Provide some checks, spawn a process and poll it until all checks pass. That way you know when the application really starts working.

from spawn_and_check import execute, check_http
process = execute('run_some_service --port 8000', [check_http('http://127.0.0.1:8000')], timeout=10)
# The process is ready at this point.

The API has not been stabilised nor defined and it is prone to change. Most probably the checks signature will change from () -> bool to subprocess.Popen -> bool.