.. _application-errors:

Application Errors
==================

.. versionadded:: 0.3

Applications fail, servers fail.  Sooner or later you will see an exception
in production.  Even if your code is 100% correct, you will still see
exceptions from time to time.  Why?  Because everything else involved will
fail.  Here are some situations where perfectly fine code can lead to server
errors:

-   the client terminated the request early and the application was still
    reading from the incoming data
-   the database server was overloaded and could not handle the query
-   a filesystem is full
-   a harddrive crashed
-   a backend server overloaded
-   a programming error in a library you are using
-   network connection of the server to another system failed

And that's just a small sample of issues you could be facing.  So how do we
deal with that sort of problem?  By default if your application runs in
production mode, Flask will display a very simple page for you and log the
exception to the :attr:`~flask.Flask.logger`.

But there is more you can do, and we will cover some better setups to deal
with errors.

Error Logging Tools
-------------------

Sending error mails, even if just for critical ones, can become
overwhelming if enough users are hitting the error and log files are
typically never looked at. This is why we recommend using `Sentry
<https://sentry.io/>`_ for dealing with application errors.  It's
available as an Open Source project `on GitHub
<https://github.com/getsentry/sentry>`_ and is also available as a `hosted version
<https://sentry.io/signup/>`_ which you can try for free. Sentry
aggregates duplicate errors, captures the full stack trace and local
variables for debugging, and sends you mails based on new errors or
frequency thresholds.

To use Sentry you need to install the `sentry-sdk` client with extra `flask` dependencies::

    $ pip install sentry-sdk[flask]

And then add this to your Flask app::

    import sentry_sdk
    from sentry_sdk.integrations.flask import FlaskIntegration

    sentry_sdk.init('YOUR_DSN_HERE',integrations=[FlaskIntegration()])

The `YOUR_DSN_HERE` value needs to be replaced with the DSN value you get
from your Sentry installation.

After installation, failures leading to an Internal Server Error
are automatically reported to Sentry and from there you can
receive error notifications.

Follow-up reads:

* Sentry also supports catching errors from your worker queue (RQ, Celery) in a
  similar fashion.  See the `Python SDK docs
  <https://docs.sentry.io/platforms/python/>`_ for more information.
* `Getting started with Sentry <https://docs.sentry.io/quickstart/?platform=python>`_
* `Flask-specific documentation <https://docs.sentry.io/platforms/python/flask/>`_.

.. _error-handlers:

Error handlers
--------------

You might want to show custom error pages to the user when an error occurs.
This can be done by registering error handlers.

An error handler is a normal view function that returns a response, but instead
of being registered for a route, it is registered for an exception or HTTP
status code that would be raised while trying to handle a request.

Registering
```````````

Register handlers by decorating a function with
:meth:`~flask.Flask.errorhandler`. Or use
:meth:`~flask.Flask.register_error_handler` to register the function later.
Remember to set the error code when returning the response. ::

    @app.errorhandler(werkzeug.exceptions.BadRequest)
    def handle_bad_request(e):
        return 'bad request!', 400

    # or, without the decorator
    app.register_error_handler(400, handle_bad_request)

:exc:`werkzeug.exceptions.HTTPException` subclasses like
:exc:`~werkzeug.exceptions.BadRequest` and their HTTP codes are interchangeable
when registering handlers. (``BadRequest.code == 400``)

Non-standard HTTP codes cannot be registered by code because they are not known
by Werkzeug. Instead, define a subclass of
:class:`~werkzeug.exceptions.HTTPException` with the appropriate code and
register and raise that exception class. ::

    class InsufficientStorage(werkzeug.exceptions.HTTPException):
        code = 507
        description = 'Not enough storage space.'

    app.register_error_handler(InsufficientStorage, handle_507)

    raise InsufficientStorage()

Handlers can be registered for any exception class, not just
:exc:`~werkzeug.exceptions.HTTPException` subclasses or HTTP status
codes. Handlers can be registered for a specific class, or for all subclasses
of a parent class.

Handling
````````

When an exception is caught by Flask while handling a request, it is first
looked up by code. If no handler is registered for the code, it is looked up
by its class hierarchy; the most specific handler is chosen. If no handler is
registered, :class:`~werkzeug.exceptions.HTTPException` subclasses show a
generic message about their code, while other exceptions are converted to a
generic 500 Internal Server Error.

For example, if an instance of :exc:`ConnectionRefusedError` is raised,
and a handler is registered for :exc:`ConnectionError` and
:exc:`ConnectionRefusedError`,
the more specific :exc:`ConnectionRefusedError` handler is called with the
exception instance to generate the response.

Handlers registered on the blueprint take precedence over those registered
globally on the application, assuming a blueprint is handling the request that
raises the exception. However, the blueprint cannot handle 404 routing errors
because the 404 occurs at the routing level before the blueprint can be
determined.


