Getting notified about window title changes

Well, thanks to @Basile's comment, I learned a lot and came up with following working sample:

#!/usr/bin/python3
import Xlib
import Xlib.display

disp = Xlib.display.Display()
root = disp.screen().root

NET_WM_NAME = disp.intern_atom('_NET_WM_NAME')
NET_ACTIVE_WINDOW = disp.intern_atom('_NET_ACTIVE_WINDOW')

root.change_attributes(event_mask=Xlib.X.FocusChangeMask)
while True:
    try:
        window_id = root.get_full_property(NET_ACTIVE_WINDOW, Xlib.X.AnyPropertyType).value[0]
        window = disp.create_resource_object('window', window_id)
        window.change_attributes(event_mask=Xlib.X.PropertyChangeMask)
        window_name = window.get_full_property(NET_WM_NAME, 0).value
    except Xlib.error.XError:
        window_name = None
    print(window_name)
    event = disp.next_event()

Rather than running xdotool naively, it listens synchronously to events generated by X which is exactly what I was after.

I couldn't get your focus-change approach to work reliably under Kwin 4.x, but modern window managers maintain a _NET_ACTIVE_WINDOW property on the root window that you can listen for changes to.

Here's a Python implementation of just that:

#!/usr/bin/python
from contextlib import contextmanager
import Xlib
import Xlib.display

disp = Xlib.display.Display()
root = disp.screen().root

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 }

@contextmanager
def window_obj(win_id):
    """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 Xlib.error.XError:
            pass
    yield window_obj

def get_active_window():
    win_id = root.get_full_property(NET_ACTIVE_WINDOW,
                                       Xlib.X.AnyPropertyType).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=Xlib.X.NoEventMask)

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

    return win_id, focus_changed

def _get_window_name_inner(win_obj):
    """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
                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):
    if not win_id:
        last_seen['title'] = "<no window id>"
        return last_seen['title']

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

    return last_seen['title'], title_changed

def handle_xevent(event):
    if event.type != Xlib.X.PropertyNotify:
        return

    changed = False
    if event.atom == NET_ACTIVE_WINDOW:
        if get_active_window()[1]:
            changed = changed or get_window_name(last_seen['xid'])[1]
    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):
    """Replace this with whatever you want to actually do"""
    print(new_state)

if __name__ == '__main__':
    root.change_attributes(event_mask=Xlib.X.PropertyChangeMask)

    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())

The more fully-commented version I wrote as an example for someone is in this gist.

UPDATE: Now, it also demonstrates the second half (listening to _NET_WM_NAME) to do exactly what was requested.

UPDATE #2: ...and the third part: Falling back to WM_NAME if something like xterm hasn't set _NET_WM_NAME. (The latter is UTF-8 encoded while the former is supposed to use a legacy character coding called compound text but, since nobody seems to know how to work with it, you get programs throwing whatever stream of bytes they have in there and xprop just assuming it'll be ISO-8859-1.)