>>> import mock
>>> m = mock.Mock()
>>> m.assert_called_once()  # Old version of Mock? This may not be what you expect.
<Mock name='mock.assert_called_once()' id='4558624144'>

>>> assert_called_once = mock.Mock.assert_called_once  # Doesn't exist. Now you know!
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: type object 'Mock' has no attribute 'assert_called_once'

>>> assert_called_once_with = mock.Mock.assert_called_once_with  # There it is.

>>> assert_called_once_with(m)  # This does the right thing.
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File ".../mock.py", line 845, in assert_called_once_with
    raise AssertionError(msg)
AssertionError: Expected to be called once. Called 0 times.