A driver run with sudo sends no keys: "Can't open display"

Symptoms

A script that reads game memory as root and steers with xdotool prints Failed creating new xdo instance and Error: Can't open display: (null); the game gets no keys.

Cause

sudo resets the environment and drops DISPLAY (observed).

What to do

Pass it through: sudo env DISPLAY=:1 python3 follow.py ....