Closes Issue 21262: New method assert_not_called for Mock.

It raises AssertionError if the mock has been called.

Lib/unittest/mock.py

@@ -758,6 +758,14 @@ class NonCallableMock(Base):
         else:
             return _call
 
+    def assert_not_called(_mock_self, *args, **kwargs):
+        """assert that the mock was never called.
+        """
+        self = _mock_self
+        if self.call_count != 0:
+            msg = ("Expected '%s' to not have been called. Called %s times." %
+                   (self._mock_name or 'mock', self.call_count))
+            raise AssertionError(msg)
 
     def assert_called_with(_mock_self, *args, **kwargs):
         """assert that the mock was called with the specified arguments.

Lib/unittest/test/testmock/testmock.py

@@ -1198,6 +1198,15 @@ class MockTest(unittest.TestCase):
         m.assert_foo_call()
         m.assret_foo_call()
 
+    #Issue21262
+    def test_assert_not_called(self):
+        m = Mock()
+        m.hello.assert_not_called()
+        m.hello()
+        with self.assertRaises(AssertionError):
+            m.hello.assert_not_called()
+
+
     def test_mock_add_spec(self):
         class _One(object):
             one = 1