.Model):
  3.     def __init__(self, username, remote_addr=None):
  4.         self.username = username
  5.         if remote_addr is None and has_request_context():
  6.             remote_addr = request.remote_addr
  7.         self.remote_addr = remote_addr

Alternatively you can also just test any of the context bound objects (such as request or g) for truthness:

  1. class User(db.Model):
  3.     def __init__(self, username, remote_addr=None):
  4.         self.username = username
  5.         if remote_addr is None and request:
  6.             remote_addr = request.remote_addr
  7.         self.remote_addr = remote_addr

Changelog

New in version 0.7.

flask.copy_current_request_context(f)

A helper function that decorates a function to retain the current request context. This is useful when working with greenlets. The moment the function is decorated a copy of the request context is created and then pushed when the function is called. The current session is also included in the copied request context.

Example:

  1. import gevent
  2. from flask import copy_current_request_context
  4. @app.route('/')
  5. def index():
  6.     @copy_current_request_context
  7.     def do_some_work():
  8.         # do some work here, it can access flask.request or
  9.         # flask.session like you would otherwise in the view function.
  10.         ...
  11.     gevent.spawn(do_some_work)
  12.     return 'Regular response'

Changelog

New in version 0.10.

flask.has_app_context()

Works like has_request_context() but for the application context. You can also just do a boolean check on the current_app object instead.

Changelog

New in version 0.9.

flask.url_for(endpoint, \*values*)

Generates a URL to the given endpoint with the method provided.

Variable arguments that are unknown to the target endpoint are appended to the generated URL as query arguments. If the value of a query argument is None, the whole pair is skipped. In case blueprints are active you can shortcut references to the same blueprint by prefixing the local endpoint with a dot (.).

This will reference the index function local to the current blueprint:

  1. url_for('.index')

See URL Building.

Configuration values APPLICATION_ROOT and SERVER_NAME are only used when generating URLs outside of a request context.

To integrate applications, Flask has a hook to intercept URL build errors through Flask.url_build_error_handlers. The url_for function results in a BuildError when the current app does not have a URL for the given endpoint and values. When it does, the current_app calls its url_build_error_handlers if it is not None, which can return a string to use as the result of url_for (instead of url_for’s default to raise the BuildError exception) or re-raise the exception. An example:

  1. def external_url_handler(error, endpoint, values):
  2.     "Looks up an external URL when `url_for` cannot build a URL."
  3.     # This is an example of hooking the build_error_handler.
  4.     # Here, lookup_url is some utility function you've built
  5.     # which looks up the endpoint in some external URL registry.
  6.     url = lookup_url(endpoint, **values)
  7.     if url is None:
  8.         # External lookup did not have a URL.
  9.         # Re-raise the BuildError, in context of original traceback.
  10.         exc_type, exc_value, tb = sys.exc_info()
  11.         if exc_value is error:
  12.             raise exc_type(exc_value).with_traceback(tb)
  13.         else:
  14.             raise error
  15.     # url_for will use this result, instead of raising BuildError.
  16.     return url
  18. app.url_build_error_handlers.append(external_url_handler)

Here, error is the instance of BuildError, and endpoint and values are the arguments passed into url_for. Note that this is for building URLs outside the current application, and not for handling 404 NotFound errors.

Changelog

New in version 0.10: The _scheme parameter was added.

New in version 0.9: The _anchor and _method parameters were added.

New in version 0.9: Calls Flask.handle_build_error() on BuildError.

  • Parameters

    • endpoint (str) – the endpoint of the URL (name of the function)

    • values (Any) – the variable arguments of the URL rule

    • _external – if set to True, an absolute URL is generated. Server address can be changed via SERVER_NAME configuration variable which falls back to the Host header, then to the IP and port of the request.

    • _scheme – a string specifying the desired URL scheme. The _external parameter must be set to True or a ValueError is raised. The default behavior uses the same scheme as the current request, or PREFERRED_URL_SCHEME if no request context is available. This also can be set to an empty string to build protocol-relative URLs.

    • _anchor – if provided this is added as anchor to the URL.

    • _method – if provided this explicitly specifies an HTTP method.

    Return type

    str

flask.abort(status, \args, **kwargs*)

Raises an HTTPException for the given status code or WSGI application.

If a status code is given, it will be looked up in the list of exceptions and will raise that exception. If passed a WSGI application, it will wrap it in a proxy WSGI exception and raise that:

  1. abort(404)  # 404 Not Found
  2. abort(Response('Hello World'))

flask.redirect(location, code=302, Response=None)

Returns a response object (a WSGI application) that, if called, redirects the client to the target location. Supported codes are 301, 302, 303, 305, 307, and 308. 300 is not supported because it’s not a real redirect and 304 because it’s the answer for a request with a request with defined If-Modified-Since headers.

Changelog

New in version 0.10: The class used for the Response object can now be passed in.

New in version 0.6: The location can now be a unicode string that is encoded using the iri_to_uri() function.

  • Parameters

    • location (str) – the location the response should redirect to.

    • code (int) – the redirect status code. defaults to 302.

    • Response (class) – a Response class to use when instantiating a response. The default is werkzeug.wrappers.Response if unspecified.

    Return type

    Response

flask.make_response(\args*)

Sometimes it is necessary to set additional headers in a view. Because views do not have to return response objects but can return a value that is converted into a response object by Flask itself, it becomes tricky to add headers to it. This function can be called instead of using a return and you will get a response object which you can use to attach headers.

If view looked like this and you want to add a new header:

  1. def index():
  2.     return render_template('index.html', foo=42)

You can now do something like this:

  1. def index():
  2.     response = make_response(render_template('index.html', foo=42))
  3.     response.headers['X-Parachutes'] = 'parachutes are cool'
  4.     return response

This function accepts the very same arguments you can return from a view function. This for example creates a response with a 404 error code:

  1. response = make_response(render_template('not_found.html'), 404)

The other use case of this function is to force the return value of a view function into a response which is helpful with view decorators:

  1. response = make_response(view_function())
  2. response.headers['X-Parachutes'] = 'parachutes are cool'

Internally this function does the following things:

Changelog

New in version 0.6.

flask.after_this_request(f)

Executes a function after this request. This is useful to modify response objects. The function is passed the response object and has to return the same or a new one.

Example:

  1. @app.route('/')
  2. def index():
  3.     @after_this_request
  4.     def add_header(response):
  5.         response.headers['X-Foo'] = 'Parachute'
  6.         return response
  7.     return 'Hello World!'

