Multiprocessing output differs between Linux and Windows - Why?

Question

I am attempting to pass a shared secret to child processes. In a Linux environment this works. In a Windows environment the child does not receive the shared secret. The three files below are a simple example of what I'm trying to do:

main.py

import multiprocessing
import module1
import module2

if __name__ == "__main__":
    module1.init()
    process = multiprocessing.Process(target=module2.start)
    process.start()
    process.join()

module1.py

import ctypes
import multiprocessing

x = None

def init():
    global x
    x = multiprocessing.Value(ctypes.c_wchar_p, "asdf")

module2.py

import module1

def start():
    print(module1.x.value)

---

In an Ubuntu 14.04 environment, on Python 3.5, I receive the following output:

$ python3 main.py
asdf

In a CentOS 7 environment, I receiving the following output:

$ python3 main.py
asdf

Using the Windows Subsystem for Linux on Windows 10 (both before and after the Creator Update, so Ubuntu 14.04 and 16.04) I get the following output:

$ python3 main.py
asdf

However, in both Windows 7 and Windows 10 environments, using either 3.5 or 3.6, I am getting an AttributeError instead of the above output:

Process Process-1:
Traceback (most recent call last):
  File "C:\Python\Python35\lib\multiprocessing\process.py", line 249, in _bootstrap
    self.run()
  File "C:\Python\Python35\lib\multiprocessing\process.py", line 93, in run
    self._target(*self._args, **self._kwargs)
  File "H:\Development\replicate-bug\module2.py", line 5, in start
    print(module1.x.value)
AttributeError: 'NoneType' object has no attribute 'value'

---

I am using a shared ctype. This value should be inherited by the child process.

Why do I receive this AttributeError in a Windows environment, but not a Linux environment?

Answer 1

As mentioned in one of the posts automatically linked on the sidebar, windows does not have the fork systemcall present on *NIX systems.

This implies that instead of sharing global state (like NIX Processes can do), a Windows child process is basically completely separate. This includes modules.

What I suspect is happening is that the module gets loaded anew and the module1 you access inside module2.start isn't quite the module you expected.

The multiprocessing guidelines explicitly mention that module-level constants are exempt from the rule: "variables may not contain what you expect". Well in either case, the solution is to explicitly pass the module you want to the child process like so:

module 2

def start(mod1):
    print(mod1.x.value)

main.py

if __name__ == '__main__':
    module1.init()
    process = multiprocessing.Process(target=module2.start, args=(module1,))
    process.start()
    process.join()