Mark the most-recently-focused i3 window with _last

marklastwindow.py

#!/usr/bin/env python3

# Mark the most-recently-focused i3 window with _last
#
# Add this to your i3 config:
# exec_always --no-startup-id /path/to/script
# bindsym <your binding here> [con_mark=_last] focus
#
# Inspired by:
# - https://www.reddit.com/r/i3wm/comments/4d4luc/how_to_focus_the_last_window/
#   + without the CPU overhead
#   + without stomping on other marks that might be in use
# - https://github.com/acrisci/i3ipc-python/blob/master/examples/focus-last.py
#   + without the socket
#
# Written in 2018 by Trapier Marshall
#
# To the extent possible under law, the author(s) have dedicated all copyright
# and related and neighboring rights to this software to the public domain
# worldwide. This software is distributed without any warranty.
#
# You should have received a copy of the CC0 Public Domain Dedication along
# with this software. If not, see
# <http://creativecommons.org/publicdomain/zero/1.0/>.

import i3ipc

class LastMarker:
    def __init__(self):
        self.lastid = ''
        self.i3 = i3ipc.Connection()
        self.i3.on('window::focus', self.on_window_focus)

    def on_window_focus(self, i3, event):
        tree = i3.get_tree()
        for c in tree.find_marked('_last'):
            c.command('mark --toggle _last')
        last=tree.find_by_id(self.lastid)
        if last:
            last.command('mark --add _last')
        self.lastid = event.container.id

    def run(self):
        self.lastid = self.i3.get_tree().find_focused().id
        self.i3.main()

if __name__ == '__main__':
    last_marker = LastMarker()
    last_marker.run()