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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 mode(self, mode):
 """Sets the mode of this BraintreeGateway. :param mode: The mode of this BraintreeGateway. :type: str """

        allowed_values = ["test", "live"]
        if mode is not None and mode not in allowed_values:
            raise ValueError(
                "Invalid value for `mode` ({0}), must be one of {1}"
                .format(mode, allowed_values)
            )

        self._mode = mode