This is more useful if a function other than the view function wants to modify a response. For instance think of a decorator that wants to add some headers without converting the return value into a response object.

Changelog

New in version 0.9.

  • Parameters

    f (Callable[[flask.typing.ResponseClass], flask.typing.ResponseClass**]) –

    Return type

    Callable[[flask.typing.ResponseClass], flask.typing.ResponseClass]

flask.send_file(path_or_file, mimetype=None, as_attachment=False, download_name=None, attachment_filename=None, conditional=True, etag=True, add_etags=None, last_modified=None, max_age=None, cache_timeout=None)

Send the contents of a file to the client.

The first argument can be a file path or a file-like object. Paths are preferred in most cases because Werkzeug can manage the file and get extra information from the path. Passing a file-like object requires that the file is opened in binary mode, and is mostly useful when building a file in memory with io.BytesIO.

Never pass file paths provided by a user. The path is assumed to be trusted, so a user could craft a path to access a file you didn’t intend. Use send_from_directory() to safely serve user-requested paths from within a directory.

If the WSGI server sets a file_wrapper in environ, it is used, otherwise Werkzeug’s built-in wrapper is used. Alternatively, if the HTTP server supports X-Sendfile, configuring Flask with USE_X_SENDFILE = True will tell the server to send the given path, which is much more efficient than reading it in Python.

  • Parameters

    • path_or_file (Union[os.PathLike, str, BinaryIO]) – The path to the file to send, relative to the current working directory if a relative path is given. Alternatively, a file-like object opened in binary mode. Make sure the file pointer is seeked to the start of the data.

    • mimetype (Optional[str]) – The MIME type to send for the file. If not provided, it will try to detect it from the file name.

    • as_attachment (bool) – Indicate to a browser that it should offer to save the file instead of displaying it.

    • download_name (Optional[str]) – The default name browsers will use when saving the file. Defaults to the passed file name.

    • conditional (bool) – Enable conditional and range responses based on request headers. Requires passing a file path and environ.

    • etag (Union[bool, str]) – Calculate an ETag for the file, which requires passing a file path. Can also be a string to use instead.

    • last_modified (Optional[Union[datetime.datetime, int, float]**]) – The last modified time to send for the file, in seconds. If not provided, it will try to detect it from the file path.

    • max_age (Optional[Union[int, Callable[**[Optional[str]], Optional[int]]]**]) – How long the client should cache the file, in seconds. If set, Cache-Control will be public, otherwise it will be no-cache to prefer conditional caching.

    • attachment_filename (Optional[str]) –

    • add_etags (Optional[bool]) –

    • cache_timeout (Optional[int]) –

Changelog

Changed in version 2.0: download_name replaces the attachment_filename parameter. If as_attachment=False, it is passed with Content-Disposition: inline instead.

Changed in version 2.0: max_age replaces the cache_timeout parameter. conditional is enabled and max_age is not set by default.

Changed in version 2.0: etag replaces the add_etags parameter. It can be a string to use instead of generating one.

Changed in version 2.0: Passing a file-like object that inherits from TextIOBase will raise a ValueError rather than sending an empty file.

New in version 2.0: Moved the implementation to Werkzeug. This is now a wrapper to pass some Flask-specific arguments.

Changed in version 1.1: filename may be a PathLike object.

Changed in version 1.1: Passing a BytesIO object supports range requests.

Changed in version 1.0.3: Filenames are encoded with ASCII instead of Latin-1 for broader compatibility with WSGI servers.

Changed in version 1.0: UTF-8 filenames as specified in RFC 2231 are supported.

Changed in version 0.12: The filename is no longer automatically inferred from file objects. If you want to use automatic MIME and etag support, pass a filename via filename_or_fp or attachment_filename.

Changed in version 0.12: attachment_filename is preferred over filename for MIME detection.

Changed in version 0.9: cache_timeout defaults to Flask.get_send_file_max_age().

Changed in version 0.7: MIME guessing and etag support for file-like objects was deprecated because it was unreliable. Pass a filename if you are able to, otherwise attach an etag yourself.

Changed in version 0.5: The add_etags, cache_timeout and conditional parameters were added. The default behavior is to add etags.

New in version 0.2.

flask.send_from_directory(directory, path, filename=None, \*kwargs*)

Send a file from within a directory using send_file().

  1. @app.route("/uploads/<path:name>")
  2. def download_file(name):
  3.     return send_from_directory(
  4.         app.config['UPLOAD_FOLDER'], name, as_attachment=True
  5.     )

This is a secure way to serve files from a folder, such as static files or uploads. Uses safe_join() to ensure the path coming from the client is not maliciously crafted to point outside the specified directory.

If the final path does not point to an existing regular file, raises a 404 NotFound error.

Changelog

Changed in version 2.0: path replaces the filename parameter.

New in version 2.0: Moved the implementation to Werkzeug. This is now a wrapper to pass some Flask-specific arguments.

New in version 0.5.

flask.escape()

Replace the characters &, <, >, ', and " in the string with HTML-safe sequences. Use this if you need to display text that might contain such characters in HTML.

If the object has an __html__ method, it is called and the return value is assumed to already be safe for HTML.

  • Parameters

    s – An object to be converted to a string and escaped.

    Returns

    A Markup string with the escaped text.

class flask.Markup(base=’’, encoding=None, errors=’strict’)

A string that is ready to be safely inserted into an HTML or XML document, either because it was escaped or because it was marked safe.

Passing an object to the constructor converts it to text and wraps it to mark it safe without escaping. To escape the text, use the escape() class method instead.

  1. >>> Markup("Hello, <em>World</em>!")
  2. Markup('Hello, <em>World</em>!')
  3. >>> Markup(42)
  4. Markup('42')
  5. >>> Markup.escape("Hello, <em>World</em>!")
  6. Markup('Hello &lt;em&gt;World&lt;/em&gt;!')

This implements the __html__() interface that some frameworks use. Passing an object that implements __html__() will wrap the output of that method, marking it safe.

  1. >>> class Foo:
  2. ...     def __html__(self):
  3. ...         return '<a href="/foo">foo</a>'
  4. ...
  5. >>> Markup(Foo())
  6. Markup('<a href="/foo">foo</a>')

This is a subclass of str. It has the same methods, but escapes their arguments and returns a Markup instance.

