configparser: whitespace-ending delimiter with an empty value cannot be parsed (write() output is unreadable)

Bug report

The whitespace-in-delimiters feature added in gh-156353 (3.16) cannot parse an option that has an empty value when the delimiter ends in whitespace — including its own write() output.

_read matches option lines against line.clean, which is val.strip() (both sides) with comments removed. For a whitespace delimiter, the delimiter is the trailing whitespace, so strip() erases it and OPTCRE no longer matches.

Reproduction (on main, 3.16.0a0)

import configparser, io

# (1) plain read — rejected, though the option regex itself matches "key ":
cp = configparser.ConfigParser(delimiters=(' ',))
cp.read_string("[s]\nkey \n")          # -> ParsingError   (expected: {'key': ''})

# (2) round-trip — write() emits a line read_string() then refuses to parse:
w = configparser.ConfigParser(delimiters=(' ',)); w['s'] = {'key': ''}
buf = io.StringIO(); w.write(buf)
print(repr(buf.getvalue()))            # '[s]\nkey   \n\n'
configparser.ConfigParser(delimiters=(' ',)).read_string(buf.getvalue())  # ParsingError

A serializer that cannot read its own output is a clear self-inconsistency. Empty values are a supported, first-class feature for every other delimiter (key=, key:, key->, key|| all read back as {'key': ''}), so whitespace delimiters should behave the same.

Bug class

Any delimiter whose string ends in whitespace combined with an empty (or all-whitespace) value: ' ', '\t', ' ', 'x ', '= ' all raise; delimiters without trailing whitespace ('->', ' x') work.

Why it's not caught

The gh-156353 tests (test_space_delimiter, test_any_delimiter) only use non-empty values, so this edge is unpinned.

Fix

Match the option regex against a form that preserves trailing whitespace (the delimiter). optval is already stripped after the match, so ordinary values keep no trailing whitespace and the other uses of line.clean are unaffected. I have a small patch + tests and will open a PR referencing this issue.

Found with AI assistance; I've reproduced and verified the behaviour and fix on a locally built interpreter and understand them.

Linked PRs

• gh-157457

Start with ConfigParser._read and the existing test_space_delimiter and test_any_delimiter coverage described in the issue. Reproduce the empty-value cases with read_string() and write(), then extend the tests so whitespace-ending delimiters and empty values parse consistently and the round trip succeeds.