Making a Queue for a function so it only runs once at a time in python

Question

I have a multithreaded function, which all write to the same log file. How can I make this function (maybe with a function decorator) to add the execution of writing to the log file to a queue. Small example:

#!/usr/bin/python

import thread
import time

# Define a function for the thread
def print_time( threadName, delay):
   count = 0
   while count < 5:
      time.sleep(delay)
      count += 1
      writeToLog(threadName, time.ctime(time.time()))
      print "%s: %s" % ( threadName, time.ctime(time.time()) )

# Create two threads as follows
try:
   thread.start_new_thread( print_time, ("Thread-1", 2, ) )
   thread.start_new_thread( print_time, ("Thread-2", 4, ) )
except:
   print "Error: unable to start thread"

def writeToLog(threadName, time):
   self.fileWriter = open("log.txt", "w")
   self.fileWriter.write("ThreadName: " + threadName + "\n")
   self.fileWriter.write("Time: " + time + "\n")
   self.fileWriter.close()

How can I make this function writeToLog add to a queue when executed? Now I get every time both threads call the writeToLog function an error because the other writeToLog function (from the other thread) already closed the file. When having a global variable for this writer, which is closed in the end, I get output like this:

ThreadName: thread1
ThreadName: thread2
Time: 9:50AM
Time: 9:50AM

And the output I always want has to look like this:

ThreadName: Thread-1
Time: 9:50AM
ThreadName: Thread-2
Time: 9:50AM

Answer 1

Concurrency access to a shared resource is a well known problem. Python thread provide some mechanism to avoid issues. Use python locks : http://docs.python.org/2/library/threading.html#lock-objects Lock are used to synchronize access to a shared resource :

lock = Lock()

lock.acquire() # will block if lock is already held
... access shared resource
lock.release()

More information : http://effbot.org/zone/thread-synchronization.htm

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