def sort_func(self, key):
        """Sorting logic for `Quantity` objects."""
        if key == self._KEYS.VALUE:
            return 'aaa'
        if key == self._KEYS.SOURCE:
            return 'zzz'
        return key
def sort_func ( self , key ) : if key == self . _KEYS . VALUE : return 'aaa' if key == self . _KEYS . SOURCE : return 'zzz' return key
Sorting logic for `Quantity` objects.