Added documentation on API error handlers

docs/patterns/apierrors.rst

@@ -0,0 +1,63 @@
+Implementing API Exceptions
+===========================
+
+It's very common to implement RESTful APIs on top of Flask.  One of the
+first thing that developers run into is the realization that the builtin
+exceptions are not expressive enough for APIs and that the content type of
+``text/html`` they are emitting is not very useful for API consumers.
+
+The better solution than using ``abort`` to signal errors for invalid API
+usage is to implement your own exception type and install an error handler
+for it that produces the errors in the format the user is expecting.
+
+Simple Exception Class
+----------------------
+
+The basic idea is to introduce a new exception 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.
+
+This is a simple example::
+
+    from flask import jsonfiy
+
+    class InvalidUsage(Exception):
+        status_code = 400
+
+        def __init__(self, message, status_code=None, payload=None):
+            Exception.__init__(self)
+            self.message = message
+            if status_code is not None:
+                self.status_code = status_code
+            self.payload = payload
+
+        def to_dict(self):
+            rv = dict(self.payload or ())
+            rv['message'] = self.message
+            return rv
+
+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.
+
+Registering an Error Handler
+----------------------------
+
+At that point views can raise that error, but it would immediately result
+in an internal server error.  The reason for this is that there is no
+handler registered for this error class.  That however is easy to add::
+
+    @app.errorhandler(InvalidAPIUsage)
+    def handle_invalid_usage(error):
+        response = jsonify(error.to_dict())
+        response.status_code = error.status_code
+        return response
+
+Usage in Views
+--------------
+
+Here is how a view can use that functionality::
+
+    @app.route('/foo')
+    def get_foo():
+        raise InvalidUsage('This view is gone', status_code=410)

docs/patterns/index.rst

@@ -19,6 +19,7 @@ Snippet Archives <http://flask.pocoo.org/snippets/>`_.
    packages
    appfactories
    appdispatch
+   apierrors
    urlprocessors
    distribute
    fabric