CODEWITHSUNDEEP
pythonmultiprocessing
ThunderJack
ThunderJack

Reputation: 83

Does Python Multiprocessing Copy All Modules From The Parent?

I noticed that I can access functions and modules in a child process that were outside of the child process function/target. So I'm wondering when I create a child process in python does it copy everything from current process? Why is it I can access functions and imported modules that are outside the child target?

from multiprocessing import Process, Pipe

def test1():
         return "hello"

def simpleChildProcess( childPipe ):
       # simpleChildProcess can access test1 function
       foo = test1()
       childPipe.send( foo )

parentPipe, childPipe = Pipe()
childProcess = Process( target=simpleChildProcess, args=(childPipe,) )

childProcess.start()

print "Pipe Contains: %s" % parentPipe.recv()

Upvotes: 7

Views: 1824

Answers (1)

unutbu
unutbu

Reputation: 879909

On Unix-like OSes, multiprocessing.Process uses os.fork to spawn new processes. fork creates a new process which is a copy of the parent process, and the forked process resumes from the point where fork was called.

Since Windows lacks fork, multiprocessing.Process starts a new Python process and imports the calling module. On Windows, calls to Process have to be inside if __name__ == '__main__' to prevent Process from being called repeatedly every time the calling module is imported. (It's a good practice even on Unix to include if __name__ == '__main__': to prevent your code from causing runaway process-spawning).

Thus, the child process has access to functions and modules that have been defined by the calling module up to the point where Process had been called (in the case of Unix) or after importation of the calling module (in the case of Windows).

Upvotes: 9

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