execute_process: how to pass argument to python script

Usage
1 
execute_process(COMMAND ${Python3_EXECUTABLE} my_script.py my_arg)

doesn’t work as intended: my_script.py is executed, but does not see the argument my_arg.

Whereas on the command line

$ python3 my_script.py my_args

does work.

As a minimal example, suppose my_script.py is

import sys
print("my_script found argument", sys.argv[1])