<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def set_units(self, unit):
 """Set the unit for this data point Unit, as with data_type, are actually associated with the stream and not the individual data point. As such, changing this within a stream is not encouraged. Setting the unit on the data point is useful when the stream might be created with the write of a data point. """

        self._units = validate_type(unit, type(None), *six.string_types)