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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def assert_called_once(_mock_self):
 """assert that the mock was called only once. """

        self = _mock_self
        if not self.call_count == 1:
            msg = ("Expected '%s' to have been called once. Called %s times." %
                   (self._mock_name or 'mock', self.call_count))
            raise AssertionError(msg)