clock.py

import datetime, threading

def get_time():
  t = threading.Timer(1,get_time)
  t.start()
  r = datetime.datetime.today()
  s = '{:02d}:{:02d}:{:02d}.{:3d}'.format(r.hour, r.minute, r.second, int(r.microsecond / 1000), '2s')
  print(s)
get_time()

#output
#22:09:10.144
#22:09:11.144
#22:09:12.145
#22:09:13.145
#22:09:14.146
#22:09:15.147