  1. >>> Markup("<em>%s</em>") % ("foo & bar",)
  2. Markup('<em>foo &amp; bar</em>')
  3. >>> Markup("<em>Hello</em> ") + "<foo>"
  4. Markup('<em>Hello</em> &lt;foo&gt;')

  • Parameters

    Return type

    Markup

  • classmethod escape(s)

    Escape a string. Calls escape() and ensures that for subclasses the correct type is returned.

  • striptags()

    unescape() the markup, remove tags, and normalize whitespace to single spaces.

    1. >>> Markup("Main &raquo;        <em>About</em>").striptags()
    2. 'Main » About'

    • Return type

      str

  • unescape()

    Convert escaped markup back into a text string. This replaces HTML entities with the characters they represent.

    1. >>> Markup("Main &raquo; <em>About</em>").unescape()
    2. 'Main » <em>About</em>'

    • Return type

      str

Message Flashing

flask.flash(message, category=’message’)

Flashes a message to the next request. In order to remove the flashed message from the session and to display it to the user, the template has to call get_flashed_messages().

Changelog

Changed in version 0.3: category parameter added.

  • Parameters

    • message (str) – the message to be flashed.

    • category (str) – the category for the message. The following values are recommended: 'message' for any kind of message, 'error' for errors, 'info' for information messages and 'warning' for warnings. However any kind of string can be used as category.

    Return type

    None

flask.get_flashed_messages(with_categories=False, category_filter=())

Pulls all flashed messages from the session and returns them. Further calls in the same request to the function will return the same messages. By default just the messages are returned, but when with_categories is set to True, the return value will be a list of tuples in the form (category, message) instead.

Filter the flashed messages to one or more categories by providing those categories in category_filter. This allows rendering categories in separate html blocks. The with_categories and category_filter arguments are distinct:

  • with_categories controls whether categories are returned with message text (True gives a tuple, where False gives just the message text).

  • category_filter filters the messages down to only those matching the provided categories.

See Message Flashing for examples.

Changelog

Changed in version 0.9: category_filter parameter added.

Changed in version 0.3: with_categories parameter added.

  • Parameters

    • with_categories (bool) – set to True to also receive categories.

    • category_filter (Iterable[str]) – filter of categories to limit return values. Only categories in the list will be returned.

    Return type

    Union[List[str], List[Tuple[str, str]]]

JSON Support

Flask uses the built-in json module for handling JSON. It will use the current blueprint’s or application’s JSON encoder and decoder for easier customization. By default it handles some extra data types:

Jinja’s |tojson filter is configured to use Flask’s dumps() function. The filter marks the output with |safe automatically. Use the filter to render data inside <script> tags.

  1. <script>
  2.     const names = {{ names|tosjon }};
  3.     renderChart(names, {{ axis_data|tojson }});
  4. </script>

flask.json.jsonify(\args, **kwargs*)

Serialize data to JSON and wrap it in a Response with the application/json mimetype.

Uses dumps() to serialize the data, but args and kwargs are treated as data rather than arguments to json.dumps().

  1. Single argument: Treated as a single value.

  2. Multiple arguments: Treated as a list of values. jsonify(1, 2, 3) is the same as jsonify([1, 2, 3]).

  3. Keyword arguments: Treated as a dict of values. jsonify(data=data, errors=errors) is the same as jsonify({"data": data, "errors": errors}).

  4. Passing both arguments and keyword arguments is not allowed as it’s not clear what should happen.

  1. from flask import jsonify
  3. @app.route("/users/me")
  4. def get_current_user():
  5.     return jsonify(
  6.         username=g.user.username,
  7.         email=g.user.email,
  8.         id=g.user.id,
  9.     )

Will return a JSON response like this:

  1. {
  2.   "username": "admin",
  3.   "email": "admin@localhost",
  4.   "id": 42
  5. }

The default output omits indents and spaces after separators. In debug mode or if JSONIFY_PRETTYPRINT_REGULAR is True, the output will be formatted to be easier to read.

Changelog

Changed in version 2.0.2: decimal.Decimal is supported by converting to a string.

Changed in version 0.11: Added support for serializing top-level arrays. This introduces a security risk in ancient browsers. See JSON Security.

New in version 0.2.

flask.json.dumps(obj, app=None, \*kwargs*)

Serialize an object to a string of JSON.

Takes the same arguments as the built-in json.dumps(), with some defaults from application configuration.

  • Parameters

    • obj (Any) – Object to serialize to JSON.

    • app (Optional[Flask]) – Use this app’s config instead of the active app context or defaults.

    • kwargs (Any) – Extra arguments passed to json.dumps().

    Return type

    str

Changelog

Changed in version 2.0.2: decimal.Decimal is supported by converting to a string.

Changed in version 2.0: encoding is deprecated and will be removed in Flask 2.1.

Changed in version 1.0.3: app can be passed directly, rather than requiring an app context for configuration.

flask.json.dump(obj, fp, app=None, \*kwargs*)

Serialize an object to JSON written to a file object.

Takes the same arguments as the built-in json.dump(), with some defaults from application configuration.

  • Parameters

    • obj (Any) – Object to serialize to JSON.

    • fp (IO[str]) – File object to write JSON to.

    • app (Optional[Flask]) – Use this app’s config instead of the active app context or defaults.

    • kwargs (Any) – Extra arguments passed to json.dump().

    Return type

    None

Changelog

Changed in version 2.0: Writing to a binary file, and the encoding argument, is deprecated and will be removed in Flask 2.1.

flask.json.loads(s, app=None, \*kwargs*)

Deserialize an object from a string of JSON.

Takes the same arguments as the built-in json.loads(), with some defaults from application configuration.

  • Parameters

    • s (Union[str, bytes]) – JSON string to deserialize.

    • app (Optional[Flask]) – Use this app’s config instead of the active app context or defaults.

    • kwargs (Any) – Extra arguments passed to json.loads().

    Return type

    Any

Changelog

Changed in version 2.0: encoding is deprecated and will be removed in Flask 2.1. The data must be a string or UTF-8 bytes.

Changed in version 1.0.3: app can be passed directly, rather than requiring an app context for configuration.

flask.json.load(fp, app=None, \*kwargs*)

Deserialize an object from JSON read from a file object.

Takes the same arguments as the built-in json.load(), with some defaults from application configuration.

  • Parameters

    • fp (IO[str]) – File object to read JSON from.

    • app (Optional[Flask]) – Use this app’s config instead of the active app context or defaults.

    • kwargs (Any) – Extra arguments passed to json.load().

