Expose Tcl's external event loop API in tkinter

Feature or enhancement

Proposal:

Tkinter runs Tcl's event loop by calling Tcl_DoOneEvent repeatedly, which is the usual way of running the event loop in Tcl/Tk. This puts Tcl/Tk in charge of the event loop.

Tcl also supports running external event loops via the function Tcl_SetNotifier and associated functions (Tcl_QueueEvent and Tcl_ServiceAll in particular). This part of Tcl's API is currently not exposed in tkinter.

Exposing Tcl_SetNotifier and associated functions would allow Tcl to use the event loop of a different GUI toolkit (e.g. Glib via pygobject) as an external event loop. Alternatively, a single event loop managed by Python can be used as an external event loop both by tkinter and other GUI toolkits at the same time.

This has two advantages:

• It allows the use of multiple GUI toolkits at the same time, without the need for threads or polling (i.e., no busy-sleep loop, which is wasteful, slower, and fragile).
• It makes threading easier, as it avoids the need for ENTER_TCL / LEAVE_TCL and ENTER_PYTHON / LEAVE_PYTHON (in the case of tkinter).

Below is a minimal script demonstrating how two GUI toolkits can run at the same time using a single event loop managed by Python (using select from the Python standard library, just to keep it simple).

import os
import select
import sys
import time

import tkinter
from tkinter import _tkinter

import gi
gi.require_version('Gtk', '3.0')
from gi.repository import Gtk, Gdk, GLib


# --- Create an event source for tkinter

class TkinterEventHandler:

    def __init__(self, callback):
        self.callback = callback
        self.ready_events = 0

    def process_event(self, flags):
        if not (flags & _tkinter.FILE_EVENTS):
            return False

        events = self.ready_events
        self.ready_events = 0
        if events != 0:
            self.callback(_tkinter.READABLE)

        return True

    def __call__(self, events):
        if self.ready_events == 0:
            _tkinter.queue_event(self.process_event)
        self.ready_events = events


class TkinterEventSource:

    def __init__(self):
        self.handlers = {}
        self.timer_abstime = 0

    def set_timer(self, interval_usec=None):
        mode = _tkinter.set_service_mode(_tkinter.SERVICE_ALL)
        if interval_usec is None:
            self.timer_abstime = 0
        else:
            now = time.perf_counter_ns() // 1000
            self.timer_abstime = now + interval_usec

    def wait_for_event(self, timeout):
        return 0

    def init_notifier(self):
        return

    def create_file_handler(self, fd, mask, callback):
        event_handler = TkinterEventHandler(callback)
        if mask & _tkinter.READABLE:
            self.handlers[fd] = event_handler

    def delete_file_handler(self, fd):
        try:
            del self.handlers[fd]
        except KeyError:
            pass

    def finalize_notifier(self, clientData):
        return

    def alert_notifier(self, clientData):
        return

    def service_mode_hook(self, mode):
        return
    
    def setup(self):
        timer_abstime = self.timer_abstime
        if timer_abstime > 0:
            now = time.perf_counter_ns() // 1000
            timeout_usec = timer_abstime - now
            if timeout_usec < 0:
                timeout = 0
            else:
                timeout = timeout_usec / 1000000
        else:
            timeout = None
        fds = list(self.handlers.keys())
        return timeout, fds

    def check(self, ready_events):
        timer_abstime = self.timer_abstime
        result = 0;
        now = time.perf_counter_ns() // 1000
        if timer_abstime < now:
            self.timer_abstime = 0
        ready_rlist, ready_wlist, ready_xlist = ready_events
        for fd in ready_rlist:
            handler = self.handlers.get(fd)
            if handler is None:
                continue
            handler(_tkinter.READABLE)
        _tkinter.service_all()

# --- Tell Tcl to use TkinterEventSource as its notifier

tkinter_event_source = TkinterEventSource()
_tkinter.set_notifier(tkinter_event_source.set_timer,
                      tkinter_event_source.wait_for_event,
                      tkinter_event_source.create_file_handler,
                      tkinter_event_source.delete_file_handler,
                      tkinter_event_source.init_notifier,
                      tkinter_event_source.finalize_notifier,
                      tkinter_event_source.alert_notifier,
                      tkinter_event_source.service_mode_hook
                     )


# --- Create an event source for GTK

class GTKEventSource:

    def setup(self):
        context = GLib.MainContext.default()
        if not context.acquire():
            raise RuntimeError("Failed to acquire the context")
        ready, priority = context.prepare()
        timeout_msec, self.fds = context.query(priority)
        fds = [fd.fd for fd in self.fds]
        if timeout_msec == -1:
            timeout = None
        else:
            timeout = timeout_msec / 1000
        context.release()
        return timeout, fds

    def check(self, ready_events):
        context = GLib.MainContext.default()
        if not context.acquire():
            raise RuntimeError("Failed to acquire the context")
        ready_rlist, ready_wlist, ready_xlist = ready_events
        ready_rlist = set(ready_rlist)
        for fd in self.fds:
            if fd.fd in ready_rlist:
                fd.revents |= GLib.IO_IN
        some_ready = context.check(GLib.MAXINT, self.fds)
        context.dispatch()
        context.release()

gtk_event_source = GTKEventSource()


