$ python
Python 3.8.2 | packaged by conda-forge | (default, Apr 24 2020, 07:56:27)
[Clang 9.0.1 ] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from multiprocessing import Pool
>>>
>>> def f(x):
...     return x*x
...
 
>>> with Pool(5) as p:
...     print(p.map(f, [1, 2, 3]))
Process SpawnPoolWorker-1:
Process SpawnPoolWorker-2:
Process SpawnPoolWorker-3:
Traceback (most recent call last):
  File "/Users/bussonniermatthias/miniconda3/lib/python3.8/multiprocessing/process.py", line 315, in _bootstrap
    self.run()
  File "/Users/bussonniermatthias/miniconda3/lib/python3.8/multiprocessing/process.py", line 108, in run
    self._target(*self._args, **self._kwargs)
  File "/Users/bussonniermatthias/miniconda3/lib/python3.8/multiprocessing/pool.py", line 114, in worker
    task = get()
  File "/Users/bussonniermatthias/miniconda3/lib/python3.8/multiprocessing/queues.py", line 358, in get
    return _ForkingPickler.loads(res)
AttributeError: Can't get attribute 'f' on <module '__main__' (built-in)>
Traceback (most recent call last):
...

This is likely due to https://bugs.python.org/issue33725 (use spawn on MacOS), we we can't use `fork()`.

To be unpickle-able, the code for f needs to be imported, which it can't be from the repl. Windows has this same issue, due to also not using fork().

From https://docs.python.org/3/library/pickle.html#what-can-be-pickled-and-unpickled: "Thus the defining module must be importable in the unpickling environment, and the module must contain the named object, otherwise an exception will be raised."

I'll wait a while to see who else chimes in, but I think this should be closed as "not a bug".

Thanks.

That's an annoying side effect of having spawn by default on macOS then. 

Could it be possible in `pool.map()` to detect that some of the objects are from main and would fail and then raise an error message in the parent process before starting the children ?

There is a documentation ticket already open : https://bugs.python.org/issue33553

Some related note in documentation about repl with commit : https://github.com/python/cpython/commit/73dd030c8b71a7080648554652912982054b1177

> Note Functionality within this package requires that the __main__ module be importable by the children. This is covered in Programming guidelines however it is worth pointing out here. This means that some examples, such as the multiprocessing.pool.Pool examples will not work in the interactive interpreter.