bpo-36280: Add Constant.kind field

[gvanrossum]

The value is a string for string and byte literals, None otherwise.
It is 'u' for u"..." literals, 'b' for b"..." literals, '' for "..." literals.
The 'r' (raw) prefix is ignored.
Does not apply to f-strings.

This appears sufficient to make mypy capable of using the stdlib ast module instead of typed_ast (assuming a mypy patch I'm working on).

WIP: I need to make the tests pass. @ilevkivskyi @serhiy-storchaka

https://bugs.python.org/issue36280

[serhiy-storchaka]

Do you need indications for triple-quote strings?

Why do you need 'b' for bytes if you can use isinstance(node.value, bytes)?

What kind should be for concatenated '' and u'' literals? E.g.: 'abc' u'def'.

[gvanrossum]

Do you need indications for triple-quote strings?

Not for this use case.

Why do you need 'b' for bytes if you can use isinstance(node.value, bytes)?

Good point. (I believe I was just copying that behavior from the Python 2 part of typed_ast, where it was considered useful.)

What kind should be for concatenated '' and u'' literals? E.g.: 'abc' u'def'.

For my use case it doesn't really matter; I'd rather not waste time trying to do something smarter than looking for the first segment.

Here's what I propose to do. I'll set it to 'u' if the first segment starts with 'u' (since ru'...' isn't allowed anyway), else I'll set it to NULL/None.

Sadly none of that helps me fixing the unit tests -- I believe the problems are all caused by the changes to the repr() of ast.Constant.

[miss-islington]

@gvanrossum: Status check is done, and it's a success ✅ .

[miss-islington]

Sorry, I can't merge this PR. Reason: Pull Request is not mergeable.