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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 set_parallelism(self, num_instances):
 """Sets the number of instances for the source operator of the stream. Attributes: num_instances (int):
 The level of parallelism for the source operator of the stream. """

        assert (num_instances > 0)
        self.env._set_parallelism(self.src_operator_id, num_instances)
        return self