sysconfig.get_config_var("py_version_nodot") should return 3_10

[mattip]

BPO	40747
Nosy	@vstinner, @ambv, @markshannon, @mattip, @zooba, @The-Compiler, @pablogsal
PRs	bpo-40747: Make py_version_nodot 3_10 not 310 #20333 bpo-40747: Make py_version_nodot 3_10 not 310 (PEP 641) #22858

^{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 = None
closed_at = <Date 2020-12-11.00:42:46.047>
created_at = <Date 2020-05-23.20:47:46.058>
labels = ['type-bug', 'expert-installation', '3.10']
title = 'sysconfig.get_config_var("py_version_nodot") should return 3_10'
updated_at = <Date 2020-12-11.00:42:46.046>
user = 'https://github.com/mattip'

bugs.python.org fields:

activity = <Date 2020-12-11.00:42:46.046>
actor = 'steve.dower'
assignee = 'none'
closed = True
closed_date = <Date 2020-12-11.00:42:46.047>
closer = 'steve.dower'
components = ['Installation']
creation = <Date 2020-05-23.20:47:46.058>
creator = 'mattip'
dependencies = []
files = []
hgrepos = []
issue_num = 40747
keywords = ['patch']
message_count = 4.0
messages = ['369750', '380344', '382797', '382857']
nosy_count = 7.0
nosy_names = ['vstinner', 'lukasz.langa', 'Mark.Shannon', 'mattip', 'steve.dower', 'The Compiler', 'pablogsal']
pr_nums = ['20333', '22858']
priority = 'normal'
resolution = 'rejected'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue40747'
versions = ['Python 3.10']

[mattip]

Over in packaging, that code expects sysconfig.get_config_var("py_version_nodot") to be consistent with tags._version_nodot, which expects 3_10 for python 3.10. See pypa/packaging#308. The current value of sysconfig.get_config_var("py_version_nodot") (which comes from https://github.com/python/cpython/blob/master/Lib/sysconfig.py#L89) is 310 which is ambiguous.

[vstinner]

Links about the "AssertionError: would build wheel with unsupported tag ('cp310', 'cp310', 'linux_x86_64')" error:

• PEP-641 -- Using an underscore in the version portion of Python 3.10 compatibility tags
  https://www.python.org/dev/peps/pep-0641/
• PEP-641 discussion:
  https://discuss.python.org/t/pep-641-using-an-underscore-in-the-version-portion-of-python-3-10-compatibility-tags/5513
• CPython PR 20333: "bpo-40747: Make py_version_nodot 3_10 not 310"
  bpo-40747: Make py_version_nodot 3_10 not 310 #20333
• wheel: "Fails to build wheel for Python 3.10"
  Fails to build wheel for Python 3.10 pypa/wheel#354
• python3-setuptools: "python-setuptools fails to build with Python 3.10: AssertionError: would build wheel with unsupported tag ('cp310', 'cp310', 'linux_x86_64')"
  https://bugzilla.redhat.com/show_bug.cgi?id=1891840

[markshannon]

What's blocking this?
It is a real pain not to be able to install packages for 3.10.

I don't much care what the format is, but since there seems to be some difficulty in changing it, why not leave the output as 310?

It isn't ambiguous, as long as we specify how to parse the number:
2 digits: XY. X = major, Y = minor
3 digits: XYY. X = major, YY = minor
4 digits: XXYY. XX = major, YY = minor

So 31.0 will be 3100.

That should be good for a millenium or so :)

[zooba]

This has been blocking it: https://discuss.python.org/t/pep-641-using-an-underscore-in-the-version-portion-of-python-3-10-compatibility-tags/5513

Pablo (PEP delegate) has just posted his rejection though, so it will be fixed when packaging changes back to "310" from "3_10" and updates percolate out through the ecosystem.

Since moving to "4.x" is a breaking change, we can change to any system we like at that point. Well in advance of 31.0 being ambiguous.