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

[plonknimbuzz]

or, if there is no available method for my case.
at least, how can i find my previous display?

something like this

def run(action):
    disp = Display() #idk how to attach old display
    if(disp.is_alive):
        if(action=='start'):
            print('continue existing display')
            return disp
        else:
            disp.stop()
            print('stop existing display')
            return None
    else:
        if(action=='start'):
            print('create new display')
            return disp #return something that i can use 1 hour later
        else:
            print('display not found. Cannot stop the display')
            return None

[The-Compiler]

What's the point? The whole idea behind this library is starting and managing an Xvfb process. If you already have one, there's nothing left for it to do?