urwid text layout that fills with underscores on the right (works with wide and combining characters too!)

gistfile1.py

import urwid

class UnderscoreRight(urwid.StandardTextLayout):
    def layout(self, text, width, align, wrap):
        s = urwid.StandardTextLayout.layout(self, text, width, align, wrap)
        out = []
        last_offset = 0
        for row in s:
            used = 0
            for seg in row:
                used += seg[0]
                if len(seg) == 3:
                    last_offset = seg[2]
            if used == width:
                out.append(row)
                continue
            fill = width - used
            out.append(row + [(fill, last_offset, '_'*fill)])
        return out

urwid.MainLoop(urwid.Filler(
    urwid.Edit(
        'stuff: ',
        'edit here',
        layout=UnderscoreRight(),
        multiline=True,
        )
    )).run()