Warn on None responses.

flask.py

@@ -687,7 +687,7 @@ class Flask(object):
         """Converts the return value from a view function to a real
         response object that is an instance of :attr:`response_class`.
 
-        The following types are allowd for `rv`:
+        The following types are allowed for `rv`:
 
         ======================= ===========================================
         :attr:`response_class`  the object is returned unchanged
@@ -703,6 +703,11 @@ class Flask(object):
 
         :param rv: the return value from the view function
         """
+        if rv is None:
+            from warnings import warn
+            warn(Warning('View function did not return a response'),
+                 stacklevel=2)
+            return u''
         if isinstance(rv, self.response_class):
             return rv
         if isinstance(rv, basestring):

tests/flask_tests.py

@@ -15,6 +15,7 @@ import sys
 import flask
 import unittest
 import tempfile
+import warnings
 
 
 example_path = os.path.join(os.path.dirname(__file__), '..', 'examples')
@@ -224,6 +225,19 @@ class BasicFunctionalityTestCase(unittest.TestCase):
             assert flask.url_for('static', filename='index.html') \
                 == '/static/index.html'
 
+    def test_none_response(self):
+        warnings.filterwarnings('error', 'View function did not return')
+        app = flask.Flask(__name__)
+        @app.route('/')
+        def test():
+            return None
+        try:
+            app.test_client().get('/')
+        except Warning:
+            pass
+        else:
+            assert "Expected warning"
+
 
 class JSONTestCase(unittest.TestCase):