def union(self, iterable):
        """adds elements with id's not already in the model"""
        _dict = self._dict
        append = self.append
        for i in iterable:
            if i.id not in _dict:
                append(i)
def union(self, iterable):
        """adds elements with id's not already in the model"""
        _dict = self._dict
        append = self.append
        for i in iterable:
            if i.id not in _dict:
                append(i)
def union ( self , iterable ) : _dict = self . _dict append = self . append for i in iterable : if i . id not in _dict : append ( i )
adds elements with id's not already in the model
adds elements with id s not already in the model