making animation with the ghostscript interpreter

anim_ps.py

#!/usr/bin/env python3
# -*- coding: utf8 -*-

from subprocess import Popen, PIPE, STDOUT
import sys, os
from time import sleep
import select
from random import randint
MAXX=512
MAXY=512
BX=350
BY=350
gs = Popen(['gs', f"-g{MAXX}x{MAXY}"], 
        stdin=PIPE,
        stdout=PIPE,
        stderr=PIPE,
        bufsize=-1,
        )
os.set_blocking(gs.stdout.fileno(), False)
os.set_blocking(gs.stdin.fileno(), False)
os.set_blocking(gs.stderr.fileno(), False)

def puts(s):
    s += "\n"
    select.select([], [gs.stdin], [])
    gs.stdin.write(s.encode("latin1"))
    gs.stdin.flush()
    try:
        back = gs.stderr.read().decode("latin1")
        if back:
            print(back)
    except: pass

vx= randint(-5,5)
vy= randint(-5,5)
x=250
y=250
r = 10
puts("newpath")

def draw_oval_at(x,y, r):
    global vx, vy, MAXY, MAXX
    puts("newpath")
    puts(f"{x} {y} {r} 0 360 arc closepath")
    puts("0.7 setgray fill")
    puts("stroke")

puts("1 setlinewidth")
while True:
    puts(f"newpath {r} {r} moveto {BX-r} {r} lineto  closepath stroke")
    puts(f"newpath {r} {r} moveto {r} {BY-r} lineto closepath stroke")
    puts(f"newpath {BX-r} {r} moveto {BX-r} {BY-r} lineto closepath stroke")
    puts(f"newpath {r} {BY-r} moveto {BX-r} {BY-r} lineto closepath stroke")
    if not  r < x +vx < (BX -r):
        vx = -vx
    if not r < y +vy < (BY -r):
        vy = -vy
    x+=vx
    y+=vy

    draw_oval_at(x, y, r)
    sleep(.01)
    puts("erasepage")