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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_id(self, unit_id=None):
 """Get or set unit ID field :param unit_id: unit ID (0 to 255) or None for get value :type unit_id: int or None :returns: unit ID or None if set fail :rtype: int or None """

        if unit_id is None:
            return self.__unit_id
        if 0 <= int(unit_id) < 256:
            self.__unit_id = int(unit_id)
            return self.__unit_id
        else:
            return None