AttributeError: Can't get attribute 'initializer' on <module 'loky.backend.popen_loky_posix' ...

[vvvhung]

I run example at https://loky.readthedocs.io/en/stable/auto_examples/reuseable_executor.html#sphx-glr-download-auto-examples-reuseable-executor-py but error occur

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LokyProcess-2 failed with traceback: 
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Traceback (most recent call last):
  File "/Users/louis/Documents/venv_flax_py3/lib/python3.6/site-packages/loky/backend/popen_loky_posix.py", line 198, in <module>
    process_obj = pickle.load(from_parent)
AttributeError: Can't get attribute 'initializer' on <module 'loky.backend.popen_loky_posix' from '/Users/louis/Documents/venv_flax_py3/lib/python3.6/site-packages/loky/backend/popen_loky_posix.py'>


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LokyProcess-1 failed with traceback: 
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Traceback (most recent call last):
  File "/Users/louis/Documents/venv_flax_py3/lib/python3.6/site-packages/loky/backend/popen_loky_posix.py", line 198, in <module>
    process_obj = pickle.load(from_parent)
AttributeError: Can't get attribute 'initializer' on <module 'loky.backend.popen_loky_posix' from '/Users/louis/Documents/venv_flax_py3/lib/python3.6/site-packages/loky/backend/popen_loky_posix.py'>


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Traceback (most recent call last):
  File "/Users/louis/Documents/test/scripts/multi_loky.py", line 31, in <module>
    assert executor.submit(test_initializer).result() == 'initialized'
  File "/Users/louis/Documents/venv_flax_py3/lib/python3.6/site-packages/loky/_base.py", line 431, in result
    return self.__get_result()
  File "/Users/louis/Documents/venv_flax_py3/lib/python3.6/site-packages/loky/_base.py", line 382, in __get_result
    raise self._exception
loky.process_executor.BrokenProcessPool: A process in the executor was terminated abruptly while the future was running or pending.

[tomMoral]

Thanks for reporting this.
I can indeed reproduce. Here, the initializer is not picklable as the main module is not initialized. It would need to be serialized with cloudpickle.

[vvvhung]

It will be great if we have a trick to overcome it right now. Thank you.

[tomMoral]

A quick fix would be to use the following script, and launch it with the environment variable LOKY_PICKLER='pickle'.

from time import sleep
from loky import get_reusable_executor


INITIALIZER_STATUS = 'uninitialized'


def initializer(x):
    global INITIALIZER_STATUS
    INITIALIZER_STATUS = x


def return_initializer_status(delay=0):
    sleep(delay)

    global INITIALIZER_STATUS
    return INITIALIZER_STATUS


if __name__ == '__main__':
    executor = get_reusable_executor(
        max_workers=2, initializer=initializer, initargs=('initialized',),
        context="loky_init_main")

    assert executor.submit(return_initializer_status).result() == 'initialized'

    # With reuse=True, the executor use the same initializer
    executor = get_reusable_executor(max_workers=4, reuse=True)
    for x in executor.map(return_initializer_status, [.5] * 4):
        assert x == 'initialized'

    # With reuse='auto', the initializer is not used anymore as a new executor
    # is created.
    executor = get_reusable_executor(max_workers=4, context='loky_init_main')
    for x in executor.map(return_initializer_status, [.1] * 4):
        assert x == 'uninitialized'

After investigating, the issue here is that we do not handle consistently the pickling of the initializer in loky. The fix is non-trivial as it is linked to cloudpipe/cloudpickle#187.