python 多线程中join()的作用

# encoding: utf-8
"""
author: yangyi@youzan.com
time: 2019/7/30 11:20 AM
func:
"""

from multiprocessing import Process
import os
import time

def now():
  return str(time.strftime('%Y-%m-%d %H:%M:%S', time.localtime()))


def func_1(name):
  print(now() + ' Run child process %s (%s)...' % (name, os.getpid()))
  time.sleep(4)
  print(now() + ' Stop child process %s (%s)...\n' % (name, os.getpid()))


def func_2(name):
  print(now() + ' Run child process %s (%s)...' % (name, os.getpid()))
  time.sleep(8)
  print(now() + ' hello world!')
  print(now() + ' Stop child process %s (%s)...\n' % (name, os.getpid()))


if __name__ == '__main__':
  print ('Parent process %s.' % os.getpid())
  p1 = Process(target=func_1, args=('func_1',))
  p2 = Process(target=func_2, args=('func_2',))
  print now() + ' Process start.'
  p1.start()
  p2.start()
  p1.join()
  p2.join()
  print now() + ' Process end .'

if __name__ == '__main__':
  print ('Parent process %s.' % os.getpid())
  p1 = Process(target=func_1, args=('func_1',))
  p2 = Process(target=func_2, args=('func_2',))
  print now() + ' Process start.'
  p1.start()
  p2.start()
  print now() + ' Process end .'

if __name__ == '__main__':
  print ('Parent process %s.' % os.getpid())
  p1 = Process(target=func_1, args=('func_1',))
  p2 = Process(target=func_2, args=('func_2',))
  print now() + ' Process start.'
  p1.start()
  p2.start()
  p1.join() ### 在p1 执行完之后 。不等待p2 执行，主进程结束。
  print now() + ' Process end .'