Debugging Application Errors

In Production

Do not run the development server, or enable the built-in debugger, in a production environment. The debugger allows executing arbitrary Python code from the browser. It’s protected by a pin, but that should not be relied on for security.

Use an error logging tool, such as Sentry, as described in Error Logging Tools, or enable logging and notifications as described in Logging.

If you have access to the server, you could add some code to start an external debugger if request.remote_addr matches your IP. Some IDE debuggers also have a remote mode so breakpoints on the server can be interacted with locally. Only enable a debugger temporarily.

The Built-In Debugger

The built-in Werkzeug development server provides a debugger which shows an interactive traceback in the browser when an unhandled error occurs during a request. This debugger should only be used during development.

screenshot of debugger in action

Warning

The debugger allows executing arbitrary Python code from the browser. It is protected by a pin, but still represents a major security risk. Do not run the development server or debugger in a production environment.

To enable the debugger, run the development server with the FLASK_ENV environment variable set to development. This puts Flask in debug mode, which changes how it handles some errors, and enables the debugger and reloader.

BashFishCMDPowershell

  1. $ export FLASK_ENV=development
  2. $ flask run
  1. $ set -x FLASK_ENV development
  2. $ flask run
  1. > set FLASK_ENV=development
  2. > flask run
  1. > $env:FLASK_ENV = "development"
  2. > flask run

FLASK_ENV can only be set as an environment variable. When running from Python code, passing debug=True enables debug mode, which is mostly equivalent. Debug mode can be controlled separately from FLASK_ENV with the FLASK_DEBUG environment variable as well.

  1. app.run(debug=True)

Development Server and Command Line Interface have more information about running the debugger, debug mode, and development mode. More information about the debugger can be found in the Werkzeug documentation.

External Debuggers

External debuggers, such as those provided by IDEs, can offer a more powerful debugging experience than the built-in debugger. They can also be used to step through code during a request before an error is raised, or if no error is raised. Some even have a remote mode so you can debug code running on another machine.

When using an external debugger, the app should still be in debug mode, but it can be useful to disable the built-in debugger and reloader, which can interfere.

When running from the command line:

BashFishCMDPowershell

  1. $ export FLASK_ENV=development
  2. $ flask run --no-debugger --no-reload
  1. $ set -x FLASK_ENV development
  2. $ flask run --no-debugger --no-reload
  1. > set FLASK_ENV=development
  2. > flask run --no-debugger --no-reload
  1. > $env:FLASK_ENV = "development"
  2. > flask run --no-debugger --no-reload

When running from Python:

  1. app.run(debug=True, use_debugger=False, use_reloader=False)

Disabling these isn’t required, an external debugger will continue to work with the following caveats. If the built-in debugger is not disabled, it will catch unhandled exceptions before the external debugger can. If the reloader is not disabled, it could cause an unexpected reload if code changes during debugging.