title: AssertionError "not called" on simple mock in Python
body: <p>I'm trying to use the Python mock module (downloaded using pip) for the first time. I'm having problems setting an assert, I've narrowed it down to this code:</p>
  
  <pre><code>class TestUsingMock(unittest.TestCase):
  
      def setUp(self):
          self.fake_client = mock.Mock()
  
      def test_mock(self):
          self.fake_client.copy = mock.Mock()
          self.fake_client.copy("123")
          self.fake_client.assert_called_with("123")
  
  if __name__ == "__main__":
      unittest.main()
  </code></pre>
  
  <p>This is the error I get:</p>
  
  <pre><code>F
  ======================================================================
  FAIL: test_mock (__main__.TestVCSDriver)
  ----------------------------------------------------------------------
  Traceback (most recent call last):
    File "./mock_test.py", line 17, in test_mock
      self.fake_client.assert_called_with("123")
    File "/Library/Python/2.6/site-packages/mock.py", line 859, in assert_called_with
      raise AssertionError('Expected call: %s\nNot called' % (expected,))
  AssertionError: Expected call: mock('123')
  Not called
  </code></pre>
  
  <p>Without the assertion, everything works fine. What am I doing wrong?</p>