Python's threading and multiprocess lock not working

Question

I have a toy example that demonstrates multithreading and locks. Without locks, I obviously get a bad value for the global counter variable. However, when I put in the threading.Lock in, it leads to even more inconsistencies. Also, multiprocessing.Lock() doesn't completely fix the problem either.

import threading 
from multiprocessing import Process, Lock

num_experiments = 200
num_threads = 5 
iterations_in_one_thread = 500 

def f(): 

    global counter 
    for i in range(iterations_in_one_thread): 
        with lock: 
            counter += 1 

bad_count = 0 
# lock = threading.Lock()
lock = Lock() 

for x in range(num_experiments): 

    counter = 0 
    threads = [] 
    for i in range(num_threads):
        t = threading.Thread(target=f)
        threads.append(t)
        t.start()

    for i in threads: 
        t.join() 

    if counter != num_threads * iterations_in_one_thread: 
        bad_count += 1 
        print counter 


print "Bad count:", bad_count
print "Total runs:", num_experiments 

Output I expect: Bad count: 0 Output I get: Bad count: 3 (or sometimes upto 6)

I'm on python 2.7.3

Python 2.7.13 |Anaconda custom (x86_64)| (default, Dec 20 2016, 23:05:08)
[GCC 4.2.1 Compatible Apple LLVM 6.0 (clang-600.0.57)] on darwin

Answer 1

I think that you just made a simple typo.

for i in threads: 
    t.join()

should be

for i in threads: 
    i.join()

Otherwise, you're only joining on the last thread. Also, you probably shouldn't mix the multiprocessing library with the threading library. Stick with threading.Lock .