    Return type

    Any

Changelog

Changed in version 2.0: encoding is deprecated and will be removed in Flask 2.1. The file must be text mode, or binary mode with UTF-8 bytes.

class flask.json.JSONEncoder(**, skipkeys=False, ensure_ascii=True, check_circular=True, allow_nan=True, sort_keys=False, indent=None, separators=None, default=None*)

The default JSON encoder. Handles extra types compared to the built-in json.JSONEncoder.

Assign a subclass of this to flask.Flask.json_encoder or flask.Blueprint.json_encoder to override the default.

  • default(o)

    Convert o to a JSON serializable type. See json.JSONEncoder.default(). Python does not support overriding how basic types like str or list are serialized, they are handled before this method.

    • Parameters

      o (Any) –

      Return type

      Any

class flask.json.JSONDecoder(**, object_hook=None, parse_float=None, parse_int=None, parse_constant=None, strict=True, object_pairs_hook=None*)

The default JSON decoder.

This does not change any behavior from the built-in json.JSONDecoder.

Assign a subclass of this to flask.Flask.json_decoder or flask.Blueprint.json_decoder to override the default.

Tagged JSON

A compact representation for lossless serialization of non-standard JSON types. SecureCookieSessionInterface uses this to serialize the session data, but it may be useful in other places. It can be extended to support other types.

class flask.json.tag.TaggedJSONSerializer

Serializer that uses a tag system to compactly represent objects that are not JSON types. Passed as the intermediate serializer to itsdangerous.Serializer.

The following extra types are supported:

  • dict

  • tuple

  • bytes

  • Markup

  • UUID

  • datetime

  • Return type

    None

  • default_tags = [<class ‘flask.json.tag.TagDict’>, <class ‘flask.json.tag.PassDict’>, <class ‘flask.json.tag.TagTuple’>, <class ‘flask.json.tag.PassList’>, <class ‘flask.json.tag.TagBytes’>, <class ‘flask.json.tag.TagMarkup’>, <class ‘flask.json.tag.TagUUID’>, <class ‘flask.json.tag.TagDateTime’>]

    Tag classes to bind when creating the serializer. Other tags can be added later using register().

  • dumps(value)

    Tag the value and dump it to a compact JSON string.

    • Parameters

      value (Any) –

      Return type

      str

  • loads(value)

    Load data from a JSON string and deserialized any tagged objects.

    • Parameters

      value (str) –

      Return type

      Any

  • register(tag_class, force=False, index=None)

    Register a new tag with this serializer.

    • Parameters

      • tag_class (Type[flask.json.tag.JSONTag]) – tag class to register. Will be instantiated with this serializer instance.

      • force (bool) – overwrite an existing tag. If false (default), a KeyError is raised.

      • index (Optional[int]) – index to insert the new tag in the tag order. Useful when the new tag is a special case of an existing tag. If None (default), the tag is appended to the end of the order.

      Raises

      KeyError – if the tag key is already registered and force is not true.

      Return type

      None

  • tag(value)

    Convert a value to a tagged representation if necessary.

  • untag(value)

    Convert a tagged representation back to the original type.

class flask.json.tag.JSONTag(serializer)

Base class for defining type tags for TaggedJSONSerializer.

  • Parameters

    serializer (TaggedJSONSerializer) –

    Return type

    None

  • check(value)

    Check if the given value should be tagged by this tag.

    • Parameters

      value (Any) –

      Return type

      bool

  • key: Optional[str] = None

    The tag to mark the serialized object with. If None, this tag is only used as an intermediate step during tagging.

  • tag(value)

    Convert the value to a valid JSON type and add the tag structure around it.

    • Parameters

      value (Any) –

      Return type

      Any

  • to_json(value)

    Convert the Python object to an object that is a valid JSON type. The tag will be added later.

    • Parameters

      value (Any) –

      Return type

      Any

  • to_python(value)

    Convert the JSON representation back to the correct type. The tag will already be removed.

    • Parameters

      value (Any) –

      Return type

      Any

Let’s see an example that adds support for OrderedDict. Dicts don’t have an order in JSON, so to handle this we will dump the items as a list of [key, value] pairs. Subclass JSONTag and give it the new key ' od' to identify the type. The session serializer processes dicts first, so insert the new tag at the front of the order since OrderedDict must be processed before dict.

  1. from flask.json.tag import JSONTag
  3. class TagOrderedDict(JSONTag):
  4.     __slots__ = ('serializer',)
  5.     key = ' od'
  7.     def check(self, value):
  8.         return isinstance(value, OrderedDict)
  10.     def to_json(self, value):
  11.         return [[k, self.serializer.tag(v)] for k, v in iteritems(value)]
  13.     def to_python(self, value):
  14.         return OrderedDict(value)
  16. app.session_interface.serializer.register(TagOrderedDict, index=0)

Template Rendering

flask.render_template(template_name_or_list, \*context*)

Renders a template from the template folder with the given context.

flask.render_template_string(source, \*context*)

Renders a template from the given template source string with the given context. Template variables will be autoescaped.

  • Parameters

    • source (str) – the source code of the template to be rendered

    • context (Any) – the variables that should be available in the context of the template.

    Return type

    str

flask.get_template_attribute(template_name, attribute)

Loads a macro (or variable) a template exports. This can be used to invoke a macro from within Python code. If you for example have a template named _cider.html with the following contents:

  1. {% macro hello(name) %}Hello {{ name }}!{% endmacro %}

You can access this from Python code like this:

  1. hello = get_template_attribute('_cider.html', 'hello')
  2. return hello('World')

Changelog

New in version 0.2.

  • Parameters

    • template_name (str) – the name of the template

    • attribute (str) – the name of the variable of macro to access

    Return type

    Any

Configuration

class flask.Config(root_path, defaults=None)

Works exactly like a dict but provides ways to fill it from files or special dictionaries. There are two common patterns to populate the config.

Either you can fill the config from a config file:

  1. app.config.from_pyfile('yourconfig.cfg')

Or alternatively you can define the configuration options in the module that calls from_object() or provide an import path to a module that should be loaded. It is also possible to tell it to use the same module and with that provide the configuration values just before the call:

  1. DEBUG = True
  2. SECRET_KEY = 'development key'
  3. app.config.from_object(__name__)

In both cases (loading from any Python file or loading from modules), only uppercase keys are added to the config. This makes it possible to use lowercase values in the config file for temporary values that are not added to the config or to define the config keys in the same file that implements the application.

