Add a BuildError hook to url_for, #456.

flask/app.py

@@ -329,6 +329,17 @@ 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`.
+        #:
+        #: .. versionadded:: 0.9
+        self.build_error_handler = None
+
         #: 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
         #: the blueprint this function is active for, `None` for all requests.
@@ -1473,6 +1484,15 @@ class Flask(_PackageBoundObject):
         for func in funcs:
             func(endpoint, values)
 
+    def handle_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:
+            raise error
+        return self.build_error_handler(error, endpoint, **values)
+
     def preprocess_request(self):
         """Called before the actual request dispatching and will
         call every as :meth:`before_request` decorated function.

flask/helpers.py

@@ -20,6 +20,7 @@ import mimetypes
 from time import time
 from zlib import adler32
 from threading import RLock
+from werkzeug.routing import BuildError
 from werkzeug.urls import url_quote
 
 # try to load the best simplejson implementation available.  If JSON
@@ -214,6 +215,10 @@ def url_for(endpoint, **values):
     .. versionadded:: 0.9
        The `_anchor` and `_method` parameters were added.
 
+    .. versionadded:: 0.9
+       Calls :meth:`Flask.handle_build_error` on
+       :exc:`~werkzeug.routing.BuildError`.
+
     :param endpoint: the endpoint of the URL (name of the function)
     :param values: the variable arguments of the URL rule
     :param _external: if set to `True`, an absolute URL is generated.
@@ -260,6 +265,15 @@ def url_for(endpoint, **values):
     anchor = values.pop('_anchor', None)
     method = values.pop('_method', None)
     appctx.app.inject_url_defaults(endpoint, values)
+    try:
+        rv = url_adapter.build(endpoint, values, method=method,
+                               force_external=external)
+    except BuildError, error:
+        values['_external'] = external
+        values['_anchor'] = anchor
+        values['_method'] = method
+        return appctx.app.handle_build_error(error, endpoint, **values)
+
     rv = url_adapter.build(endpoint, values, method=method,
                            force_external=external)
     if anchor is not None:

flask/testsuite/basic.py

@@ -19,6 +19,7 @@ from threading import Thread
 from flask.testsuite import FlaskTestCase, emits_module_deprecation_warning
 from werkzeug.exceptions import BadRequest, NotFound
 from werkzeug.http import parse_date
+from werkzeug.routing import BuildError
 
 
 class BasicFunctionalityTestCase(FlaskTestCase):
@@ -695,6 +696,17 @@ class BasicFunctionalityTestCase(FlaskTestCase):
             self.assert_equal(flask.url_for('hello', name='test x', _external=True),
                               'http://localhost/hello/test%20x')
 
+    def test_build_error_handler(self):
+        app = flask.Flask(__name__)
+        with app.test_request_context():
+            self.assertRaises(BuildError, flask.url_for, 'spam')
+        def handler(error, endpoint, **values):
+            # Just a test.
+            return '/test_handler/'
+        app.build_error_handler = handler
+        with app.test_request_context():
+            self.assert_equal(flask.url_for('spam'), '/test_handler/')
+
     def test_custom_converters(self):
         from werkzeug.routing import BaseConverter
         class ListConverter(BaseConverter):