NameError: name 'lock' is not defined

Question

I have a python program as follows

def send_request(data):
    global lock
    lock.acquire()
    print(data)
    lock.release()

if __name__ == '__main__':
    data_list = ['data1', 'data2', 'data3']
    lock = multiprocessing.Lock()
    pool = multiprocessing.Pool(3)

    pool.map(send_request, data_list)
    pool.close()
    pool.join()

Why did this error occur? NameError: name 'lock' is not defined.

updated

In the answer below, @Jean-François Fabre said that the reason is because “When running your subprocesses, python is "forking" and doesn't see the lock declaration, because it doesn't execute the __main__ part in subprocesses.” But in the following example, the subprocess should not see the lock definition too, but why is the program working fine?

import multiprocessing

def send_request(data):
   lock.acquire()
   print(data,' ',os.getpid())
   lock.release()

def init(l):
   global lock
   lock = l

if __name__ == '__main__':
   data_list = ['data1', 'data2', 'data3']
   lock = multiprocessing.Lock()
   pool = multiprocessing.Pool(8, initializer=init, initargs=(lock,))
   pool.map(send_request, data_list)
   pool.close()
   pool.join()

See [Python sharing a lock between processes](https://stackoverflow.com/q/25557686/222914) — Janne Karila, Sep 09 '19 at 13:38

Jean-François Fabre · Answer 1 · 2019-09-17T20:04:13.867

In the context of multiprocessing, you have to do more than that.

When running your subprocesses, python is "forking" and doesn't see the lock declaration, because it doesn't execute the __main__ part in subprocesses.

Plus, on Windows (which doesn't have fork, the forking is emulated, leading to different behaviour compared to Unix-like platforms: in a nutshell, fork is able to resume the new process where the old process started, but on Windows, Python has to run a new process from the beginning and take control afterwards, this leads to side effects)

You have to create your lock as a global variable outside the __main__ test (and you can drop global keyword, it will work without it)

import multiprocessing

lock = multiprocessing.Lock()

def send_request(data):
    lock.acquire()
    print(data)
    lock.release()

with those modifications your program prints

data1
data2
data3

as expected.

NameError: name 'lock' is not defined

updated

1 Answers1