column_pack.py

import urwid

class MyEdit(urwid.Edit):
    def keypress(self, size, key):
        if key == 'enter':
            return True

        urwid.Edit.keypress(self, size, key)

edit = MyEdit(edit_text='Press ENTER to see my problem!')

left = urwid.Text('left left left')
right = urwid.Text('right right right')

header = urwid.Columns([
    ('pack', urwid.AttrMap(left, 'left')),
    urwid.AttrMap(edit, 'center'),
    ('pack', urwid.AttrMap(right, 'right')),
], dividechars=1)

frame = urwid.Frame(body=urwid.ListBox([]), header=header)


def unhandled_input(input_):
    left.set_text('This does not fit into the left column :(, how can I keep it packed?')

palette = [
    ('left', '', '', '', '', '#600'),
    ('center', '', '', '', '', '#006'),
    ('right', '', '', '', '', '#060'),
]

frame.set_focus('header')
loop = urwid.MainLoop(frame, palette, unhandled_input=unhandled_input)
loop.screen.set_terminal_properties(colors=256)
loop.run()