deadlock.py

# based on the example in https://docs.python.org/2.7/library/multiprocessing.html
from multiprocessing import Process, Queue, Manager

def f(q):
    q.put('X' * 1000000)

if __name__ == '__main__':
    manager = Manager()
    queue = manager.Queue()
    p = Process(target=f, args=(queue,))
    p.start()
    print("we can join process...")
    p.join()                    # manager avoids deadlock
    print("we can join process...done")
    obj = queue.get()
    print(obj == 'X' * 1000000)
    
    queue = Queue()
    p = Process(target=f, args=(queue,))
    p.start()
    print("we cannot join process...")
    p.join()                    # this deadlocks
    print("we cannot join process...done")
    obj = queue.get()
    print(obj == 'X' * 1000000)