Closes Issue 21262: New method assert_not_called for Mock.

It raises AssertionError if the mock has been called.

Doc/library/unittest.mock.rst

@@ -321,6 +321,20 @@ the `new_callable` argument to `patch`.
             >>> calls = [call(4), call(2), call(3)]
             >>> mock.assert_has_calls(calls, any_order=True)
 
+    .. method:: assert_not_called(*args, **kwargs)
+
+        Assert the mock was never called.
+
+            >>> m = Mock()
+            >>> m.hello.assert_not_called()
+            >>> obj = m.hello()
+            >>> m.hello.assert_not_called()
+            Traceback (most recent call last):
+              ...
+            AssertionError: Expected 'hello' to not have been called. Called 1 times.
+
+        .. versionadded:: 3.5
+
 
     .. method:: reset_mock()
 

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

Misc/NEWS

@@ -50,6 +50,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #21262: New method assert_not_called for Mock.
+  It raises AssertionError if the mock has been called.
+
 - Issue #21238: New keyword argument `unsafe` to Mock. It raises
   `AttributeError` incase of an attribute startswith assert or assret.