Added parsing of ignore annotation for import statements.

[spkersten]

This implements @JukkaL's suggestion of adding # mypy: ignore annotations in #500.

In this PR, only ignore annotations on import statements are allowed, but I think it would be easily extendable to other statements as well. Every statement Node could have an annotations field that could be used during semantic analysis or type checking.

[JukkaL]

Should this refer to AnnotationParseError?

[JukkaL]

Cool, thanks for implementing this! A few notes:

• I recommend adding a few test cases for invalid annotations and no-op annotations.
• Test what happens if we refer to an ignored imported module in code. The module object should probably have type Any if the import was ignored. What do you think?

I'm going to consider this as an experimental feature, since this feature hasn't received much discussion yet. However, a feature like this is probably going to be almost essential.

[JukkaL]

Missing empty line

[spkersten]

Good to see that I'm not the only one who cares about theses things 😄

[spkersten]

I'd forgotten about from x import y. Makes perfect sense to me to give y (and a in import a) the Any type in these cases. I'll add it.

[spkersten]

Actually, it doesn't seem necessary to define those imported symbols with type Any (or it already happens). I did some quick checks and after from x import y # mypy: ignore, y can be used as if it has Any type (as far as I can see).

There still an open issue. Annotations after all other statements are ignored. That doesn't seem like a big deal to me since, but it might be confusing. Since this is indeed an experimental feature, I propose to leave that for what it is.

[JukkaL]

Yeah, we should definitely complain about annotations in unsupported contexts. We can address that later.

I'm going to play around with this a little and think this through before merging, but this is probably okay as long we clearly document that this is an experimental feature that hasn't been fully implemented and that may be changed in the future.

[JukkaL]

Let's wait and see if PEP 484 will pick a notation for this: python/typing#16

[JukkaL]

I did a more general implementation of this functionality (the magic comment is # type: ignore, from PEP 484). Now you can ignore errors on arbitrary lines. Thanks for the inspiration!