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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 key_by(self, key_selector):
 """Applies a key_by operator to the stream. Attributes: key_attribute_index (int):
 The index of the key attributed (assuming tuple records). """

        op = Operator(
            _generate_uuid(),
            OpType.KeyBy,
            "KeyBy",
            other=key_selector,
            num_instances=self.env.config.parallelism)
        return self.__register(op)