Raise exceptions if a function is overridden by a new endpoint. This fixes #570

flask/app.py

@@ -942,8 +942,13 @@ class Flask(_PackageBoundObject):
 
         rule = self.url_rule_class(rule, methods=methods, **options)
         rule.provide_automatic_options = provide_automatic_options
+
         self.url_map.add(rule)
         if view_func is not None:
+            old_func = self.view_functions.get(endpoint)
+            if old_func is not None and old_func is not view_func:
+                raise AssertionError('View function mapping is overwriting an '
+                                     'existing endpoint function: %s' % endpoint)
             self.view_functions[endpoint] = view_func
 
     def route(self, rule, **options):

flask/testsuite/basic.py

@@ -387,7 +387,7 @@ class BasicFunctionalityTestCase(FlaskTestCase):
             return ''
 
         @app.route('/test_filters_without_returning_categories/')
-        def test_filters():
+        def test_filters2():
             messages = flask.get_flashed_messages(category_filter=['message', 'warning'])
             self.assert_equal(len(messages), 2)
             self.assert_equal(messages[0], u'Hello World')

flask/testsuite/views.py

@@ -145,6 +145,23 @@ class ViewTestCase(FlaskTestCase):
         self.assert_equal(rv.data, '')
         self.assert_equal(rv.headers['X-Method'], 'HEAD')
 
+    def test_endpoint_override(self):
+        app = flask.Flask(__name__)
+        app.debug = True
+
+        class Index(flask.views.View):
+            methods = ['GET', 'POST']
+            def dispatch_request(self):
+                return flask.request.method
+
+        app.add_url_rule('/', view_func=Index.as_view('index'))
+
+        with self.assert_raises(AssertionError):
+            app.add_url_rule('/', view_func=Index.as_view('index'))
+
+        # But these tests should still pass. We just log a warning.
+        self.common_test(app)
+
 
 def suite():
     suite = unittest.TestSuite()