当我使用 threading.Lock 锁定线程操作时，为什么它的输出没有 0？

Question

import threading

COUNTER = 100000

lock = threading.Lock()

def add():
  global x
  with lock:
    for i in range(COUNTER):
      x += 1

def subtract():
  global x
  with lock:
    for i in range(COUNTER):
      x -= 1
x = 0
t0 = threading.Thread(target = add)
t1 = threading.Thread(target = subtract)
t0.start()
t1.start()
print(x)
# The output shall be zero isn't it?

我根据我在课程中看到的代码编写了这段代码，但实际上效果并不好。共享变量时，线程对数据的处理非常错误

Answer 1

更正后的代码是：

import threading

COUNTER = 100000

lock = threading.Lock()


def add():
    global x
    with lock:
        for i in range(COUNTER):
            x += 1


def subtract():
    global x
    with lock:
        for i in range(COUNTER):
            x -= 1


x = 0
t0 = threading.Thread(target=add)
t1 = threading.Thread(target=subtract)
t0.start()
t1.start()
# Added the below lines
t0.join() 
t1.join()
# END
print(x)

您的错误原因是您的主线程没有等待您的子线程完成那里的进程。这就是为什么在打印介于两者之间的 x 值时会得到异常输出的原因。