+ =运算符在Python中是线程安全的吗? [英] Is the += operator thread-safe in Python?

问题描述

我想创建一个非线程安全的代码块进行实验，这些是2个线程将要调用的函数.

I want to create a non-thread-safe chunk of code for experimentation, and those are the functions that 2 threads are going to call.

c = 0

def increment():
  c += 1

def decrement():
  c -= 1

此代码线程安全吗?

如果没有，我可以理解为什么它不是线程安全的，以及哪种类型的语句通常会导致非线程安全的操作.

If not, may I understand why it is not thread safe, and what kind of statements usually lead to non-thread-safe operations.

如果它是线程安全的，如何使它显式地成为非线程安全的?

If it is thread-safe, how can I make it explicitly non-thread-safe?

推荐答案

由于GIL，单个操作码是线程安全的，但除此之外:

Single opcodes are thread-safe because of the GIL but nothing else:

import time
class something(object):
    def __init__(self,c):
        self.c=c
    def inc(self):
        new = self.c+1 
        # if the thread is interrupted by another inc() call its result is wrong
        time.sleep(0.001) # sleep makes the os continue another thread
        self.c = new


x = something(0)
import threading

for _ in range(10000):
    threading.Thread(target=x.inc).start()

print x.c # ~900 here, instead of 10000

由多个线程共享的每个资源必须具有锁定.

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