open piepwire screencast node for you to use in gst pipeline

open_screencast.py

#!/usr/bin/env python3

import sys
import signal
import dbus
from dbus.mainloop.glib import DBusGMainLoop
from gi.repository import GLib

REQUEST_IFACE = "org.freedesktop.portal.Request"

def get_response(bus, req_path):
    loop = GLib.MainLoop()
    result = [None]

    def signal_callback(*args, **kwargs):
        if (kwargs.get('dbus_interface') == REQUEST_IFACE and
            kwargs.get('member') == "Response" and
            kwargs.get('path') == req_path):
            result[0] = args
            loop.quit()

    bus.add_signal_receiver(
        signal_callback,
        interface_keyword='dbus_interface',
        path_keyword='path',
        member_keyword='member'
    )

    GLib.timeout_add_seconds(30, lambda: loop.quit() or True)
    loop.run()
    return result[0]

def main():
    DBusGMainLoop(set_as_default=True)
    loop = GLib.MainLoop()

    signal.signal(signal.SIGINT, lambda *_: loop.quit())
    signal.signal(signal.SIGTERM, lambda *_: loop.quit())

    bus = dbus.SessionBus()
    portal = bus.get_object("org.freedesktop.portal.Desktop",
                            "/org/freedesktop/portal/desktop")
    screencast = dbus.Interface(portal, "org.freedesktop.portal.ScreenCast")

    resp = get_response(bus, screencast.CreateSession(
        {"session_handle_token": "u2", "handle_token": "u1"}))
    if not resp or resp[0] != 0:
        print("Failed to create session", file=sys.stderr)
        return 1

    session = dbus.ObjectPath(resp[1]['session_handle'])

    resp = get_response(bus, screencast.SelectSources(
        session, {"multiple": False, "types": dbus.UInt32(1 | 2)}))
    if not resp or resp[0] != 0:
        print("Failed to select sources", file=sys.stderr)
        return 1

    resp = get_response(bus, screencast.Start(session, "", {}))
    if not resp or resp[0] != 0:
        print("Failed to start screencast", file=sys.stderr)
        return 1

    node_id = resp[1]['streams'][0][0]
    print(node_id, flush=True)
    print(f"Screencast open on node {node_id}, waiting...", file=sys.stderr)

    loop.run()
    print("Closing screencast session.", file=sys.stderr)
    return 0

if __name__ == "__main__":
    sys.exit(main())