Added a pattern for custom error pages.

docs/errorhandling.rst

@@ -1,3 +1,5 @@
+.. _application-errors:
+
 Handling Application Errors
 ===========================
 

docs/patterns/errorpages.rst

@@ -0,0 +1,77 @@
+Custom Error Pages
+==================
+
+Flask comes with a handy :func:`~flask.abort` function that aborts a
+request with an HTTP error code early.  It will also provide a plain black
+and white error page for you with a basic description, but nothing fancy.
+
+Depening on the error code it is less or more likely for the user to
+actually see such an error.
+
+Common Error Codes
+------------------
+
+The following error codes are some that are often displayed to the user,
+even if the application behaves correctly:
+
+*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.
+
+*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 he tries to access a resource he cannot access.
+
+*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 he was
+    looking for was deleted for all ethernity.
+
+*500 Internal Server Error*
+    Usually happens on programming errors or if the server is overloaded.
+    A terrible good idea to have a nice page there, because your
+    application *will* fail sooner or later (see also:
+    :ref:`application-errors`).
+
+
+Error Handlers
+--------------
+
+An error handler is a function, just like a view function, but it is
+called when an error happens and is passed that error.  The error is most
+likely a :exc:`~werkzeug.exceptions.HTTPException`, but in one case it
+can be a different error: a handler for internal server errors will be
+passed other exception instances as well if they are uncought.
+
+An error handler is registered with the :meth:`~flask.Flask.errorhandler`
+decorator and the error code of the exception.  Keep in mind that Flask
+will *not* set the error code for you, so make sure to also provide the
+HTTP status code when returning a response.
+
+Here an example implementation for a "404 Page Not Found" exception::
+
+    from flask import render_template
+
+    @app.errorhandler(404)
+    def page_not_found(e):
+        return render_template('404.html'), 404
+
+An example template might be this:
+
+.. sourcecode:: 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 %}

docs/patterns/index.rst

@@ -27,3 +27,4 @@ Snippet Archives <http://flask.pocoo.org/snippets/>`_.
    templateinheritance
    flashing
    jquery
+   errorpages