Get active window title in X

xdotool can do that.

xdotool getactivewindow

I modified your solution slightly so it should run more efficiently (it passes parameters to xprop so only the data it needs is returned). Also, I'm not sure it's necessary to buffer the output of xprop so I took that out. It should also correct return "Active window not found" if for some reason it can't find the active window.

def get_active_window_title(self):
    root = Popen(['xprop', '-root', '_NET_ACTIVE_WINDOW'], stdout=PIPE)

    for line in root.stdout:
        m = re.search('^_NET_ACTIVE_WINDOW.* ([\w]+)$', line)
        if m != None:
            id_ = m.group(1)
            id_w = Popen(['xprop', '-id', id_, 'WM_NAME'], stdout=PIPE)
            break

    if id_w != None:
        for line in id_w.stdout:
            match = re.match("WM_NAME\(\w+\) = (?P<name>.+)$", line)
            if match != None:
                return match.group("name")

    return "Active window not found"

You can get the active window title with xdotool:

$ xdotool getactivewindow getwindowname