多线程资源访问 - 我在哪里放锁？

Question

我有线程代码,每个线程需要写入同一个文件.为了防止并发问题,我使用的是Lock对象.

我的问题是我是否正确使用了锁.如果我从每个线程中设置锁定,该锁定是全局的还是仅特定于该特定线程？

基本上,我应该先创建一个Lock并将其引用传递给每个线程,还是可以像在此处一样在线程内设置它:

import time
from threading import Thread, Lock

def main():
    for i in range(20):
        agent = Agent(i)
        agent.start()

class Agent(Thread):
    def __init__(self, thread_num):
        Thread.__init__(self)
        self.thread_num = thread_num

    def run(self):
        while True:
            print 'hello from thread %s' % self.thread_num
            self.write_result()   

    def write_result(self):
        lock = Lock()
        lock.acquire()
        try:
            f = open('foo.txt', 'a')
            f.write('hello from thread %s\n' % self.thread_num)
            f.flush()
            f.close()
        finally:
            lock.release()

if __name__ == '__main__':
    main()

Answer 1

对于您的用例,一种方法可能是编写一个file锁定的子类:

class LockedWrite(file):
    """ Wrapper class to a file object that locks writes """
    def __init__(self, *args, **kwds):
        super(LockedWrite, self).__init__(*args, **kwds)
        self._lock = Lock()

    def write(self, *args, **kwds):
        self._lock.acquire()
        try:
            super(LockedWrite, self).write(*args, **kwds)
        finally:
            self._lock.release()

要在代码中使用,只需替换以下函数:

def main():
    f = LockedWrite('foo.txt', 'a')

    for i in range(20):
        agent = Agent(i, f)
        agent.start()

class Agent(Thread):
    def __init__(self, thread_num, fileobj):
        Thread.__init__(self)
        self.thread_num = thread_num
        self._file = fileobj    

#   ...

    def write_result(self):
        self._file.write('hello from thread %s\n' % self.thread_num)

这种方法将文件锁定放在文件本身,这似乎更清洁恕我直言

Answer 2

在方法外部创建锁。

class Agent(Thread):
    mylock = Lock()
    def write_result(self):
        self.mylock.acquire()
        try:
            ...
        finally:
            self.mylock.release()

或者如果使用 python >= 2.5：

class Agent(Thread):
    mylock = Lock()
    def write_result(self):
        with self.mylock:
            ...

要在 python 2.5 中使用它，您必须从 future 导入该语句：

from __future__ import with_statement