# --- Make one example window for tkinter

class TkinterWindow:

    def __init__(self):
        self.window = tkinter.Tk()
        self.window.title("Tkinter")

        self.button1 = tkinter.Button(self.window,
                                      text="Click me",
                                      command=self.clicked,
                                      font='Helvetica 20',
                                      width=20,
                                      background='pale green')
        self.button1.pack()

        pipe_read_fd, self.pipe_write_fd = os.pipe()
        self.window.tk.createfilehandler(pipe_read_fd,
                                         tkinter.READABLE,
                                         self.read_from_pipe)
        self.button2 = tkinter.Button(self.window,
                                      text="Trigger the file descriptor",
                                      command=self.write_to_pipe,
                                      font='Helvetica 20',
                                      width=20,
                                      background='lightblue')
        self.button2.pack()

        self.button3 = tkinter.Button(self.window,
                                      text="Start the timer (1 sec)",
                                      command=self.update_timer,
                                      font='Helvetica 20',
                                      width=20,
                                      background='steelblue')
        self.button3.pack()

    def clicked(self):
        print("Tkinter button clicked")

    def write_to_pipe(self):
        message = "Tkinter file descriptor triggered"
        n = os.write(self.pipe_write_fd, message.encode())

    def read_from_pipe(self, fd, mask):
        message = os.read(fd, 1024)
        print(message.decode())

    def update_timer(self, counter=0):
        print("Tkinter timer update %d" % counter)
        self.window.after(1000, self.update_timer, counter+1)

tkinter_window = TkinterWindow()

# --- Make one example window for GTK

settings = Gtk.Settings.get_default()
settings.props.gtk_theme_name = "Adwaita"

class GTKWindow(Gtk.ApplicationWindow):

    def __init__(self, **kargs):
        super().__init__(**kargs, title='GTK')

        self.grid = Gtk.Grid()
        self.add(self.grid)

        button1 = self.create_button(1, text="Click me", color="red")
        button1.connect("clicked", self.clicked)
        self.grid.attach(button1, 0, 0, 1, 1)

        pipe_read_fd, self.pipe_write_fd = os.pipe()
        GLib.io_add_watch(pipe_read_fd, GLib.IOCondition.IN, self.read_from_pipe)
        button2 = self.create_button(2, text="Trigger the file descriptor", color='orchid')
        button2.connect("clicked", self.write_to_pipe)
        self.grid.attach(button2, 0, 1, 1, 1)

        button3 = self.create_button(3, text="Start the timer (2 sec)", color='salmon')
        button3.connect("clicked", self.start_timer)
        self.grid.attach(button3, 0, 2, 1, 1)

        self.show_all()

    def create_button(self, number, text, color):
        button = Gtk.Button.new_with_label(text)
        key = "custom-button%d" % number
        button.get_style_context().add_class(key)
        css_provider = Gtk.CssProvider()
        css_provider.load_from_data(b"""
            .%s {
                font-family: Helvetica;
                font-size: 20pt;
                background: %s;
                color: black;
            }
        """ % (key.encode(),  color.encode()))
        # Apply the CSS to the display
        style_context = button.get_style_context()
        style_context.add_provider(css_provider, Gtk.STYLE_PROVIDER_PRIORITY_USER)
        style_context.add_class(key) # Add the CSS class to the button
        return button

    def clicked(self, button):
        print("GTK button clicked")

    def write_to_pipe(self, button):
        message = "GTK file descriptor triggered"
        n = os.write(self.pipe_write_fd, message.encode())

    def read_from_pipe(self, fd, condition):
        if condition & GLib.IOCondition.IN:
            message = os.read(fd, 1024)
            print(message.decode())
            return True

    def update_timer(self):
        print("GTK timer update %d" % self.counter)
        self.counter += 1
        return True

    def start_timer(self, button):
        self.counter = 0
        GLib.timeout_add(2000, self.update_timer)


glib_window = GTKWindow()


# --- run the event loop with the two GUI frameworks as event sources

sources = [tkinter_event_source, gtk_event_source ]


while True:
    rlist = []
    wlist = []
    xlist = []
    shortest_timeout = None
    for source in sources:
        timeout, fds = source.setup()
        if timeout is not None:
            if shortest_timeout is None:
                shortest_timeout = timeout
            else:
                if timeout < shortest_timeout:
                    shortest_timeout = timeout
        rlist.extend(fds)
    ready_events = select.select(rlist, wlist, xlist, shortest_timeout)
    for source in sources:
        source.check(ready_events)

Has this already been discussed elsewhere?

I have already discussed this feature proposal on Discourse

Links to previous discussion of this feature:

https://discuss.python.org/t/connecting-asyncio-and-tkinter-event-loops/14722/40

Linked PRs

• gh-155805

Start by reviewing tkinter and _tkinter's existing event-loop integration, then compare it with Tcl_SetNotifier and the associated Tcl_QueueEvent and Tcl_ServiceAll APIs. Read the linked discussion and PR gh-155805 first; the work is done when the external notifier API is exposed and its event-loop behavior is validated.