How to use existing display / set display number?

[plonknimbuzz]

Hi, there

i have this service:

#/etc/systemd/system/xvfb.service

[Unit]
Description=X virtual framebuffer

[Service]
Type=simple
User=root

ExecStart=/usr/bin/Xvfb :99 -ac

[Install]
WantedBy=multi-user.target

ps -ef|grep Xvfb
root     15802     1  0 Jul28 ?        00:00:00 /usr/bin/Xvfb :99 -ac

as u can see that i used :99 as display number but i failed to set it using this script

from pyvirtualdisplay import Display
import os
print(os.environ.get("DISPLAY"))
os.environ["DISPLAY"] = ":99"
print(os.environ.get("DISPLAY"))
with Display() as disp:
    # display is active
    print(disp.is_alive()) # True
    print(os.environ.get("DISPLAY"))
# display is stopped
print(disp.is_alive())

result

None
:99
True
:0 ==> failed
False

my question is: how to set display number? because i want to use display that created from outside of this library