Probably the most interesting way to load configurations is from an environment variable pointing to a file:

  1. app.config.from_envvar('YOURAPPLICATION_SETTINGS')

In this case before launching the application you have to set this environment variable to the file you want to use. On Linux and OS X use the export statement:

  1. export YOURAPPLICATION_SETTINGS='/path/to/config/file'

On windows use set instead.

  • Parameters

    • root_path (str) – path to which files are read relative from. When the config object is created by the application, this is the application’s root_path.

    • defaults (Optional[dict]) – an optional dictionary of default values

    Return type

    None

  • from_envvar(variable_name, silent=False)

    Loads a configuration from an environment variable pointing to a configuration file. This is basically just a shortcut with nicer error messages for this line of code:

    1. app.config.from_pyfile(os.environ['YOURAPPLICATION_SETTINGS'])

    • Parameters

      • variable_name (str) – name of the environment variable

      • silent (bool) – set to True if you want silent failure for missing files.

      Returns

      True if the file was loaded successfully.

      Return type

      bool

  • from_file(filename, load, silent=False)

    Update the values in the config from a file that is loaded using the load parameter. The loaded data is passed to the from_mapping() method.

    1. import json
    2. app.config.from_file("config.json", load=json.load)
    4. import toml
    5. app.config.from_file("config.toml", load=toml.load)

    • Parameters

      • filename (str) – The path to the data file. This can be an absolute path or relative to the config root path.

      • load (Callable[[Reader], Mapping] where Reader implements a read method.) – A callable that takes a file handle and returns a mapping of loaded data from the file.

      • silent (bool) – Ignore the file if it doesn’t exist.

      Returns

      True if the file was loaded successfully.

      Return type

      bool

    Changelog

    New in version 2.0.

  • from_mapping(mapping=None, \*kwargs*)

    Updates the config like update() ignoring items with non-upper keys. :return: Always returns True.

    Changelog

    New in version 0.11.

  • from_object(obj)

    Updates the values from the given object. An object can be of one of the following two types:

    • a string: in this case the object with that name will be imported

    • an actual object reference: that object is used directly

    Objects are usually either modules or classes. from_object() loads only the uppercase attributes of the module/class. A dict object will not work with from_object() because the keys of a dict are not attributes of the dict class.

    Example of module-based configuration:

    1. app.config.from_object('yourapplication.default_config')
    2. from yourapplication import default_config
    3. app.config.from_object(default_config)

    Nothing is done to the object before loading. If the object is a class and has @property attributes, it needs to be instantiated before being passed to this method.

    You should not use this function to load the actual configuration but rather configuration defaults. The actual config should be loaded with from_pyfile() and ideally from a location not within the package because the package might be installed system wide.

    See Development / Production for an example of class-based configuration using from_object().

    • Parameters

      obj (Union[object, str]) – an import name or object

      Return type

      None

  • from_prefixed_env(prefix=’FLASK’, **, loads=<function loads>*)

    Load any environment variables that start with FLASK_, dropping the prefix from the env key for the config key. Values are passed through a loading function to attempt to convert them to more specific types than strings.

    Keys are loaded in sorted() order.

    The default loading function attempts to parse values as any valid JSON type, including dicts and lists.

    Specific items in nested dicts can be set by separating the keys with double underscores (__). If an intermediate key doesn’t exist, it will be initialized to an empty dict.

    • Parameters

      • prefix (str) – Load env vars that start with this prefix, separated with an underscore (_).

      • loads (Callable[**[str]**, Any]) – Pass each string value to this function and use the returned value as the config value. If any error is raised it is ignored and the value remains a string. The default is json.loads().

      Return type

      bool

    New in version 2.1.

  • from_pyfile(filename, silent=False)

    Updates the values in the config from a Python file. This function behaves as if the file was imported as module with the from_object() function.

    • Parameters

      • filename (str) – the filename of the config. This can either be an absolute filename or a filename relative to the root path.

      • silent (bool) – set to True if you want silent failure for missing files.

      Returns

      True if the file was loaded successfully.

      Return type

      bool

    Changelog

    New in version 0.7: silent parameter.

  • get_namespace(namespace, lowercase=True, trim_namespace=True)

    Returns a dictionary containing a subset of configuration options that match the specified namespace/prefix. Example usage:

    1. app.config['IMAGE_STORE_TYPE'] = 'fs'
    2. app.config['IMAGE_STORE_PATH'] = '/var/app/images'
    3. app.config['IMAGE_STORE_BASE_URL'] = 'http://img.website.com'
    4. image_store_config = app.config.get_namespace('IMAGE_STORE_')

    The resulting dictionary image_store_config would look like:

    1. {
    2.     'type': 'fs',
    3.     'path': '/var/app/images',
    4.     'base_url': 'http://img.website.com'
    5. }

    This is often useful when configuration options map directly to keyword arguments in functions or class constructors.

    • Parameters

      • namespace (str) – a configuration namespace

      • lowercase (bool) – a flag indicating if the keys of the resulting dictionary should be lowercase

      • trim_namespace (bool) – a flag indicating if the keys of the resulting dictionary should not include the namespace

      Return type

      Dict[str, Any]

    Changelog

    New in version 0.11.

Stream Helpers

flask.stream_with_context(generator_or_function)

Request contexts disappear when the response is started on the server. This is done for efficiency reasons and to make it less likely to encounter memory leaks with badly written WSGI middlewares. The downside is that if you are using streamed responses, the generator cannot access request bound information any more.

This function however can help you keep the context around for longer:

  1. from flask import stream_with_context, request, Response
  3. @app.route('/stream')
  4. def streamed_response():
  5.     @stream_with_context
  6.     def generate():
  7.         yield 'Hello '
  8.         yield request.args['name']
  9.         yield '!'
  10.     return Response(generate())

Alternatively it can also be used around a specific generator:

  1. from flask import stream_with_context, request, Response
  3. @app.route('/stream')
  4. def streamed_response():
  5.     def generate():
  6.         yield 'Hello '
  7.         yield request.args['name']
  8.         yield '!'
  9.     return Response(stream_with_context(generate()))

Changelog

New in version 0.9.

Useful Internals

class flask.ctx.RequestContext(app, environ, request=None, session=None)

The request context contains all request relevant information. It is created at the beginning of the request and pushed to the _request_ctx_stack and removed at the end of it. It will create the URL adapter and request object for the WSGI environment provided.

Do not attempt to use this class directly, instead use test_request_context() and request_context() to create this object.

When the request context is popped, it will evaluate all the functions registered on the application for teardown execution (teardown_request()).

