GIL锁

import threading
total = 0
def add():
    global total
    for i in range(1000000):
        total += 1


def sub():
    global total
    for i in range(1000000):
        total -= 1

if __name__ == ‘__main__‘:
    t1 = threading.Thread(target=add)
    t2 = threading.Thread(target=sub)
    t1.start()
    t2.start()
    t1.join()
    t2.join()
    print(total)   # 并不等于0,说明不是等t1执行完了t2再执行.