Behavior and use cases of _import_module() is unclear

[leogama]

It seems that _import_module() works much more like Unpickler.find_class() than like importlib.import_module()

Unfortunately there are a lot of undocumented non-public functions in dill. However, you can see from the code that it does import the module (or submodule), so it's an appropriate name... it's just not the dill equivalent of importlib.import_module().

Yes, it seems that there are places where using importlib.import_module() is more appropriate than _import_module().

Take a look at this listing, it doesn't look right:

>>> import dill
>>> dill.__version__
'0.3.5.1'
>>> import importlib
>>> dill_settings = importlib.import_module('dill.settings')
>>> dill_settings
<module 'dill.settings' from '/home/leogama/.local/lib/python3.8/site-packages/dill/settings.py'>
>>> dill.copy(dill_settings)
{'protocol': 4, 'byref': False, 'fmode': 0, 'recurse': False, 'ignore': False}
>>> import pickletools
>>> pickletools.dis(pickletools.optimize(dill.dumps(dill_settings, protocol=0)))
    0: c    GLOBAL     'dill._dill _import_module'
   27: (    MARK
   28: V        UNICODE    'dill.settings'
   43: t        TUPLE      (MARK at 27)
   44: R    REDUCE
   45: .    STOP
highest protocol among opcodes = 0

I think we should compare the API and implementation of this function with the ones of pickle._Unpickler.find_class(), importlib.import_module() and even logging.config._resolve() that I recently found out.

Originally posted by @leogama in #529 (comment)