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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 unit(self, unit):
 """Sets the unit of this Dimensions. :param unit: The unit of this Dimensions. :type: str """

        allowed_values = ["cm", "inch", "foot"]  # noqa: E501
        if unit is not None and unit not in allowed_values:
            raise ValueError(
                "Invalid value for `unit` ({0}), must be one of {1}"  # noqa: E501
                .format(unit, allowed_values)
            )

        self._unit = unit