将一个类变量与多个线程同步答案

【问题标题】：Sync a class variable with multiple threads将一个类变量与多个线程同步
【发布时间】：2019-11-21 11:04:00
【问题描述】：

我正在尝试创建一个装饰器，它将全局验证请求存储桶是否已满。桶大小为 40，泄漏率为每秒 2 个。我希望能够在保持 bucker sub 40 的同时处理不同请求的多个实例。 self.bucket_current 怎么可能变为负数？另外，如何调试这些线程以及它们的作用？

from time import sleep
from concurrent.futures import ThreadPoolExecutor
from math import ceil

class Counter:
    def __init__(self):
        self.bucket_size = 40
        self.bucket_current = 0
        self.bucket_leak_rate = 2


    def countdown(self, secs):
        self.bucket_current += secs
        print(self.bucket_current)
        r = ceil(self.bucket_current/self.bucket_leak_rate)
        sleep(1)
        while self.bucket_current > 0:
            for _ in range(1, r+1):
                if self.bucket_current != 1:
                    self.bucket_current -= 2
                else:
                    self.bucket_current -= 1

                print(self.bucket_current)
                sleep(1)


c = Counter()

with ThreadPoolExecutor() as executor:
    executor.submit(c.countdown, 9)
    sleep(3)
    executor.submit(c.countdown, 7)

输出：

【问题讨论】：

你的预期输出是什么？

标签： python multithreading synchronization python-decorators class-variables

【解决方案1】：

self.bucket_current 怎么可能变成负数？

答案是，根据您的代码，在您的第一行 executor.submit(c.countdown, 9) 之后，线程单独执行，3 秒后 (sleep(3)) 另一个线程加入执行 executor.submit(c.countdown, 7) 并且没有这两个线程之间的任何同步。

你需要提供一种机制来同步这两个线程，这是我有时使用的一个简单的装饰器：

from time import sleep
from concurrent.futures import ThreadPoolExecutor
from math import ceil
from functools import wraps
from threading import Lock


def synchronized(tlockname):
    """A decorator to place an instance based lock around a method """

    def _synched(func):
        @wraps(func)
        def _synchronizer(self, *args, **kwargs):
            tlock = self.__getattribute__(tlockname)
            tlock.acquire()
            try:
                return func(self, *args, **kwargs)
            finally:
                tlock.release()

        return _synchronizer

    return _synched


class Counter:
    def __init__(self):
        self.bucket_size = 40
        self.bucket_current = 0
        self.bucket_leak_rate = 2
        self.bucket_lock = Lock()

    @synchronized("bucket_lock")
    def countdown(self, secs):
        self.bucket_current += secs
        print(self.bucket_current)
        r = ceil(self.bucket_current / self.bucket_leak_rate)
        sleep(1)
        while self.bucket_current > 0:
            for _ in range(1, r + 1):
                if self.bucket_current != 1:
                    self.bucket_current -= 2
                else:
                    self.bucket_current -= 1

                print(self.bucket_current)
                sleep(1)


c = Counter()

with ThreadPoolExecutor() as executor:
    executor.submit(c.countdown, 9)
    sleep(3)
    executor.submit(c.countdown, 7)

【讨论】：