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Error Docs Revision/Consolidation
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5
docs/blueprints.rst
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@ -242,8 +242,9 @@ you can use relative redirects by prefixing the endpoint with a dot only::
This will link to ``admin.index`` for instance in case the current request
was dispatched to any other admin blueprint endpoint.
Error Handlers
--------------
Blueprint Error Handlers
------------------------
Blueprints support the ``errorhandler`` decorator just like the :class:`Flask`
application object, so it is easy to make Blueprint-specific custom error

97
docs/debugging.rst Normal file
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@ -0,0 +1,97 @@
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.
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.
.. code-block:: text
$ export FLASK_ENV=development
$ 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 controled separately from
``FLASK_ENV`` with the ``FLASK_DEBUG`` environment variable as well.
.. code-block:: python
app.run(debug=True)
:doc:`/server` and :doc:`/cli` have more information about running the
debugger, debug mode, and development 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,
but it can be useful to disable the built-in debugger and reloader,
which can interfere.
When running from the command line:
.. code-block:: text
$ export FLASK_ENV=development
$ flask run --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 debugging.

678
docs/errorhandling.rst
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@ -1,12 +1,10 @@
Handling 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
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
@ -18,8 +16,8 @@ errors:
- 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
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, and an exception is raised Flask will display a very simple
page for you and log the exception to the :attr:`~flask.Flask.logger`.
@ -27,71 +25,140 @@ But there is more you can do, and we will cover some better setups to deal
with errors including custom exceptions and 3rd party tools.
.. _common-error-codes:
.. _error-logging-tools:
Common Error Codes
``````````````````
Error Logging Tools
-------------------
The following error codes are some that are often displayed to the user,
even if the application behaves correctly:
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.
*400 Bad Request*
When the server will not process the request due to something that
the server perceives to be a client error. Such as malformed request
syntax, missing query parameters, etc.
To use Sentry you need to install the ``sentry-sdk`` client with extra
``flask`` dependencies.
*403 Forbidden*
If you have some kind of access control on your website, you will have
to send a 403 code for disallowed resources. So make sure the user
is not lost when they try to access a forbidden resource.
.. code-block:: text
*404 Not Found*
The good old "chap, you made a mistake typing that URL" message. So
common that even novices to the internet know that 404 means: damn,
the thing I was looking for is not there. It's a very good idea to
make sure there is actually something useful on a 404 page, at least a
link back to the index.
$ pip install sentry-sdk[flask]
*410 Gone*
Did you know that there the "404 Not Found" has a brother named "410
Gone"? Few people actually implement that, but the idea is that
resources that previously existed and got deleted answer with 410
instead of 404. If you are not deleting documents permanently from
the database but just mark them as deleted, do the user a favour and
use the 410 code instead and display a message that what they were
looking for was deleted for all eternity.
And then add this to your Flask app:
*500 Internal Server Error*
Usually happens on programming errors or if the server is overloaded.
A terribly good idea is to have a nice page there, because your
application *will* fail sooner or later.
.. code-block:: python
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.
See also:
- Sentry also supports catching errors from a worker queue
(RQ, Celery, etc.) 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
--------------
Default Error Handling
``````````````````````
When an error occurs in Flask, an appropriate `HTTP status code
<https://developer.mozilla.org/en-US/docs/Web/HTTP/Status>`__ will be
returned. 400-499 indicate errors with the client's request data, or
about the data requested. 500-599 indicate errors with the server or
application itself.
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 function that returns a response when a type of error is
raised, similar to how a view is a function that returns a response when a
request URL is matched. It is passed the instance of the error being handled,
which is most likely a :exc:`~werkzeug.exceptions.HTTPException`.
The status code of the response will not be set to the handler's code. Make
sure to provide the appropriate HTTP status code when returning a response from
a handler.
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.
.. code-block:: python
@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.
.. code-block:: python
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 building a Flask application you *will* run into exceptions. If some part
of your code breaks while handling a request (and you have no error handlers
registered) an "500 Internal Server Error"
registered), a "500 Internal Server Error"
(:exc:`~werkzeug.exceptions.InternalServerError`) will be returned by default.
Similarly, if a request is sent to an unregistered route a "404 Not Found"
(:exc:`~werkzeug.exceptions.NotFound`) error will occur. If a route receives an
unallowed request method a "405 Method Not Allowed"
Similarly, "404 Not Found"
(:exc:`~werkzeug.exceptions.NotFound`) error will occur if a request is sent to an unregistered route.
