CODEWITHSUNDEEP
pythonpython-3.xmultithreadingmultiprocessing
Marsilinou Zaky
Marsilinou Zaky

Reputation: 1047

Implementing code to start multiple threads on top of multiple process in python

Extending on a question I asked before about multithreads on top of multiprocesses in python

Can I do multithreads on each process of a multiprocess program?

So I'm trying to implement an example that achieves that. First I spawn 2 processes and each one will create 10 threads within it, but something doesn't look right here. I'm not implementing any kind of locks or semaphores so I expect the output to be scrambled (as in example 3). When I run this code example 1 and 2 are printed in the correct format. And for example 2 I even tried creating 2 threads to start each process to make sure they're not starting sequentially. ! Why is that happening? What am I missing and where did the coordination happen? 

import multiprocessing, threading, time

def startThreads(n):
    threads = [threading.Thread(target=printer, args=(n, i))  for i in range(10)]
    [t.start() for t in threads]
    [t.join() for t in threads]

def printer(process_num, thread_num):
    time.sleep(1)
    print(f"Process number: {process_num} thread number: {thread_num}")
    print(f"Process number: P thread number: T")

if __name__ == '__main__':

    # Example 1
    pros = [multiprocessing.Process(target=startThreads, args=(p_num, )) for p_num in range(5)]
    [p.start() for p in pros]
    [p.join() for p in pros]
    # Process number: 0 thread number: 0
    # Process number: P thread number: T
    # Process number: 0 thread number: 4
    # Process number: P thread number: T
    # Process number: 0 thread number: 1
    # Process number: P thread number: T
    # Process number: 0 thread number: 2
    # Process number: P thread number: T
    # Process number: 0 thread number: 3
    # Process number: P thread number: T
    # Process number: 1 thread number: 0
    # ...

    # Example 2
    print()
    startThreads(0)
    # Process number: 0 thread number: 1
    # Process number: P thread number: TProcess number: 0 thread number: 0
    # Process number: P thread number: T

    # Process number: 0 thread number: 2Process number: 0 thread number: 4Process number: 0 thread number: 3
    # Process number: P thread number: T

    # Process number: P thread number: T

    # Process number: P thread number: T

Notice how in example two how print behaved, on the other hand example one always prints in the correct format (safe print) while in both cases the print function is called by a thread, the same thing happens when I remove the print formatting and instead use a fixed string to be printed.

As the discussion in this question says that we need to implement some sort of safe printing method to get every print statement in a new line, but that's not the case with example 1

import multiprocessing, threading, time

def startThreads():
    threads = [threading.Thread(target=printer)  for i in range(5)]
    [t.start() for t in threads]
    [t.join() for t in threads]

def printer():
    time.sleep(0.05)
    print(f"Process number: P thread number: T")

if __name__ == '__main__':
    p = multiprocessing.Process(target=startThreads)
    p.start()
    p.join()
    print("=====================================")
    startThreads()

    # Process number: P thread number: T
    # Process number: P thread number: T
    # Process number: P thread number: T
    # Process number: P thread number: T
    # Process number: P thread number: T
    # =====================================
    # Process number: P thread number: TProcess number: P thread number: T
    # Process number: P thread number: T

    # Process number: P thread number: TProcess number: P thread number: T
    #

I tried to use only one process, but it still safe printing where every line is printing in a new line, but when I call startThreads explicitly, it doesn't behave the same and doesn't safe print, is it behaving this way?!

Upvotes: 6

Views: 405

Answers (1)

RafalS
RafalS

Reputation: 6324

And with the same code I get scrambled output:

1:20:21:3, , 0:00:1, 0:71:5, , 1:40:91:1, 1:6, , , 1:0, , 0:5, 0:4, 0:3, 0:6, 1:7, 0:8, 1:9, , 1:8, 
0:0, 0:10:2, , 0:3, 0:4, 0:50:6, 1:0, , 1:1, 0:7, 1:2, 1:3, 0:9, 0:81:5, 1:4, , 1:71:6, , 1:91:8, , 
0:0, 0:1, 0:40:3, 0:2, , 0:60:5, , 0:70:8, , 0:9,

Try running it multiple times. If 1 and 2 are always scrambled - maybe it's platform dependent.

so I expect the output to be scrambled

It's not synchronized in any way. The ordering is just random :)

Upvotes: 1

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