flask/docs/debugging.rst

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
:ref:`error-logging-tools`, or enable logging and notifications as
described in :doc:`/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.

.. image:: _static/debugger.png
   :align: center
   :class: screenshot
   :alt: 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.

The debugger is enabled by default when the development server is run in debug mode.

.. code-block:: text

    $ flask --app hello run --debug

When running from Python code, passing ``debug=True`` enables debug mode, which is
mostly equivalent.

.. code-block:: python

    app.run(debug=True)

:doc:`/server` and :doc:`/cli` have more information about running the debugger and
debug mode. More information about the debugger can be found in the `Werkzeug
documentation <https://werkzeug.palletsprojects.com/debug/>`__.


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, otherwise Flask
turns unhandled errors into generic 500 error pages. However, the built-in debugger and
reloader should be disabled so they don't interfere with the external debugger.

.. code-block:: text

    $ flask --app hello run --debug --no-debugger --no-reload

When running from Python:

.. code-block:: python

    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 a breakpoint.
-   The development server will still catch unhandled exceptions if the built-in
    debugger is disabled, otherwise it would crash on any error. If you want that (and
    usually you don't) pass ``passthrough_errors=True`` to ``app.run``.

    .. code-block:: python

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