from threading import Thread
import os
num = 100
def f1():
    global num#引入全域性變數
    num = 3
    print(num)$這裡會列印3
if __name__ = '__main__':
    t1 = Thread(target= f1,)
    t1.start()
    t1.join()#等待子執行緒執行結束才會繼續往下走
    print(num)#這裡會列印3

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