If a route receives an unallowed request method, a "405 Method Not Allowed"
(:exc:`~werkzeug.exceptions.MethodNotAllowed`) will be raised. These are all
subclasses of :class:`~werkzeug.exceptions.HTTPException` and are provided by
default in Flask.
Flask gives you to the ability to raise any HTTP exception registered by
werkzeug. However, as the default HTTP exceptions return simple exception
pages, Flask also offers the opportunity to customise these HTTP exceptions via
custom error handlers as well as to add exception handlers for builtin and
custom exceptions.
Werkzeug. However, the default HTTP exceptions return simple exception
pages. You might want to show custom error pages to the user when an error occurs.
This can be done by registering error handlers.
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
When Flask catches an exception while handling a request, it is first looked up by code.
If no handler is registered for the code, Flask looks up the error 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".
@ -107,21 +174,118 @@ because the 404 occurs at the routing level before the blueprint can be
determined.
Generic Exception Handlers
``````````````````````````
.. _handling-errors:
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.
Handling Errors
```````````````
For example, an error handler for ``HTTPException`` might be useful for turning
the default HTML errors pages into JSON. 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
An error handler for "500 Internal Server Error" will be passed uncaught exceptions in
addition to explicit 500 errors. In debug mode, a handler for "500 Internal Server Error" will not be used.
Instead, the interactive debugger will be shown.
Custom Error Pages
------------------
Sometimes when building a Flask application, you might want to raise a
:exc:`~werkzeug.exceptions.HTTPException` to signal to the user that
something is wrong with the request. Fortunately, Flask comes with a handy
:func:`~flask.abort` function that aborts a request with a HTTP error from
werkzeug as desired.
werkzeug as desired. It will also provide a plain black and white error page
for you with a basic description, but nothing fancy.
Consider the code below, we might have a user profile route, but if the user
fails to pass a username we raise a "400 Bad Request" and if the user passes a
username but we can't find it, we raise a "404 Not Found".
Depending on the error code it is less or more likely for the user to
actually see such an error.
Consider the code below, we might have a user profile route, and if the user
fails to pass a username we can raise a "400 Bad Request". If the user passes a
username and we can't find it, we raise a "404 Not Found".
.. code-block:: python
@ -143,133 +307,51 @@ username but we can't find it, we raise a "404 Not Found".
return render_template("profile.html", user=user)
.. _custom-error-handlers:
Custom error handlers
`````````````````````
The default :exc:`~werkzeug.exceptions.HTTPException` returns a black and white
error page with a basic description, but nothing fancy. Considering
these errors *will* be thrown during the lifetime of your application, it is
highly advisable to customise these exceptions to improve the user experience
of your site. 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.
It is passed the instance of the error being handled, which is most
likely an integer that represents a :exc:`~werkzeug.exceptions.HTTPException`
status code. For example 500 (an "Internal Server Error") which maps to
:exc:`~werkzeug.exceptions.InternalServerError`.
It is registered with the :meth:`~flask.Flask.errorhandler`
decorator or the :meth:`~flask.Flask.register_error_handler` to register
the function later. A handler can be registered for a status code,
like 404 or 500, or for an built-in exception class, like KeyError,
or a custom exception class that inherits from Exception or its subclasses.
The status code of the response will not be set to the handler's code. Make
sure to provide the appropriate HTTP status code when returning a response from
a handler or a 200 OK HTTP code will be sent instead.
Here is another example implementation for a "404 Page Not Found" exception:
.. code-block:: python
from werkzeug.exceptions import InternalServerError
from flask import render_template
# as a decorator with an int as the exception code
@app.errorhandler(500)
def handle_internal_server_error(e):
# returning 500 with the text sets the error handler's code
# make sure to provide the appropriate HTTP status code
# otherwise 200 will be returned as default
return 'Internal Server Error!', 500
@app.errorhandler(404)
def page_not_found(e):
# note that we set the 404 status explicitly
return render_template('404.html'), 404
# or, as a decorator with the werkzeug exception for internal server error
@app.errorhandler(InternalServerError)
def handle_internal_server_error(e):
# werkzeug exceptions have a code attribute
return 'Internal Server Error!', e.code
# or, without the decorator
app.register_error_handler(500, handle_internal_server_error)
# similarly with a werkzeug exception
app.register_error_handler(InternalServerError, handle_internal_server_error)
A handler for "500 Internal Server Error" will not be used when running in
debug mode. Instead, the interactive debugger will be shown.
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.
