gh-157456: Fix configparser parsing an empty value with a whitespace-ending delimiter

[winklemad]

The whitespace-in-delimiters feature (gh-156353) can't parse an option with an empty value when the delimiter ends in whitespace — including its own write() output.

_read matches option lines against line.clean, which is str.strip-ed on both sides (plus comment removal). For a whitespace delimiter the delimiter is the trailing whitespace separating the key from the (empty) value, so strip() removes it and OPTCRE no longer matches.

>>> import configparser, io
>>> cp = configparser.ConfigParser(delimiters=(' ',))
>>> cp.read_string("[s]\nkey \n")          # ParsingError  (before); {'s': {'key': ''}}  (after)
>>> w = configparser.ConfigParser(delimiters=(' ',)); w['s'] = {'key': ''}
>>> buf = io.StringIO(); w.write(buf); buf.getvalue()
'[s]\nkey   \n\n'
>>> configparser.ConfigParser(delimiters=(' ',)).read_string(buf.getvalue())   # ParsingError before; now OK

Empty values are first-class for every other delimiter (key=, key:, key->, key|| → {'key': ''}), so whitespace delimiters should match.

Fix: match the option regex against a form that preserves trailing whitespace (the delimiter). The matched value is .strip()-ed immediately afterwards, so ordinary values keep no trailing whitespace, and the other uses of line.clean (blank-line detection, continuation values, section headers) are unchanged. With allow_no_value=True a whitespace delimiter can't distinguish an empty value from a valueless option, so it reads as None there (unchanged, and self-consistent on round-trip); the regression test covers both modes.

• Fixes configparser: whitespace-ending delimiter with an empty value cannot be parsed (write() output is unreadable) #157456
• ./python -m test test_configparser → SUCCESS (396 tests); the new test_whitespace_delimiter_empty_value fails on main (12 errors across parser variants) and passes with the change.
• Added a Misc/NEWS.d entry.

AI-tools disclosure (per the devguide policy): I used an AI assistant to help locate the bug and draft the patch and test. I reproduced and verified the behaviour and the fix against a locally built interpreter, understand the change, and stand by it.

• Issue: configparser: whitespace-ending delimiter with an empty value cannot be parsed (write() output is unreadable) #157456