Add kind field to ast.Constant, to distinguish u"..." from "..." for type checkers

[gvanrossum]

BPO	36280
Nosy	@gvanrossum, @serhiy-storchaka, @miss-islington
PRs	bpo-36280: Add Constant.kind field #12295

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/gvanrossum'
closed_at = <Date 2019-03-13.20:01:54.579>
created_at = <Date 2019-03-13.04:00:08.733>
labels = ['type-bug', '3.8']
title = 'Add kind field to ast.Constant, to distinguish u"..." from "..." for type checkers'
updated_at = <Date 2019-03-13.20:01:54.578>
user = 'https://github.com/gvanrossum'

bugs.python.org fields:

activity = <Date 2019-03-13.20:01:54.578>
actor = 'gvanrossum'
assignee = 'gvanrossum'
closed = True
closed_date = <Date 2019-03-13.20:01:54.579>
closer = 'gvanrossum'
components = []
creation = <Date 2019-03-13.04:00:08.733>
creator = 'gvanrossum'
dependencies = []
files = []
hgrepos = []
issue_num = 36280
keywords = ['patch']
message_count = 5.0
messages = ['337834', '337853', '337854', '337872', '337873']
nosy_count = 3.0
nosy_names = ['gvanrossum', 'serhiy.storchaka', 'miss-islington']
pr_nums = ['12295']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue36280'
versions = ['Python 3.8']

[serhiy-storchaka]

Maybe use a special subclass of Constant for indicating string literals with the "u" prefix?

[gvanrossum]

Maybe use a special subclass of Constant for indicating string literals with the "u" prefix?

That would indeed be more convenient, as it requires fewer code and test changes. Are there examples of such classes in Python.asdl?

[gvanrossum]

On second thought, even though it's a subclass, it feels less consistent than an extra attribute. So I'd rather keep the current approach (kind -> 'u' or None).

[miss-islington]

New changeset 10f8ce6 by Miss Islington (bot) (Guido van Rossum) in branch 'master':
bpo-36280: Add Constant.kind field (GH-12295)
10f8ce6

[gvanrossum]

Thanks for the review Serhiy!

[vasily-v-ryabov]

Please correct me if I'm wrong. This Constant.kind field is not used in mypy any more. Previously it was used in fastparse.py (intro by this commit: python/mypy@69a5a02), but now I don't see it in mypy/fastparse.py or anywhere else. Yes, this field is present in ast module interface, and I didn't try to search if it is used by some PyPI package. Maybe you know where it is used. Thanks!

[gvanrossum]

@vasily-v-ryabov What is your question, or what action are you requesting we take?

[vasily-v-ryabov]

@gvanrossum Initially I wanted to remove this field as it looked outdated. I've even prepared draft commit vasily-v-ryabov@68906e0 but then I've found this thread and related commits, and also I understood it may potentially break backward compatibility. If there is no such risk, I'd suggest to create a PR or prepare some deprecation. Though this particular commit is not critical to me if this stuff is really used.

The reason to me is making AST more compact which should be generally good for performance. I'm interested in improvements for AST and compiler related stuff in CPython for my project (hope it will be open in the future).