Improved interface for the URL build error handler

flask/app.py

@@ -19,7 +19,7 @@ from itertools import chain
 from functools import update_wrapper
 
 from werkzeug.datastructures import ImmutableDict
-from werkzeug.routing import Map, Rule, RequestRedirect
+from werkzeug.routing import Map, Rule, RequestRedirect, BuildError
 from werkzeug.exceptions import HTTPException, InternalServerError, \
      MethodNotAllowed, BadRequest
 
@@ -341,16 +341,14 @@ class Flask(_PackageBoundObject):
         #: decorator.
         self.error_handler_spec = {None: self._error_handlers}
 
-        #: If not `None`, this function is called when :meth:`url_for` raises
-        #: :exc:`~werkzeug.routing.BuildError`, with the call signature::
-        #:
-        #:     self.build_error_handler(error, endpoint, **values)
-        #:
-        #: Here, `error` is the instance of `BuildError`, and `endpoint` and
-        #: `**values` are the arguments passed into :meth:`url_for`.
+        #: A list of functions that are called when :meth:`url_for` raises a
+        #: :exc:`~werkzeug.routing.BuildError`.  Each function registered here
+        #: is called with `error`, `endpoint` and `values`.  If a function
+        #: returns `None` or raises a `BuildError` the next function is
+        #: tried.
         #:
         #: .. versionadded:: 0.9
-        self.build_error_handler = None
+        self.url_build_error_handlers = []
 
         #: A dictionary with lists of functions that should be called at the
         #: beginning of the request.  The key of the dictionary is the name of
@@ -1490,19 +1488,24 @@ class Flask(_PackageBoundObject):
         for func in funcs:
             func(endpoint, values)
 
-    def handle_build_error(self, error, endpoint, **values):
+    def handle_url_build_error(self, error, endpoint, values):
         """Handle :class:`~werkzeug.routing.BuildError` on :meth:`url_for`.
-
-        Calls :attr:`build_error_handler` if it is not `None`.
         """
-        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
-        return self.build_error_handler(error, endpoint, **values)
+        exc_type, exc_value, tb = sys.exc_info()
+        for handler in self.url_build_error_handlers:
+            try:
+                rv = handler(error, endpoint, values)
+                if rv is not None:
+                    return rv
+            except BuildError, error:
+                pass
+
+        # At this point we want to reraise the exception.  If the error is
+        # still the same one we can reraise it with the original traceback,
+        # otherwise we raise it from here.
+        if error is exc_value:
+            raise exc_type, exc_value, tb
+        raise error
 
     def preprocess_request(self):
         """Called before the actual request dispatching and will