Automatically add Optional type to kwargs with None default

[mpariente]

I was having a look a this PR and decided to give it a try.

# In parsing_workout.py
class Dummy:
    def __init__(self, dummy: float = None):
        pass

Then execute

python parsing_workout.py --print_config > conf.yml
python parsing_workout.py --config conf.yml

I get the following error :

parsing_workout.py: error: Parser key "data.dummy": float() argument must be a string or a number, not 'NoneType'

Notably, this doesn't crash with IPython.

I think it would make sense to automatically add Optional to the type in those cases, WDYT?

[mauvilsa]

Automatically adding Optional would be an easy change. I agree it is nice that a None default is implicitly an optional, since adding the optional to the type seems kind of redundant. Though it seems that this implicit optional is discouraged and in the near future by default will not be a valid type hint, see python/mypy#9091 and python/peps#689. So I have a bit of mixed feelings about this.

[mpariente]

Wasn't aware about this. I understand.
Maybe support an --implicit-optional flag as mypy is going to make?

[mauvilsa]

I think that maybe we just add the implicit optional in jsonargparse without any option. Anyway mypy will show people the error when this is no longer a valid type. The ithe implicit optional code in jsonargparse would not be used, but at least it doesn't hurt.

[mpariente]

Sounds good.

[mauvilsa]

This is now implemented. Please check.

[mauvilsa]

This is now in release v3.2.0.

[mpariente]

Thanks! I'll have a look soon