The request context is automatically popped at the end of the request for you. In debug mode the request context is kept around if exceptions happen so that interactive debuggers have a chance to introspect the data. With 0.4 this can also be forced for requests that did not fail and outside of DEBUG mode. By setting 'flask._preserve_context' to True on the WSGI environment the context will not pop itself at the end of the request. This is used by the test_client() for example to implement the deferred cleanup functionality.

You might find this helpful for unittests where you need the information from the context local around for a little longer. Make sure to properly pop() the stack yourself in that situation, otherwise your unittests will leak memory.

  • Parameters

    Return type

    None

  • copy()

    Creates a copy of this request context with the same request object. This can be used to move a request context to a different greenlet. Because the actual request object is the same this cannot be used to move a request context to a different thread unless access to the request object is locked.

    Changelog

    Changed in version 1.1: The current session object is used instead of reloading the original data. This prevents flask.session pointing to an out-of-date object.

    New in version 0.10.

  • match_request()

    Can be overridden by a subclass to hook into the matching of the request.

    • Return type

      None

  • pop(exc=<object object>)

    Pops the request context and unbinds it by doing that. This will also trigger the execution of functions registered by the teardown_request() decorator.

    Changelog

    Changed in version 0.9: Added the exc argument.

  • push()

    Binds the request context to the current context.

    • Return type

      None

flask._request_ctx_stack

The internal LocalStack that holds RequestContext instances. Typically, the request and session proxies should be accessed instead of the stack. It may be useful to access the stack in extension code.

The following attributes are always present on each layer of the stack:

  • app

    the active Flask application.

    url_adapter

    the URL adapter that was used to match the request.

    request

    the current request object.

    session

    the active session object.

    g

    an object with all the attributes of the flask.g object.

    flashes

    an internal cache for the flashed messages.

Example usage:

  1. from flask import _request_ctx_stack
  3. def get_session():
  4.     ctx = _request_ctx_stack.top
  5.     if ctx is not None:
  6.         return ctx.session

class flask.ctx.AppContext(app)

The application context binds an application object implicitly to the current thread or greenlet, similar to how the RequestContext binds request information. The application context is also implicitly created if a request context is created but the application is not on top of the individual application context.

  • Parameters

    app (Flask) –

    Return type

    None

  • pop(exc=<object object>)

    Pops the app context.

  • push()

    Binds the app context to the current context.

    • Return type

      None

flask._app_ctx_stack

The internal LocalStack that holds AppContext instances. Typically, the current_app and g proxies should be accessed instead of the stack. Extensions can access the contexts on the stack as a namespace to store data.

Changelog

New in version 0.9.

class flask.blueprints.BlueprintSetupState(blueprint, app, options, first_registration)

Temporary holder object for registering a blueprint with the application. An instance of this class is created by the make_setup_state() method and later passed to all register callback functions.

  • Parameters

    Return type

    None

  • add_url_rule(rule, endpoint=None, view_func=None, \*options*)

    A helper method to register a rule (and optionally a view function) to the application. The endpoint is automatically prefixed with the blueprint’s name.

  • app

    a reference to the current application

  • blueprint

    a reference to the blueprint that created this setup state.

  • first_registration

    as blueprints can be registered multiple times with the application and not everything wants to be registered multiple times on it, this attribute can be used to figure out if the blueprint was registered in the past already.

  • options

    a dictionary with all options that were passed to the register_blueprint() method.

  • subdomain

    The subdomain that the blueprint should be active for, None otherwise.

  • url_defaults

    A dictionary with URL defaults that is added to each and every URL that was defined with the blueprint.

  • url_prefix

    The prefix that should be used for all URLs defined on the blueprint.

Signals

Changelog

New in version 0.6.

signals.signals_available

True if the signaling system is available. This is the case when blinker is installed.

The following signals exist in Flask:

flask.template_rendered

This signal is sent when a template was successfully rendered. The signal is invoked with the instance of the template as template and the context as dictionary (named context).

Example subscriber:

  1. def log_template_renders(sender, template, context, **extra):
  2.     sender.logger.debug('Rendering template "%s" with context %s',
  3.                         template.name or 'string template',
  4.                         context)
  6. from flask import template_rendered
  7. template_rendered.connect(log_template_renders, app)

flask.before_render_template

This signal is sent before template rendering process. The signal is invoked with the instance of the template as template and the context as dictionary (named context).

Example subscriber:

  1. def log_template_renders(sender, template, context, **extra):
  2.     sender.logger.debug('Rendering template "%s" with context %s',
  3.                         template.name or 'string template',
  4.                         context)
  6. from flask import before_render_template
  7. before_render_template.connect(log_template_renders, app)

flask.request_started

This signal is sent when the request context is set up, before any request processing happens. Because the request context is already bound, the subscriber can access the request with the standard global proxies such as request.

Example subscriber:

  1. def log_request(sender, **extra):
  2.     sender.logger.debug('Request context is set up')
  4. from flask import request_started
  5. request_started.connect(log_request, app)

flask.request_finished

This signal is sent right before the response is sent to the client. It is passed the response to be sent named response.

Example subscriber:

  1. def log_response(sender, response, **extra):
  2.     sender.logger.debug('Request context is about to close down.  '
  3.                         'Response: %s', response)
  5. from flask import request_finished
  6. request_finished.connect(log_response, app)

flask.got_request_exception

This signal is sent when an unhandled exception happens during request processing, including when debugging. The exception is passed to the subscriber as exception.

This signal is not sent for HTTPException, or other exceptions that have error handlers registered, unless the exception was raised from an error handler.

This example shows how to do some extra logging if a theoretical SecurityException was raised:

  1. from flask import got_request_exception
  3. def log_security_exception(sender, exception, **extra):
  4.     if not isinstance(exception, SecurityException):
  5.         return
  7.     security_logger.exception(
  8.         f"SecurityException at {request.url!r}",
  9.         exc_info=exception,
  10.     )
  12. got_request_exception.connect(log_security_exception, app)

flask.request_tearing_down

This signal is sent when the request is tearing down. This is always called, even if an exception is caused. Currently functions listening to this signal are called after the regular teardown handlers, but this is not something you can rely on.

Example subscriber:

  1. def close_db_connection(sender, **extra):
  2.     session.close()
  4. from flask import request_tearing_down
  5. request_tearing_down.connect(close_db_connection, app)

As of Flask 0.9, this will also be passed an exc keyword argument that has a reference to the exception that caused the teardown if there was one.

flask.appcontext_tearing_down

This signal is sent when the app context is tearing down. This is always called, even if an exception is caused. Currently functions listening to this signal are called after the regular teardown handlers, but this is not something you can rely on.

