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