import psutil
import subprocess as sp
import time

pid_list = []
pid = ''
argument = '...'
proc = sp.Popen(['python', 'main.py', argument], shell=True)
pid = str(proc.pid)
time.sleep(3) # <-- There no time.wait, but time.sleep.
while True:
    time.sleep(2)
    print(pid)
    for proc in psutil.process_iter():
        pid_list.append(str(proc.pid))
    print(pid_list)
    if pid not in pid_list:
        proc = sp.Popen(['python', 'main.py', argument], shell=True)
        pid = proc.pid
    else:
        pass
    pid_list.clear()