Ndtyjky

A surprising thing which I came across while writing a logic of saving some value during process termination was a bit strange for me. Writing a toy example program to show the problem.

import multiprocessing

import time



class A(multiprocessing.Process):

    def __init__(self):

        self.till = 0

        super(A, self).__init__()



    def run(self):

        i = 0

        while True:

            print(i)

            i += 1

            self.till = i

            time.sleep(1)



    def terminate(self):

        print("Terminating : {}".format(self.till))

        super(A, self).terminate()



if __name__ == "__main__":

    obj = A()

    obj.start()

    time.sleep(5)

    obj.terminate()

The output for the above program is -

0

1

2

3

4

Terminating : 0

Why is terminate() not printing out 4? Anything I am missing?

What you are doing is actually running terminate on the main process, look at this code:

class A(multiprocessing.Process):

def __init__(self):

    self.till = 0

    super(A, self).__init__()



def run(self):

    i = 0

    print(os.getpid())

    while True:

        print(i)

        i += 1

        self.till = i

        time.sleep(1)



def terminate(self):

    print("Terminating : {}".format(self.till))

    print(os.getpid())

    super(A, self).terminate()



if __name__ == "__main__":

    print("parent pid:")

    print(os.getpid())

    print("child pid:")

    obj = A()

    obj.start()

    time.sleep(3)

    obj.terminate()

Will lead to the output:

parent pid:

12111

child pid:

12222

0

1

2

Terminating : 0

12111

At terminate, you are actually sending SIGTERM to the child process, it is done from the parent process, thus the memory is of the parent, where there was no increments to self.till

init and terminate methods run on the main process hence the sub-process prints 0 for your terminate function. Run method only increments in the sub process. You can confirm this by using os.getpid() method in python.

Edit: This problem probably only occurs in Windows since it does not have a fork() system call like in Linux/Unix systems. Windows starts the whole module from the beginning to achieve the effect.