When using :doc:`/patterns/appfactories`:
.. code-block:: python
@app.errorhandler(InternalServerError)
def handle_500(e):
original = getattr(e, "original_exception", None)
from flask import Flask, render_template
if original is None:
# direct 500 error, such as abort(500)
return render_template("500.html"), 500
def page_not_found(e):
return render_template('404.html'), 404
# wrapped unhandled error
return render_template("500_unhandled.html", e=original), 500
def create_app(config_filename):
app = Flask(__name__)
app.register_error_handler(404, page_not_found)
return app
An example template might be this:
.. code-block:: html+jinja
{% extends "layout.html" %}
{% block title %}Page Not Found{% endblock %}
{% block body %}
<h1>Page Not Found</h1>
<p>What you were looking for is just not there.
<p><a href="{{ url_for('index') }}">go somewhere nice</a>
{% endblock %}
Registering Custom Exceptions
-----------------------------
You can create your own custom exceptions by subclassing
:exc:`werkzeug.exceptions.HTTPException`. As shown above, integer HTTP codes
are interchangable 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:
.. code-block:: python
class InsufficientStorage(werkzeug.exceptions.HTTPException):
code = 507
description = 'Not enough storage space.'
def handle_507(e):
return 'Not enough storage space!', 507
app.register_error_handler(InsufficientStorage, handle_507)
# during an request
raise InsufficientStorage()
Handlers can be registered for any exception class that inherits from Exception.
Unhandled Exceptions
--------------------
If an exception is raised in the code while Flask is handling a request and
there is no error handler registered for that exception, a "500 Internal Server
Error" will be returned instead. See :meth:`flask.Flask.handle_exception` for
information about this behavior.
Custom error pages
------------------
Further Examples
````````````````
The above examples wouldn't actually be an improvement on the default
exception pages. We can create a custom 500.html template like this:
.. sourcecode:: html+jinja
.. code-block:: html+jinja
{% extends "layout.html" %}
{% block title %}Internal Server Error{% endblock %}
@ -290,12 +372,10 @@ It can be implemented by rendering the template on "500 Internal Server Error":
# note that we set the 500 status explicitly
return render_template('500.html'), 500
When using the :doc:`/patterns/appfactories`:
When using :doc:`/patterns/appfactories`:
.. code-block:: python
from flask import Flask, render_template
def internal_server_error(e):
@ -306,7 +386,6 @@ When using the :doc:`/patterns/appfactories`:
app.register_error_handler(500, internal_server_error)
return app
When using :doc:`/blueprints`:
.. code-block:: python
@ -324,16 +403,21 @@ When using :doc:`/blueprints`:
blog.register_error_handler(500, internal_server_error)
Blueprint Error Handlers
------------------------
In blueprints errorhandlers will simply work as expected; however, there is a caveat
concerning handlers for 404 and 405 exceptions. These errorhandlers are only
invoked from an appropriate ``raise`` statement or a call to ``abort`` in another
of the blueprint's view functions; they are not invoked by, e.g., an invalid URL
access. This is because the blueprint does not "own" a certain URL space, so
the application instance has no way of knowing which blueprint error handler it
should run if given an invalid URL. If you would like to execute different
handling strategies for these errors based on URL prefixes, they may be defined
at the application level using the ``request`` proxy object:
In :doc:`/blueprints`, most error handlers will work as expected.
However, there is a caveat concerning handlers for 404 and 405
exceptions. These error handlers are only invoked from an appropriate
``raise`` statement or a call to ``abort`` in another of the blueprint's
view functions; they are not invoked by, e.g., an invalid URL access.
This is because the blueprint does not "own" a certain URL space, so
the application instance has no way of knowing which blueprint error
handler it should run if given an invalid URL. If you would like to
execute different handling strategies for these errors based on URL
prefixes, they may be defined at the application level using the
``request`` proxy object.
.. code-block:: python
@ -351,7 +435,6 @@ at the application level using the ``request`` proxy object:
# otherwise we return our generic site-wide 404 page
return render_template("404.html"), 404
@app.errorhandler(405)
def method_not_allowed(e):
# if a request has the wrong method to our API
@ -363,21 +446,16 @@ at the application level using the ``request`` proxy object:
return render_template("405.html"), 405
Returning API Errors as JSON
----------------------------
More information on error handling with blueprint can be found in
:doc:`/blueprints`.
Returning API errors as JSON
````````````````````````````
When building APIs in Flask, some developers realise that the builtin
When building APIs in Flask, some developers realise that the built-in
exceptions are not expressive enough for APIs and that the content type of
:mimetype:`text/html` they are emitting is not very useful for API consumers.
