Julian/urwid — comment on commit 2bfa5a33d331602cd27ec8fa74d1687d8cc3cff2
File: urwid/container.py line 710
By wardi:

The trouble here is that with lack of a better interface .widget_list has become the interface people use to modify Pile and Columns widgets, and outside code expects it to be just a list of widgets.

My preference would be to upgrade the new-ish .contents property to a MonitoredList with the content you have put in .widget_list to help people move to modifying .contents (with the sizing information).