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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 exec_pg_success(self, cmd):
 """ Execute a command inside a running container as the postgres user, asserting success. """

        result = self.inner().exec_run(cmd, user='postgres')
        assert result.exit_code == 0, result.output.decode('utf-8')
        return result