Generic Exception Handlers
``````````````````````````

It is possible to register error handlers for very generic base classes
such as ``HTTPException`` or even ``Exception``. However, be aware that
these will catch more than you might expect.

An error handler for ``HTTPException`` might be useful for turning
the default HTML errors pages into JSON, for example. However, this
handler will trigger for things you don't cause directly, such as 404
and 405 errors during routing. Be sure to craft your handler carefully
so you don't lose information about the HTTP error.

.. code-block:: python

    from flask import json
    from werkzeug.exceptions import HTTPException

    @app.errorhandler(HTTPException)
    def handle_exception(e):
        """Return JSON instead of HTML for HTTP errors."""
        # start with the correct headers and status code from the error
        response = e.get_response()
        # replace the body with JSON
        response.data = json.dumps({
            "code": e.code,
            "name": e.name,
            "description": e.description,
        })
        response.content_type = "application/json"
        return response


An error handler for ``Exception`` might seem useful for changing how
all errors, even unhandled ones, are presented to the user. However,
this is similar to doing ``except Exception:`` in Python, it will
capture *all* otherwise unhandled errors, including all HTTP status
codes. In most cases it will be safer to register handlers for more
specific exceptions. Since ``HTTPException`` instances are valid WSGI
responses, you could also pass them through directly.

.. code-block:: python

    from werkzeug.exceptions import HTTPException

    @app.errorhandler(Exception)
    def handle_exception(e):
        # pass through HTTP errors
        if isinstance(e, HTTPException):
            return e

        # now you're handling non-HTTP exceptions only
        return render_template("500_generic.html", e=e), 500

Error handlers still respect the exception class hierarchy. If you
register handlers for both ``HTTPException`` and ``Exception``, the
``Exception`` handler will not handle ``HTTPException`` subclasses
because it the ``HTTPException`` handler is more specific.

Unhandled Exceptions
````````````````````

When there is no error handler registered for an exception, a 500
Internal Server Error will be returned instead. See
:meth:`flask.Flask.handle_exception` for information about this
behavior.

If there is an error handler registered for ``InternalServerError``,
this will be invoked. As of Flask 1.1.0, this error handler will always
be passed an instance of ``InternalServerError``, not the original
unhandled error. The original error is available as ``e.original_exception``.
Until Werkzeug 1.0.0, this attribute will only exist during unhandled
errors, use ``getattr`` to get access it for compatibility.

.. code-block:: python

    @app.errorhandler(InternalServerError)
    def handle_500(e):
        original = getattr(e, "original_exception", None)

        if original is None:
            # direct 500 error, such as abort(500)
            return render_template("500.html"), 500

        # wrapped unhandled error
        return render_template("500_unhandled.html", e=original), 500


Logging
-------

See :doc:`/logging` for information on how to log exceptions, such as by
emailing them to admins.


Debugging Application Errors
============================

For production applications, configure your application with logging and
notifications as described in :ref:`application-errors`.  This section provides
pointers when debugging deployment configuration and digging deeper with a
full-featured Python debugger.


When in Doubt, Run Manually
---------------------------

Having problems getting your application configured for production?  If you
have shell access to your host, verify that you can run your application
manually from the shell in the deployment environment.  Be sure to run under
the same user account as the configured deployment to troubleshoot permission
issues.  You can use Flask's builtin development server with `debug=True` on
your production host, which is helpful in catching configuration issues, but
**be sure to do this temporarily in a controlled environment.** Do not run in
production with `debug=True`.


.. _working-with-debuggers:

Working with Debuggers
----------------------

To dig deeper, possibly to trace code execution, Flask provides a debugger out
of the box (see :ref:`debug-mode`).  If you would like to use another Python
debugger, note that debuggers interfere with each other.  You have to set some
options in order to use your favorite debugger:

* ``debug``        - whether to enable debug mode and catch exceptions
* ``use_debugger`` - whether to use the internal Flask debugger
* ``use_reloader`` - whether to reload and fork the process if modules
  were changed

``debug`` must be True (i.e., exceptions must be caught) in order for the other
two options to have any value.

If you're using Aptana/Eclipse for debugging you'll need to set both
``use_debugger`` and ``use_reloader`` to False.

A possible useful pattern for configuration is to set the following in your
config.yaml (change the block as appropriate for your application, of course)::

   FLASK:
       DEBUG: True
       DEBUG_WITH_APTANA: True

Then in your application's entry-point (main.py),
you could have something like::

   if __name__ == "__main__":
       # To allow aptana to receive errors, set use_debugger=False
       app = create_app(config="config.yaml")

       use_debugger = app.debug and not(app.config.get('DEBUG_WITH_APTANA'))
       app.run(use_debugger=use_debugger, debug=app.debug,
               use_reloader=use_debugger, host='0.0.0.0')
