testing mutliprocessing pool with path hooks

README.md

sys.path_hooks are not respected by multiprocessing when start mode is 'spawn' (default on Windows). However this works on Linux:

To test:

git clone https://gist.github.com/ec53217c41f965ffd6080a2632e9f944.git
cd ec53217c41f965ffd6080a2632e9f944
python test_pathtomodule.py

test_pathtomodule.py

import sys
from multiprocessing import Pool
import pickle
import testmodule

class Loader:
    
    def load_module(self, full_path):
        if full_path == 'test':
            if 'test' not in sys.modules:
                sys.modules['test'] = testmodule
            return testmodule
        

class Finder:
    def __init__(self, path):
        if path != 'placeholderpath':
            raise ImportError()
    def find_module(self, fullname, path=None):
        if fullname != 'test':
            return
        return Loader()

sys.path_hooks.append(Finder)
sys.path.insert(0, 'placeholderpath')

from test import MyClass


if __name__ == '__main__':
    pool = Pool(3)
    cl = MyClass()
    cl.myfunc(3)

    
    m = pool.map(cl.myfunc, range(3))
    print(list(m))
    pool.terminate()

testmodule.py

class MyClass:
    def myfunc(self, i):
        return i + 4