Using the same techniques as above and :func:`~flask.json.jsonify` we can return JSON
responses to API errors. :func:`~flask.abort` is called
with a ``description`` parameter. The errorhandler will
with a ``description`` parameter. The error handler will
use that as the JSON error message, and set the status code to 404.
.. code-block:: python
@ -397,10 +475,8 @@ use that as the JSON error message, and set the status code to 404.
return jsonify(resource)
We can also create custom exception classes; for instance, for an API we can
introduce a new custom exception that can take a proper human readable message,
We can also create custom exception classes. For instance, we can
introduce a new custom exception for an API that can take a proper human readable message,
a status code for the error and some optional payload to give more context
for the error.
@ -443,212 +519,20 @@ This is a simple example:
return jsonify(user.to_dict())
A view can now raise that exception with an error message. Additionally
A view can now raise that exception with an error message. Additionally
some extra payload can be provided as a dictionary through the `payload`
parameter.
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
# or using jsonify
@app.errorhandler(HTTPException)
def handle_exception(e):
return jsonify("code": e.code, "name": e.name, "description": e.description), e.code
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.
Generic Error Pages
-------------------
If we pass in the exception into a template as below:
.. code-block:: python
from werkzeug.exceptions import HTTPException
@app.errorhandler(HTTPException)
def handle_exception(e):
return render_template("exception.html", e=e), e.code
.. sourcecode:: html+jinja
{% extends "layout.html" %}
{% block title %}{{ e.name }}{% endblock %}
{% block body %}
<h1>{{ e.code }} {{ e.name }}</h1>
<p>{{ e.description }}</p>
<p><a href="{{ url_for('index') }}">Go home</a>
{% endblock %}
Debugging Application Errors
````````````````````````````
For production applications, configure your application with logging and
notifications as described in :doc:`/logging`. This section provides
pointers when debugging deployment configuration and digging deeper with a
full-featured Python debugger.
Logging
-------
See :doc:`/logging` for information on how to log exceptions, such as by
emailing them to admins.
See :doc:`/logging` for information about how to log exceptions, such as
by emailing them to admins.
Debugging
---------
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')
.. _error-logging-tools:
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/>`_.
See :doc:`/debugging` for information about how to debug errors in
development and production.

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@ -44,6 +44,7 @@ instructions for web development with Flask.
templating
testing
errorhandling
debugging
logging
config
signals

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@ -111,59 +111,43 @@ application). It will tell you what it tried to import and why it failed.
The most common reason is a typo or because you did not actually create an
``app`` object.
.. _debug-mode:
Debug Mode
----------
(Want to just log errors and stack traces? See :doc:`errorhandling`)
The ``flask run`` command can do more than just start the development
server. By enabling debug mode, the server will automatically reload if
code changes, and will show an interactive debugger in the browser if an
error occurs during a request.
The :command:`flask` script is nice to start a local development server, but
you would have to restart it manually after each change to your code.
That is not very nice and Flask can do better. If you enable debug
support the server will reload itself on code changes, and it will also
provide you with a helpful debugger if things go wrong.
.. image:: _static/debugger.png
:align: center
:class: screenshot
:alt: The interactive debugger in action.
To enable all development features (including debug mode) you can export
the ``FLASK_ENV`` environment variable and set it to ``development``
before running the server::
.. 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 all development features, set the ``FLASK_ENV`` environment
variable to ``development`` before calling ``flask run``.
.. code-block:: text
$ export FLASK_ENV=development
$ flask run
(On Windows you need to use ``set`` instead of ``export``.)
See also:
This does the following things:
1. it activates the debugger
2. it activates the automatic reloader
3. it enables the debug mode on the Flask application.
You can also control debug mode separately from the environment by
exporting ``FLASK_DEBUG=1``.
There are more parameters that are explained in :doc:`/server`.
.. admonition:: Attention
Even though the interactive debugger does not work in forking environments
(which makes it nearly impossible to use on production servers), it still
allows the execution of arbitrary code. This makes it a major security risk
and therefore it **must never be used on production machines**.
Screenshot of the debugger in action:
.. image:: _static/debugger.png
:align: center
:class: screenshot
:alt: screenshot of debugger in action
More information on using the debugger can be found in the `Werkzeug
documentation`_.
.. _Werkzeug documentation: https://werkzeug.palletsprojects.com/debug/#using-the-debugger
Have another debugger in mind? See :ref:`working-with-debuggers`.
- :doc:`/server` and :doc:`/cli` for information about running in
development mode.
- :doc:`/debugging` for information about using the built-in debugger
and other debuggers.
- :doc:`/logging` and :doc:`/errorhandling` to log errors and display
nice error pages.
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