Example subscriber:

  1. def close_db_connection(sender, **extra):
  2.     session.close()
  4. from flask import appcontext_tearing_down
  5. appcontext_tearing_down.connect(close_db_connection, app)

This will also be passed an exc keyword argument that has a reference to the exception that caused the teardown if there was one.

flask.appcontext_pushed

This signal is sent when an application context is pushed. The sender is the application. This is usually useful for unittests in order to temporarily hook in information. For instance it can be used to set a resource early onto the g object.

Example usage:

  1. from contextlib import contextmanager
  2. from flask import appcontext_pushed
  4. @contextmanager
  5. def user_set(app, user):
  6.     def handler(sender, **kwargs):
  7.         g.user = user
  8.     with appcontext_pushed.connected_to(handler, app):
  9.         yield

And in the testcode:

  1. def test_user_me(self):
  2.     with user_set(app, 'john'):
  3.         c = app.test_client()
  4.         resp = c.get('/users/me')
  5.         assert resp.data == 'username=john'

Changelog

New in version 0.10.

flask.appcontext_popped

This signal is sent when an application context is popped. The sender is the application. This usually falls in line with the appcontext_tearing_down signal.

Changelog

New in version 0.10.

flask.message_flashed

This signal is sent when the application is flashing a message. The messages is sent as message keyword argument and the category as category.

Example subscriber:

  1. recorded = []
  2. def record(sender, message, category, **extra):
  3.     recorded.append((message, category))
  5. from flask import message_flashed
  6. message_flashed.connect(record, app)

Changelog

New in version 0.10.

class signals.Namespace

An alias for blinker.base.Namespace if blinker is available, otherwise a dummy class that creates fake signals. This class is available for Flask extensions that want to provide the same fallback system as Flask itself.

  • signal(name, doc=None)

    Creates a new signal for this namespace if blinker is available, otherwise returns a fake signal that has a send method that will do nothing but will fail with a RuntimeError for all other operations, including connecting.

Class-Based Views

Changelog

New in version 0.7.

class flask.views.View

Alternative way to use view functions. A subclass has to implement dispatch_request() which is called with the view arguments from the URL routing system. If methods is provided the methods do not have to be passed to the add_url_rule() method explicitly:

  1. class MyView(View):
  2.     methods = ['GET']
  4.     def dispatch_request(self, name):
  5.         return f"Hello {name}!"
  7. app.add_url_rule('/hello/<name>', view_func=MyView.as_view('myview'))

When you want to decorate a pluggable view you will have to either do that when the view function is created (by wrapping the return value of as_view()) or you can use the decorators attribute:

  1. class SecretView(View):
  2.     methods = ['GET']
  3.     decorators = [superuser_required]
  5.     def dispatch_request(self):
  6.         ...

The decorators stored in the decorators list are applied one after another when the view function is created. Note that you can not use the class based decorators since those would decorate the view class and not the generated view function!

class flask.views.MethodView

A class-based view that dispatches request methods to the corresponding class methods. For example, if you implement a get method, it will be used to handle GET requests.

  1. class CounterAPI(MethodView):
  2.     def get(self):
  3.         return session.get('counter', 0)
  5.     def post(self):
  6.         session['counter'] = session.get('counter', 0) + 1
  7.         return 'OK'
  9. app.add_url_rule('/counter', view_func=CounterAPI.as_view('counter'))

URL Route Registrations

Generally there are three ways to define rules for the routing system:

  1. You can use the flask.Flask.route() decorator.

  2. You can use the flask.Flask.add_url_rule() function.

  3. You can directly access the underlying Werkzeug routing system which is exposed as flask.Flask.url_map.

Variable parts in the route can be specified with angular brackets (/user/<username>). By default a variable part in the URL accepts any string without a slash however a different converter can be specified as well by using <converter:name>.

Variable parts are passed to the view function as keyword arguments.

The following converters are available:

string

accepts any text without a slash (the default)

int

accepts integers

float

like int but for floating point values

path

like the default but also accepts slashes

any

matches one of the items provided

uuid

accepts UUID strings

Custom converters can be defined using flask.Flask.url_map.

Here are some examples:

  1. @app.route('/')
  2. def index():
  3.     pass
  5. @app.route('/<username>')
  6. def show_user(username):
  7.     pass
  9. @app.route('/post/<int:post_id>')
  10. def show_post(post_id):
  11.     pass

An important detail to keep in mind is how Flask deals with trailing slashes. The idea is to keep each URL unique so the following rules apply:

  1. If a rule ends with a slash and is requested without a slash by the user, the user is automatically redirected to the same page with a trailing slash attached.

  2. If a rule does not end with a trailing slash and the user requests the page with a trailing slash, a 404 not found is raised.

This is consistent with how web servers deal with static files. This also makes it possible to use relative link targets safely.

You can also define multiple rules for the same function. They have to be unique however. Defaults can also be specified. Here for example is a definition for a URL that accepts an optional page:

  1. @app.route('/users/', defaults={'page': 1})
  2. @app.route('/users/page/<int:page>')
  3. def show_users(page):
  4.     pass

This specifies that /users/ will be the URL for page one and /users/page/N will be the URL for page N.

If a URL contains a default value, it will be redirected to its simpler form with a 301 redirect. In the above example, /users/page/1 will be redirected to /users/. If your route handles GET and POST requests, make sure the default route only handles GET, as redirects can’t preserve form data.

  1. @app.route('/region/', defaults={'id': 1})
  2. @app.route('/region/<int:id>', methods=['GET', 'POST'])
  3. def region(id):
  4.    pass

Here are the parameters that route() and add_url_rule() accept. The only difference is that with the route parameter the view function is defined with the decorator instead of the view_func parameter.

rule

the URL rule as string

endpoint

the endpoint for the registered URL rule. Flask itself assumes that the name of the view function is the name of the endpoint if not explicitly stated.

view_func

the function to call when serving a request to the provided endpoint. If this is not provided one can specify the function later by storing it in the view_functions dictionary with the endpoint as key.

defaults

A dictionary with defaults for this rule. See the example above for how defaults work.

subdomain

specifies the rule for the subdomain in case subdomain matching is in use. If not specified the default subdomain is assumed.

