Here is a simple python program that uses the new forkserver feature introduced in 3.4b1:


name: checkforkserver.py
"""
import multiprocessing
import os


def do(i):
    print(i, os.getpid())


def test_forkserver():
    mp = multiprocessing.get_context('forkserver')
    mp.Pool(2).map(do, range(3))


if __name__ == "__main__":
    test_forkserver()
"""

When running this using the "python check_forkserver.py" command everything works as expected.

When running this using the nosetests launcher ("nosetests -s check_forkserver.py"), I get:

"""
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/opt/Python-HEAD/lib/python3.4/multiprocessing/forkserver.py", line 141, in main
    spawn.import_main_path(main_path)
  File "/opt/Python-HEAD/lib/python3.4/multiprocessing/spawn.py", line 252, in import_main_path
    methods.init_module_attrs(main_module)
  File "<frozen importlib._bootstrap>", line 1051, in init_module_attrs
AttributeError: 'NoneType' object has no attribute 'loader'
"""

Indeed, the spec variable in multiprocessing/spawn.py's import_main_path
function is None as the nosetests script is not a regular python module: in particular is does not have a ".py" extension.

If I copy or symlink or renamed the "nosetests" script as "nosetests.py" in the same folder, this works as expected. I am not familiar enough with the importlib machinery to suggest a fix for this bug.

Also there is a typo in the comment: "causing a psuedo fork bomb" => "causing a pseudo fork bomb".

Note: I am running CPython head updated today.