def add ( self , value ) : if value not in self . _set : self . _set . add ( value ) self . _list . add ( value )
set is immutable in python so how are we able to add new elemt
Add the element * value * to the set .
def add(self, value):
        """Add the element *value* to the set."""
        if value not in self._set:
            self._set.add(value)
            self._list.add(value)