Iterate over `self.obj` list, extracting `selector` from each
        element. The `selector` can be a simple integer index, or any valid
        key (hashable object).

def _filtered_list(self, selector):
        """Iterate over `self.obj` list, extracting `selector` from each
        element. The `selector` can be a simple integer index, or any valid
        key (hashable object).
        """
        res = []
        for elem in self.obj:
            self._append(elem, selector, res)
        return res