Inconsistent parsing of f-strings between typed_ast and the builtin ast

[PCManticore]

Hey folks!

I've noticed that typed_ast doesn't parse f-strings in the same way as the builtin ast module does. For instance in the following example, we only get a FormattedValue while the builtin ast module returns a JoinedStr:

import typed_ast.ast3 as typed_ast
import ast

f = typed_ast.parse("f'{a}'")
print(f.body[0].value)

f = ast.parse("f'{a}'")
print(f.body[0].value)

# going to print the following
<_ast3.FormattedValue object at 0x1040a09e8>
<_ast.JoinedStr object at 0x1040a0b00>

It would be great to have the same output as the builtin ast module. They both return a JoinedStr if the variable interpolation element is followed by one or more characters.

[emmatyping]

Hi, when you say "the builtin ast module" what version of Python are you talking about? It maybe be that this is due to #60 not being complete. typed_ast is a fork of the 3.6.0 builtin ast module.

[PCManticore]

@ethanhs Yes, good catch, I was referring to Python 3.6. I didn't check if this has been fixed in 3.7 though.

[msullivan]

I took a look at this, and the behavior of ast changed between 3.6.0 and 3.6.1 as a result of https://bugs.python.org/issue28739.

Since typed_ast was a fork of 3.6.0, we matched the 3.6.0 behavior which was inconsistent with later python versions. We have updated typed_ast to be based on 3.7.2, and the inconsistency is fixed.

(Though we don't have versions of typed_ast that are "bug-for-bug compatible" with 3.6.1+, which is a lacuna, if one that doesn't impact mypy's uses at least. If there's demand for such a thing, I'd be happy to review a PR that ported the patch in, though there would be serious questions about version numbers.)