`_
+* `Flask-specific documentation `_.
+
+.. _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 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. ::
+
+ @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 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".
@@ -106,22 +193,117 @@ 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.
+
+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
-.. _handling-errors:
+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.
-Handling Errors
-```````````````
+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
@@ -143,128 +325,41 @@ username but we can't find it, we raise a "404 Not Found".
return render_template("profile.html", user=user)
+Here is another example implementation for a "404 Page Not Found" exception::
+ from flask import render_template
-.. _custom-error-handlers:
+ @app.errorhandler(404)
+ def page_not_found(e):
+ # note that we set the 404 status explicitly
+ return render_template('404.html'), 404
-Custom error handlers
-`````````````````````
+When using :doc:`/patterns/appfactories`::
-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.
+ from flask import Flask, render_template
-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.
+ def page_not_found(e):
+ return render_template('404.html'), 404
-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`.
+ def create_app(config_filename):
+ app = Flask(__name__)
+ app.register_error_handler(404, page_not_found)
+ return app
-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.
+An example template might be this:
-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.
+.. sourcecode:: html+jinja
-.. code-block:: python
+ {% extends "layout.html" %}
+ {% block title %}Page Not Found{% endblock %}
+ {% block body %}
+ Page Not Found
+ What you were looking for is just not there.
+
go somewhere nice
+ {% endblock %}
- from werkzeug.exceptions import InternalServerError
-
- # 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
-
- # 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.
-
-.. 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
-
-
-
-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:
@@ -291,7 +386,7 @@ It can be implemented by rendering the template on "500 Internal Server Error":
return render_template('500.html'), 500
-When using the :doc:`/patterns/appfactories`:
+When using :doc:`/patterns/appfactories`:
.. code-block:: python
@@ -323,15 +418,18 @@ When using :doc:`/blueprints`:
# or with register_error_handler
blog.register_error_handler(500, internal_server_error)
+Blueprint Error Handling
+````````````````````````
-
-In blueprints errorhandlers will simply work as expected; however, there is a caveat
-concerning handlers for 404 and 405 exceptions. These errorhandlers are only
+In blueprints, 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
+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
+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:
@@ -364,20 +462,18 @@ at the application level using the ``request`` proxy object:
-More information on error handling with blueprint can be found in
-:doc:`/blueprints`.
-
+For more information on Blueprint Error Handling see :ref:`my-blueprint-error-label`.
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 +493,8 @@ 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.
@@ -444,118 +538,23 @@ 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 %}
-
{{ e.code }} {{ e.name }}
- {{ e.description }}
- Go home
- {% 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.
+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.
When in Doubt, Run Manually
---------------------------
@@ -608,47 +607,3 @@ you could have something like::
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
-`_ for dealing with application errors. It's
-available as an Open Source project `on GitHub
-`_ and is also available as a `hosted version
-`_ 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
- `_ for more information.
-* `Getting started with Sentry `_
-* `Flask-specific documentation `_.