Change the unit for this limit to the specified unit.  The new
        value may be specified as an integer, a string indicating the
        number of seconds, or one of the recognized unit names.

def unit(self, value):
        """
        Change the unit for this limit to the specified unit.  The new
        value may be specified as an integer, a string indicating the
        number of seconds, or one of the recognized unit names.
        """

        if isinstance(value, TimeUnit):
            self._unit = value
        else:
            self._unit = TimeUnit(value)