Document url_for BuildError hook.

flask/app.py

@@ -1492,6 +1492,7 @@ class Flask(_PackageBoundObject):
         if self.build_error_handler is None:
             exc_type, exc_value, tb = sys.exc_info()
             if exc_value is error:
+                # exception is current, raise in context of original traceback.
                 raise exc_type, exc_value, tb
             else:
                 raise error

flask/helpers.py

@@ -212,6 +212,40 @@ def url_for(endpoint, **values):
 
     For more information, head over to the :ref:`Quickstart <url-building>`.
 
+    To integrate applications, :class:`Flask` has a hook to intercept URL build
+    errors through :attr:`Flask.build_error_handler`.  The `url_for` function
+    results in a :exc:`~werkzeug.routing.BuildError` when the current app does
+    not have a URL for the given endpoint and values.  When it does, the
+    :data:`~flask.current_app` calls its :attr:`~Flask.build_error_handler` if
+    it is not `None`, which can return a string to use as the result of
+    `url_for` (instead of `url_for`'s default to raise the
+    :exc:`~werkzeug.routing.BuildError` exception) or re-raise the exception.
+    An example::
+
+        def external_url_handler(error, endpoint, **values):
+            "Looks up an external URL when `url_for` cannot build a URL."
+            # This is an example of hooking the build_error_handler.
+            # Here, lookup_url is some utility function you've built
+            # which looks up the endpoint in some external URL registry.
+            url = lookup_url(endpoint, **values)
+            if url is None:
+                # External lookup did not have a URL.
+                # Re-raise the BuildError, in context of original traceback.
+                exc_type, exc_value, tb = sys.exc_info()
+                if exc_value is error:
+                    raise exc_type, exc_value, tb
+                else:
+                    raise error
+            # url_for will use this result, instead of raising BuildError.
+            return url
+
+        app.build_error_handler = external_url_handler
+
+    Here, `error` is the instance of :exc:`~werkzeug.routing.BuildError`, and
+    `endpoint` and `**values` are the arguments passed into `url_for`.  Note
+    that this is for building URLs outside the current application, and not for
+    handling 404 NotFound errors.
+
     .. versionadded:: 0.9
        The `_anchor` and `_method` parameters were added.