python-xlib example which reacts to changing the active window

x11_watch_active_window.py

#!/usr/bin/env python3
# Based on code by Stephan Sokolow
# MIT-licensed
# Source: https://gist.github.com/ssokolow/e7c9aae63fb7973e4d64cff969a78ae8
"""python-xlib example which reacts to changing the active window/title.

Requires:
- Python
- python-xlib

Tested with Python 2.x because my Kubuntu 14.04 doesn't come with python-xlib
for Python 3.x.

Design:
-------

Any modern window manager that isn't horrendously broken maintains an X11
property on the root window named _NET_ACTIVE_WINDOW.

Any modern application toolkit presents the window title via a property
named _NET_WM_NAME.

This listens for changes to both of them and then hides duplicate events
so it only reacts to title changes once.

Known Bugs:
-----------

- Under some circumstances, I observed that the first window creation and last
  window deletion on on an empty desktop (ie. not even a taskbar/panel) would
  go ignored when using this test setup:

      Xephyr :3 &
      DISPLAY=:3 openbox &
      DISPLAY=:3 python3 x11_watch_active_window.py

      # ...and then launch one or more of these in other terminals
      DISPLAY=:3 leafpad
"""

# pylint: disable=unused-import
from contextlib import contextmanager
from typing import Any, Dict, Optional, Tuple, Union  # noqa

from Xlib import X
from Xlib.display import Display
from Xlib.error import XError, BadWindow
from Xlib.xobject.drawable import Window
from Xlib.protocol.rq import Event

# Connect to the X server and get the root window
disp = Display()
root = disp.screen().root


# Ignore BadWindow errors; they mean a window was closed while getting info
def ignore_BadWindow(e, rq):
    if not isinstance(e, BadWindow):
        disp.default_error_handler(e)


disp.set_error_handler(ignore_BadWindow)

# Prepare the property names we use so they can be fed into X11 APIs
NET_ACTIVE_WINDOW = disp.intern_atom('_NET_ACTIVE_WINDOW')
NET_WM_NAME = disp.intern_atom('_NET_WM_NAME')  # UTF-8
WM_NAME = disp.intern_atom('WM_NAME')           # Legacy encoding

last_seen = {'xid': None, 'title': None}  # type: Dict[str, Any]


@contextmanager
def window_obj(win_id: Optional[int]) -> Window:
    """Simplify dealing with BadWindow (make it either valid or None)"""
    window_obj = None
    if win_id:
        try:
            window_obj = disp.create_resource_object('window', win_id)
        except XError:
            pass
    yield window_obj


def get_active_window() -> Tuple[Optional[int], bool]:
    """Return a (window_obj, focus_has_changed) tuple for the active window."""
    response = root.get_full_property(NET_ACTIVE_WINDOW,
                                      X.AnyPropertyType)
    if not response:
        return None, False
    win_id = response.value[0]

    focus_changed = (win_id != last_seen['xid'])
    if focus_changed:
        with window_obj(last_seen['xid']) as old_win:
            if old_win:
                old_win.change_attributes(event_mask=X.NoEventMask)

        last_seen['xid'] = win_id
        with window_obj(win_id) as new_win:
            if new_win:
                new_win.change_attributes(event_mask=X.PropertyChangeMask)

    return win_id, focus_changed


def _get_window_name_inner(win_obj: Window) -> str:
    """Simplify dealing with _NET_WM_NAME (UTF-8) vs. WM_NAME (legacy)"""
    for atom in (NET_WM_NAME, WM_NAME):
        try:
            window_name = win_obj.get_full_property(atom, 0)
        except UnicodeDecodeError:  # Apparently a Debian distro package bug
            title = "<could not decode characters>"
        else:
            if window_name:
                win_name = window_name.value  # type: Union[str, bytes]
                if isinstance(win_name, bytes):
                    # Apparently COMPOUND_TEXT is so arcane that this is how
                    # tools like xprop deal with receiving it these days
                    win_name = win_name.decode('latin1', 'replace')
                return win_name
            else:
                title = "<unnamed window>"

    return "{} (XID: {})".format(title, win_obj.id)


def get_window_name(win_id: Optional[int]) -> Tuple[Optional[str], bool]:
    """Look up the window name for a given X11 window ID"""
    if not win_id:
        last_seen['title'] = None
        return last_seen['title'], True

    title_changed = False
    with window_obj(win_id) as wobj:
        if wobj:
            try:
                win_title = _get_window_name_inner(wobj)
            except XError:
                pass
            else:
                title_changed = (win_title != last_seen['title'])
                last_seen['title'] = win_title

    return last_seen['title'], title_changed


def handle_xevent(event: Event):
    """Handler for X events which ignores anything but focus/title change"""
    if event.type != X.PropertyNotify:
        return

    changed = False
    if event.atom == NET_ACTIVE_WINDOW:
        if get_active_window()[1]:
            get_window_name(last_seen['xid'])  # Rely on the side-effects
            changed = True
    elif event.atom in (NET_WM_NAME, WM_NAME):
        changed = changed or get_window_name(last_seen['xid'])[1]

    if changed:
        handle_change(last_seen)


def handle_change(new_state: dict):
    """Replace this with whatever you want to actually do"""
    title = new_state['title']
    if title:
        print(title, flush=True)


if __name__ == '__main__':
    # Listen for _NET_ACTIVE_WINDOW changes
    root.change_attributes(event_mask=X.PropertyChangeMask)

    # Prime last_seen with whatever window was active when we started this
    get_window_name(get_active_window()[0])
    handle_change(last_seen)

    while True:  # next_event() sleeps until we get an event
        handle_xevent(disp.next_event())