Based on: https://gist.github.com/ViktorNova/a13c6613dd365b2c67b0

brightness.py

#!/usr/bin/python3

import sys, subprocess, os
from subprocess import PIPE, Popen
from threading  import Thread
from queue import Queue, Empty

def enqueue_output(out, queue):
    for line in iter(out.readline, b''):
        queue.put(line)
    out.close()

def popen_stdout(args):
    p = subprocess.Popen(args, stdout=subprocess.PIPE)
    return p.communicate()[0]

def set_brightness(opval):
    int_val = int(opval)
    with open(brightness_file, 'w') as fh:
        fh.write("%d" % int_val)

brightness_file = "/sys/class/backlight/radeon_bl0/brightness"

low = 1
high = 255
curr = int(popen_stdout(["cat",brightness_file]).strip())

p = Popen([
    "zenity","--scale","--print-partial", "--value=%d" % curr,
    "--title","Screen Brightness","--text","","--min-value=%d" % low,
    "--max-value=%d" % high, "--step=1"
], stdout=PIPE, close_fds=True)
q = Queue()
t = Thread(target=enqueue_output, args=(p.stdout, q))
t.daemon = True
t.start()

while True:
    try: line = q.get_nowait()
    except Empty:
        if not t.is_alive():
            p.wait()
            if p.returncode == 1:
                set_brightness(curr)
            break
    else:
        set_brightness(line)