Example showing how to get monitor resolution and/or work area in Python 3 and Gtk 3.22+.

gtk-get-screen-workarea.py

#!/usr/bin/env python3
import gi
gi.require_version('Gtk', '3.0')
from gi.repository import Gtk, Gdk
import sys

class MyWindow(Gtk.ApplicationWindow):
    def __init__(self, app):
        Gtk.Window.__init__(self, title="Gtk get screen workarea", application=app)


class MyApplication(Gtk.Application):
    def __init__(self):
        Gtk.Application.__init__(self)

    def do_activate(self):
        win = MyWindow(self)
        print(self.get_workarea(win))
        win.show_all()

    def get_workarea(self, window):
        # Returns Gdk.Screen
        screen = window.get_screen()
        # Returns Gdk.Display
        display = screen.get_display()
        # Gets the monitor on which the application is going to be shown on
        # This prevents conflicts when the user has multiple monitors
        monitor = display.get_monitor_at_window(screen.get_root_window())
        # Requests the monitor workarea
        # Return type is GdkRectangle (x, y, width, height)
        # If you want the entire monitor resolution, replace with get_geometry()
        workarea = monitor.get_workarea()
        return workarea.width, workarea.height

    def do_startup(self):
        Gtk.Application.do_startup(self)

app = MyApplication()
exit_status = app.run(sys.argv)
sys.exit(exit_status)