Question

I have threaded code where each thread needs to write to the same file. To prevent concurrency issues, I am using a Lock object.

My question is whether I am using the Lock correctly. If I set the lock from within each thread, is that lock global or only specific to that specific thread?

Basically, should I create a Lock first and pass its reference to each thread, or is it ok to set it from within the thread like I do here:

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

Create the lock outside the method.

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

or if using python >= 2.5:

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

To use that with python 2.5 you must import the statement from the future:

from __future__ import with_statement