Implement PEP 526 Variable Annotations Syntax

[ilevkivskyi]

@ddfisher @gvanrossum @JukkaL

Here is an implementation of the PEP 526 syntax with minimal changes in typed_ast API. The only externally visible change is:

Assign(expr* targets, expr? value, expr? type_comment)

This requires a tiny change to mypy/fastparse.py, there is an accompanying PR to mypy with this change.

[gvanrossum]

Since this is just a backport of the corresponding Python 3.6 code I'm going to merge this mostly based on trust. I'll iterate once I've got most things working again.

[gvanrossum]

Never mind I don't have commit access to this repo, and David's out this week. :-(

[ilevkivskyi]

@gvanrossum
OK, no problem.
Anyway, I changed behaviour a bit in response to your latest e-mail: x: int is now equivalent to x = None # type: int and both are always OK, while x: int = None is flagged as error if --strict-optional.

Did I understand you correct?

I am now testing this and will update PRs (here and at mypy) soon.

[gvanrossum]

Those behaviors sound okay. I am now testing too. Two administrative nits I found:

• This PR should bump the version in setup.py
• Should we rename ast35 to ast36???

[ilevkivskyi]

@gvanrossum

This PR should bump the version in setup.py

OK, will do this

Should we rename ast35 to ast36???

There is also a third option to keep both (see my comment at mypy PR)

[ddfisher]

This isn't compatible with the current mypy, so I'll need to wait until python/mypy#2131 is pushed to PyPI before releasing this.

[ilevkivskyi]

@ddfisher I thought that you are supposed to review and merge python/mypy#2131 That one should work with both old and new versions of typed_ast.

[ddfisher]

Yep! That's the plan. I just wanted to make a note here about the need to wait for a mypy release before releasing this.

[ddfisher]

Actually, I chatted with Guido a bit about this, and we'd like to remove that dependency problem. Here's how that could be done: instead of putting the assignment type annotation in the same field as the type comment, leave the type comment field as is and add a new one for the type annotation. This is similar to how function annotations/type comments are handled. Also, I think this would get rid of the need for the new_syntax field, which would be a nice simplification. Would you mind making these changes?

[ilevkivskyi]

OK, I will do this. I will push commits here and to mypy PR together with implementing your comments (not sure that I will have time to do all this today).

[ddfisher]

Thanks!

[ilevkivskyi]

@ddfisher I updated all the three PRs (here, mypy, and typeshed) taking into account your comments.
Now all four combinations of mypy and typed_ast (old-old, old-new, new-old, new-new) should work.
Please double check.

[ddfisher]

Had to make some minor tweaks, but this looks good now. Thanks!