Detects external monitor by polling xrandr and sets up my custom layout saved from arandr because XFCE didn't do it for me.

auto_apply_screen_layout.py

from time import sleep
from subprocess import check_output
from os import system as run

def shell(command):
    return check_output(command, shell=True)

def display_count():
    return str(shell("xrandr -q")).count(' connected')

def setup_display():
    if display_count() == 2:
        print('Display changed - dual screen detected, running layout script')
        run('/home/user/bin/dual_screen.sh')
    else:
        print('Display changed detected but no setup is needed, since the default is okay')

last_state = display_count()

while True:
    if last_state != display_count():
        last_state = display_count()
        setup_display()
    sleep(0.8)