def __sub__(self, other):
        if isinstance(other, self.__class__):
            return self.__class__(default_unit=self._default_unit,
                **{self.STANDARD_UNIT: (self.standard - other.standard)})
        else:
            raise TypeError('%(class)s must be subtracted from %(class)s' % {"class":pretty_name(self)})
dataset/ETHPy150Open AppScale/appscale/AppServer/lib/django-1.5/django/contrib/gis/measure.py/MeasureBase.__sub__/original