urwid.ListBox

[Abhirup-99]

Add a len property in ListBox to return the length of the items that are being stacked.

[penguinolog]

Problem with ListBox, that it's not required to be a list based: It uses ListWalker interface and technically can use wrapped generator without real length.
__len__ can be added to fixed-size container, like Pile, Columns, Grid and Overlay (the last one is a special case with fixed length == 2)