**options

the options to be forwarded to the underlying Rule object. A change to Werkzeug is handling of method options. methods is a list of methods this rule should be limited to (GET, POST etc.). By default a rule just listens for GET (and implicitly HEAD). Starting with Flask 0.6, OPTIONS is implicitly added and handled by the standard request handling. They have to be specified as keyword arguments.

View Function Options

For internal usage the view functions can have some attributes attached to customize behavior the view function would normally not have control over. The following attributes can be provided optionally to either override some defaults to add_url_rule() or general behavior:

  • __name__: The name of a function is by default used as endpoint. If endpoint is provided explicitly this value is used. Additionally this will be prefixed with the name of the blueprint by default which cannot be customized from the function itself.

  • methods: If methods are not provided when the URL rule is added, Flask will look on the view function object itself if a methods attribute exists. If it does, it will pull the information for the methods from there.

  • provide_automatic_options: if this attribute is set Flask will either force enable or disable the automatic implementation of the HTTP OPTIONS response. This can be useful when working with decorators that want to customize the OPTIONS response on a per-view basis.

  • required_methods: if this attribute is set, Flask will always add these methods when registering a URL rule even if the methods were explicitly overridden in the route() call.

Full example:

  1. def index():
  2.     if request.method == 'OPTIONS':
  3.         # custom options handling here
  4.         ...
  5.     return 'Hello World!'
  6. index.provide_automatic_options = False
  7. index.methods = ['GET', 'OPTIONS']
  9. app.add_url_rule('/', index)

Changelog

New in version 0.8: The provide_automatic_options functionality was added.

Command Line Interface

class flask.cli.FlaskGroup(add_default_commands=True, create_app=None, add_version_option=True, load_dotenv=True, set_debug_flag=True, \*extra*)

Special subclass of the AppGroup group that supports loading more commands from the configured Flask app. Normally a developer does not have to interface with this class but there are some very advanced use cases for which it makes sense to create an instance of this. see Custom Scripts.

  • Parameters

    • add_default_commands – if this is True then the default run and shell commands will be added.

    • add_version_option – adds the --version option.

    • create_app – an optional callback that is passed the script info and returns the loaded app.

    • load_dotenv – Load the nearest .env and .flaskenv files to set environment variables. Will also change the working directory to the directory containing the first file found.

    • set_debug_flag – Set the app’s debug flag based on the active environment

Changelog

Changed in version 1.0: If installed, python-dotenv will be used to load environment variables from .env and .flaskenv files.

  • get_command(ctx, name)

    Given a context and a command name, this returns a Command object if it exists or returns None.

  • list_commands(ctx)

    Returns a list of subcommand names in the order they should appear.

  • main(\args, **kwargs*)

    This is the way to invoke a script with all the bells and whistles as a command line application. This will always terminate the application after a call. If this is not wanted, SystemExit needs to be caught.

    This method is also available by directly calling the instance of a Command.

    • Parameters

      • args – the arguments that should be used for parsing. If not provided, sys.argv[1:] is used.

      • prog_name – the program name that should be used. By default the program name is constructed by taking the file name from sys.argv[0].

      • complete_var – the environment variable that controls the bash completion support. The default is "_<prog_name>_COMPLETE" with prog_name in uppercase.

      • standalone_mode – the default behavior is to invoke the script in standalone mode. Click will then handle exceptions and convert them into error messages and the function will never return but shut down the interpreter. If this is set to False they will be propagated to the caller and the return value of this function is the return value of invoke().

      • windows_expand_args – Expand glob patterns, user dir, and env vars in command line args on Windows.

      • extra – extra keyword arguments are forwarded to the context constructor. See Context for more information.

  1. Changed in version 8.0.1: Added the `windows_expand_args` parameter to allow disabling command line arg expansion on Windows.
  3. Changed in version 8.0: When taking arguments from `sys.argv` on Windows, glob patterns, user dir, and env vars are expanded.
  5. Changed in version 3.0: Added the `standalone_mode` parameter.

class flask.cli.AppGroup(name=None, commands=None, \*attrs*)

This works similar to a regular click Group but it changes the behavior of the command() decorator so that it automatically wraps the functions in with_appcontext().

Not to be confused with FlaskGroup.

class flask.cli.ScriptInfo(app_import_path=None, create_app=None, set_debug_flag=True)

Helper object to deal with Flask applications. This is usually not necessary to interface with as it’s used internally in the dispatching to click. In future versions of Flask this object will most likely play a bigger role. Typically it’s created automatically by the FlaskGroup but you can also manually create it and pass it onwards as click object.

  • app_import_path

    Optionally the import path for the Flask application.

  • create_app

    Optionally a function that is passed the script info to create the instance of the application.

  • data

    A dictionary with arbitrary data that can be associated with this script info.

  • load_app()

    Loads the Flask app (if not yet loaded) and returns it. Calling this multiple times will just result in the already loaded app to be returned.

flask.cli.load_dotenv(path=None)

Load “dotenv” files in order of precedence to set environment variables.

If an env var is already set it is not overwritten, so earlier files in the list are preferred over later files.

This is a no-op if python-dotenv is not installed.

  • Parameters

    path – Load the file at this location instead of searching.

    Returns

    True if a file was loaded.

Changelog

Changed in version 2.0: When loading the env files, set the default encoding to UTF-8.

Changed in version 1.1.0: Returns False when python-dotenv is not installed, or when the given path isn’t a file.

New in version 1.0.

flask.cli.with_appcontext(f)

Wraps a callback so that it’s guaranteed to be executed with the script’s application context. If callbacks are registered directly to the app.cli object then they are wrapped with this function by default unless it’s disabled.

flask.cli.pass_script_info(f)

Marks a function so that an instance of ScriptInfo is passed as first argument to the click callback.

  • Parameters

    f (click.decorators.F) –

    Return type

    click.decorators.F

flask.cli.run_command = <Command run>

Run a local development server.

This server is for development purposes only. It does not provide the stability, security, or performance of production WSGI servers.

The reloader and debugger are enabled by default if FLASK_ENV=development or FLASK_DEBUG=1.

  • Parameters

    • args (Any) –

    • kwargs (Any) –

    Return type

    Any

flask.cli.shell_command = <Command shell>

Run an interactive Python shell in the context of a given Flask application. The application will populate the default namespace of this shell according to its configuration.

This is useful for executing small snippets of management code without having to manually configure the application.

  • Parameters

    • args (Any) –

    • kwargs (Any) –

    Return type

    Any