Infer magic methods

[orsinium]

Thank you for the project. I tried it on the project, and it works. At least, doesn't break anything :)

The return type for some magic methods can be also auto-annotated. A few examples:

1. __str__ and __repr__ return str
2. __init__ returns None
3. iter returns typing.Iterable

[JelleZijlstra]

Thanks, that's great to hear! I'm going to do some more work on in this in the next couple of weeks. I agree with __str__ and __repr__, but I don't think we can do __iter__, because you really want Iterable[SomeType] and autotyper is not nearly smart enough to know what that type should be.

Other methods we could infer:

• __exit__ arguments should be Optional[Type[BaseException]], Optional[BaseException], Optional[TracebackType]
• __len__ should return int
• __del__ should return None
• __bool__ should return bool
• __bytes__ should return bytes
• __format__ should return str
  Other methods in https://docs.python.org/3/reference/datamodel.html don't look safe to infer.

[orsinium]

but I don't think we can do iter, because you really want Iterable[SomeType]

This is an interesting topic to discuss, BTW. I think autotyper can partially infer some types because it is better than nothing. In this case, typing.Iterator (or more explicitly, typing.Iterator[typing.Any]) will already allow mypy to find some bugs. The same applies to generic types like typing.List (or just list).

[JelleZijlstra]

That's a fair point. It will also introduce new mypy errors if you have --disallow-any-generics on, but on a codebase that you use autotyper on you might not want to worry about strictness options yet. :)

[JelleZijlstra]

Just merged support for most of this, except for __exit__ which I split out into #8.

Let me know if you have more ideas for improvements!

[orsinium]

Great! Thank you. I'll give it a try :)

Beautiful project, BTW, I always wanted something like this. I've added it into awesome-python-typing.