code: def assert_called_once_with(_mock_self, *args, **kwargs):
          
          self = _mock_self
          if not self.call_count == 1:
              msg = ("Expected '%s' to be called once. Called %s times." %
                     (self._mock_name or 'mock', self.call_count))
              raise AssertionError(msg)
          return self.assert_called_with(*args, **kwargs)
docs: assert that the mock was called exactly once and with the specified
          arguments.