变量在线程中更新，但更新后的值不反映在循环内

Question

I'm studying multithreading in Programming Python by Mark Lutz, and encountered the following example:

import _thread as thread

stdoutmutex = thread.allocate_lock()
exitmutexes = [thread.allocate_lock() for i in range(5)]

def counter(myId, count):
    for i in range(count):
        stdoutmutex.acquire()
        print('[%s] => %s' % (myId, i))
        stdoutmutex.release()
    exitmutexes[myId].acquire()


for i in range(5):
    thread.start_new_thread(counter, (i, 20))

for mutex in exitmutexes:
    while not mutex.locked(): pass
print('Main thread exiting.')

The above code works fine. It uses mutex for each child thread and tacks them onto a global exitmutexes list. On exit each thread signals the main thread by switching its lock on.

I thought I could use a general boolean flag, instead of allocate_lock() . So I have modified the above code into this:

import _thread as thread

stdoutmutex = thread.allocate_lock()
exitmutexes = [False for i in range(5)]

def counter(myId, count):
    for i in range(count):
        stdoutmutex.acquire()
        print('[%s] => %s' % (myId, i))
        stdoutmutex.release()
    exitmutexes[myId] = True


for i in range(5):
    thread.start_new_thread(counter, (i, 20))

for mutex in exitmutexes:
    while not mutex: print(exitmutexes)
print('Main thread exiting.')

My version doesn't work. It just looping on and on. Why is a simple boolean flag not working here? Thanks.

Answer 1

mutex is a loop variable. It receives a snapshot of the value in exitmutexes[i] at the i ^th iteration, so that when exitmutexes[i] is updated, the change is not seen in mutex . So,

while not mutex

Will constantly test the old value of that entry even after it is updated. You should iterate over the indices instead:

for i in range(len(exitmutexes)):
    while not exitmutexes[i]: print(exitmutexes[i]) 

Alternatively, with enumerate :

for i, mutex in enumerate(exitmutexes):
    while not exitmutexes[i]: print(mutex)