Templates

Flask leverages Jinja2 as its template engine. You are obviously free to use a different template engine, but you still have to install Jinja2 to run Flask itself. This requirement is necessary to enable rich extensions. An extension can depend on Jinja2 being present.

This section only gives a very quick introduction into how Jinja2 is integrated into Flask. If you want information on the template engine’s syntax itself, head over to the official Jinja2 Template Documentation for more information.

Jinja Setup

Unless customized, Jinja2 is configured by Flask as follows:

• autoescaping is enabled for all templates ending in .html, .htm, .xml, .xhtml, as well as .svg when using render_template().

• autoescaping is enabled for all strings when using render_template_string().

• a template has the ability to opt in/out autoescaping with the {% autoescape %} tag.

• Flask inserts a couple of global functions and helpers into the Jinja2 context, additionally to the values that are present by default.

Standard Context

The following global variables are available within Jinja2 templates by default:

config

The current configuration object (flask.Flask.config)

Changelog

Changed in version 0.10: This is now always available, even in imported templates.

New in version 0.6.

request

The current request object (flask.request). This variable is unavailable if the template was rendered without an active request context.

session

The current session object (flask.session). This variable is unavailable if the template was rendered without an active request context.

g

The request-bound object for global variables (flask.g). This variable is unavailable if the template was rendered without an active request context.

url_for()

The flask.url_for() function.

get_flashed_messages()

The flask.get_flashed_messages() function.

The Jinja Context Behavior

These variables are added to the context of variables, they are not global variables. The difference is that by default these will not show up in the context of imported templates. This is partially caused by performance considerations, partially to keep things explicit.

What does this mean for you? If you have a macro you want to import, that needs to access the request object you have two possibilities:

1. you explicitly pass the request to the macro as parameter, or the attribute of the request object you are interested in.

2. you import the macro “with context”.

Importing with context looks like this:

{% from '_helpers.html' import my_macro with context %}

Controlling Autoescaping

Autoescaping is the concept of automatically escaping special characters for you. Special characters in the sense of HTML (or XML, and thus XHTML) are &, >, <, " as well as '. Because these characters carry specific meanings in documents on their own you have to replace them by so called “entities” if you want to use them for text. Not doing so would not only cause user frustration by the inability to use these characters in text, but can also lead to security problems. (see Cross-Site Scripting (XSS))

Sometimes however you will need to disable autoescaping in templates. This can be the case if you want to explicitly inject HTML into pages, for example if they come from a system that generates secure HTML like a markdown to HTML converter.

There are three ways to accomplish that:

• In the Python code, wrap the HTML string in a Markup object before passing it to the template. This is in general the recommended way.

• Inside the template, use the |safe filter to explicitly mark a string as safe HTML ({{ myvariable|safe }})

• Temporarily disable the autoescape system altogether.

To disable the autoescape system in templates, you can use the {% autoescape %} block:

{% autoescape false %}
    <p>autoescaping is disabled here
    <p>{{ will_not_be_escaped }}
{% endautoescape %}

Whenever you do this, please be very cautious about the variables you are using in this block.

Registering Filters

If you want to register your own filters in Jinja2 you have two ways to do that. You can either put them by hand into the jinja_env of the application or use the template_filter() decorator.

The two following examples work the same and both reverse an object:

@app.template_filter('reverse')
def reverse_filter(s):
    return s[::-1]
def reverse_filter(s):
    return s[::-1]
app.jinja_env.filters['reverse'] = reverse_filter

In case of the decorator the argument is optional if you want to use the function name as name of the filter. Once registered, you can use the filter in your templates in the same way as Jinja2’s builtin filters, for example if you have a Python list in context called mylist:

{% for x in mylist | reverse %}
{% endfor %}

Context Processors

To inject new variables automatically into the context of a template, context processors exist in Flask. Context processors run before the template is rendered and have the ability to inject new values into the template context. A context processor is a function that returns a dictionary. The keys and values of this dictionary are then merged with the template context, for all templates in the app:

@app.context_processor
def inject_user():
    return dict(user=g.user)

The context processor above makes a variable called user available in the template with the value of g.user. This example is not very interesting because g is available in templates anyways, but it gives an idea how this works.

Variables are not limited to values; a context processor can also make functions available to templates (since Python allows passing around functions):

@app.context_processor
def utility_processor():
    def format_price(amount, currency="€"):
        return f"{amount:.2f}{currency}"
    return dict(format_price=format_price)

The context processor above makes the format_price function available to all templates:

{{ format_price(0.33) }}

You could also build format_price as a template filter (see Registering Filters), but this demonstrates how to pass functions in a context processor.

Streaming

It can be useful to not render the whole template as one complete string, instead render it as a stream, yielding smaller incremental strings. This can be used for streaming HTML in chunks to speed up initial page load, or to save memory when rendering a very large template.

The Jinja2 template engine supports rendering a template piece by piece, returning an iterator of strings. Flask provides the stream_template() and stream_template_string() functions to make this easier to use.

from flask import stream_template
@app.get("/timeline")
def timeline():
    return stream_template("timeline.html")

These functions automatically apply the stream_with_context() wrapper if a request is active, so that it remains available in the template.