Python, Windows, and Multiprocessing

You can work around this by using an absolute import for OtherCustomClass:

from base_module import OtherCustomClass

I'm not exactly sure why, but it seems that when multiprocessing spawns a new process and imports your __main__, it's not able to handle the implicit relative import you're using with OtherCustomClass. If you explicitly import it from base_module, it works fine. My guess is that the spawned child process is not recognized as being part of the base_module package, so the implicit import fails, but that's just a guess.

Note that you shouldn't be using implicit relative imports anyway (they're altogether removed from Python 3), so switching to an absolute import isn't a bad thing.

Also of note, that doing an explicit relative import works on Python 3.4:

from . import OtherCustomClass

But it fails on Python 2.7:

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "C:\python27\lib\multiprocessing\forking.py", line 380, in main
    prepare(preparation_data)
  File "C:\python27\lib\multiprocessing\forking.py", line 495, in prepare
    '__parents_main__', file, path_name, etc
  File "C:\Users\oreild1\Desktop\base_module\Launcher.py", line 5, in <module>
    from . import OtherCustomClass
ValueError: Attempted relative import in non-package
error:
Traceback (most recent call last):
  File "C:\Users\oreild1\Desktop\base_module\Launcher.py", line 18, in main
    basmod = BaseModule(argv[0], argv[1])
  File "C:\Users\oreild1\Desktop\base_module\Launcher.py", line 10, in __init__
    self.manager.start()
  File "C:\python27\lib\multiprocessing\managers.py", line 528, in start
    self._address = reader.recv()
EOFError