多线程在python中修改全局列表 [英] multi threads modify a global list in python

问题描述

我想在一个线程中每2秒将一个项目添加到全局列表中， 并将列表保存到数据库中，然后每3秒在另一个线程中清空该列表.

我创建了两个局部变量来监视添加的项总数和保存的项总数，它们应该每6秒钟相等，但事实并非如此. 这是我的代码:

import datetime
import psutil,os,time
from threading import *
class AddToList(Thread):
    totalAdded=0
    def run(self):
        lock=RLock()
        lock.acquire()
        while True:
            entryList.append("AddToList at "+str(datetime.datetime.now()))
            self.totalAdded=self.totalAdded+len(entryList)
            print("totalAdded:"+str(self.totalAdded))
            time.sleep(2)
        lock.release()
class SaveList(Thread):
    totalSaved=0
    '''save entry to server'''
    def __init__(self):
        Thread.__init__(self)
    def run(self):
        lock=RLock()
        lock.acquire()
        while True:
            #save list to database,then empty the list
            self.totalSaved=self.totalSaved+len(entryList)
            del entryList[:]
            print("totalSaved:"+str(self.totalSaved))
            time.sleep(3)
        lock.release()

if __name__=="__main__":
    global entryList
    entryList=[]
    addClass= AddToList()
    addClass.start()
    saveClass=SaveList()
    saveClass.start()

结果:

totalAdded:2
totalSaved:2
totalAdded:3
totalSaved:3totalAdded:4

totalAdded:6
totalSaved:5
totalAdded:7
totalSaved:6
totalAdded:8
totalAdded:10
totalSaved:8
totalAdded:11
totalSaved:9
totalAdded:12
totalAdded:14
totalSaved:11
totalAdded:15
totalSaved:12
...........
...........
totalAdded:51
totalSaved:39totalAdded:52

totalAdded:54
totalSaved:41
totalAdded:55
totalSaved:42
totalAdded:56
totalAdded:58
totalSaved:44
totalAdded:59
totalSaved:45totalAdded:60
......
......

我是python的新手，并在线程，锁和RLock方面进行了大量搜索，但是没有运气. 哪里错了?

解决方案

要使Lock和RLock工作，必须在每个线程中使用 same 对象.锁定对象必须具有与您要保护"的对象相同的可见性".

这是您的代码的新版本，应该可以使用.它还避免使用诸如全局变量等之类的东西.

import datetime
import time
import threading

class AddToList(threading.Thread):

    def __init__(self, lock, entryList):
        threading.Thread.__init__(self)
        self.totalAdded = 0
        self.entryList = entryList
        self.lock = lock

    def run(self):
        while True:
            self.lock.acquire()
            entryList.append("AddToList at {}".format(datetime.datetime.now()))
            self.totalAdded += 1
            self.lock.release()
            print("totalAdded: {}".format(self.totalAdded))
            time.sleep(2)


class SaveList(threading.Thread):
    def __init__(self, lock, entryList):
        threading.Thread.__init__(self)
        self.totalSaved = 0
        self.entryList = entryList
        self.lock = lock

    def run(self):
        while True:
            self.lock.acquire()
            self.totalSaved += len(self.entryList)
            del self.entryList[:]
            self.lock.release()
            print("totalSaved: {}".format(self.totalSaved))
            time.sleep(3)


if __name__=="__main__":
    lock=threading.Lock()
    entryList=[]

    addClass = AddToList(lock, entryList)
    addClass.start()

    saveClass = SaveList(lock, entryList)
    saveClass.start()

一些注意事项:

1. 当您没有特殊需要时，请使用Lock而不是RLock. RLock慢得多.
2. 正如某人所指出的，最好避免在不需要时使用全局变量.同样，只有在合理的情况下，才应使用Class变量.
3. 使用锁时，应尽量限制获取和释放之间的代码.在以前的代码中，您永远不会释放锁.

i want to add an item into a global list every 2 seconds in one thread, and save the list into database before empty it every 3 seconds in another thread.

i create two local varibles to monitor the total added items and total saveditems, they should be equal every 6 senconds,but it is not. here is my code:

import datetime
import psutil,os,time
from threading import *
class AddToList(Thread):
    totalAdded=0
    def run(self):
        lock=RLock()
        lock.acquire()
        while True:
            entryList.append("AddToList at "+str(datetime.datetime.now()))
            self.totalAdded=self.totalAdded+len(entryList)
            print("totalAdded:"+str(self.totalAdded))
            time.sleep(2)
        lock.release()
class SaveList(Thread):
    totalSaved=0
    '''save entry to server'''
    def __init__(self):
        Thread.__init__(self)
    def run(self):
        lock=RLock()
        lock.acquire()
        while True:
            #save list to database,then empty the list
            self.totalSaved=self.totalSaved+len(entryList)
            del entryList[:]
            print("totalSaved:"+str(self.totalSaved))
            time.sleep(3)
        lock.release()

if __name__=="__main__":
    global entryList
    entryList=[]
    addClass= AddToList()
    addClass.start()
    saveClass=SaveList()
    saveClass.start()

result:

totalAdded:2
totalSaved:2
totalAdded:3
totalSaved:3totalAdded:4

totalAdded:6
totalSaved:5
totalAdded:7
totalSaved:6
totalAdded:8
totalAdded:10
totalSaved:8
totalAdded:11
totalSaved:9
totalAdded:12
totalAdded:14
totalSaved:11
totalAdded:15
totalSaved:12
...........
...........
totalAdded:51
totalSaved:39totalAdded:52

totalAdded:54
totalSaved:41
totalAdded:55
totalSaved:42
totalAdded:56
totalAdded:58
totalSaved:44
totalAdded:59
totalSaved:45totalAdded:60
......
......

i anm new to python and searched a lot about threading ,Lock and RLock ,but with no luck. where am wrong?

解决方案

To make Lock and RLock work you must use the same object in every thread. The lock objects must have the same "visibility" of the object that you want to "protect".

Here is a new version of you code which should work. It also avoid using things like global variables etc.

import datetime
import time
import threading

class AddToList(threading.Thread):

    def __init__(self, lock, entryList):
        threading.Thread.__init__(self)
        self.totalAdded = 0
        self.entryList = entryList
        self.lock = lock

    def run(self):
        while True:
            self.lock.acquire()
            entryList.append("AddToList at {}".format(datetime.datetime.now()))
            self.totalAdded += 1
            self.lock.release()
            print("totalAdded: {}".format(self.totalAdded))
            time.sleep(2)


class SaveList(threading.Thread):
    def __init__(self, lock, entryList):
        threading.Thread.__init__(self)
        self.totalSaved = 0
        self.entryList = entryList
        self.lock = lock

    def run(self):
        while True:
            self.lock.acquire()
            self.totalSaved += len(self.entryList)
            del self.entryList[:]
            self.lock.release()
            print("totalSaved: {}".format(self.totalSaved))
            time.sleep(3)


if __name__=="__main__":
    lock=threading.Lock()
    entryList=[]

    addClass = AddToList(lock, entryList)
    addClass.start()

    saveClass = SaveList(lock, entryList)
    saveClass.start()

Some things to note:

1. Use Lock instead of RLock when you don't have any particular needs. RLock is much slower.
2. As already pointed out by someone it is better avoid using global variables when not needed. Also Class variables should be used only when it makes sense.
3. When you use a lock you should try to limit as much as possible the code between acquire and release. In you previous code you never release the lock.

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