configparser fails to reject comment-prefixed multiline values that cannot round-trip

[augusto-rehfeldt]

Bug report

Bug description:

Documented behaviour: The public documentation for InvalidWriteError says: "Raised when attempting to write data that the parser would read back differently."

Expected: RawConfigParser.write() raises InvalidWriteError for a value whose continuation line would be discarded as a comment.

Actual: write() succeeds, emitting '[s]\nkey = a\n\t#b\n\n'; an identically configured parser reads the value as 'a'.

import configparser as c
import io

data = {"section": "s", "option": "key", "value": "a\n#b"}
p = c.RawConfigParser()
try:
    # Documented input types: string section, option and value.
    p.add_section(data["section"])
    p.set(data["section"], data["option"], data["value"])
except (TypeError, ValueError, c.Error) as e:
    print("REFUTATION REJECTED:", "invalid input:", repr(e))
else:
    out = io.StringIO()
    try:
        p.write(out)
    except Exception as e:
        print("REFUTATION REJECTED:", "write raised", type(e).__name__)
    else:
        text = out.getvalue()
        q = c.RawConfigParser()
        q.read_string(text)
        restored = q.get(data["section"], data["option"])
        actual = ("write succeeded", text, restored)
        # Independently test the documented round-trip promise.
        if restored != data["value"]:
            print("REFUTATION CONFIRMED:", data, "actual:", actual,
                  "expected:", ("exception", "InvalidWriteError"))
        else:
            print("REFUTATION REJECTED:", "round trip preserved the value")

Output on Python 3.14.6 (Windows-11-10.0.26220-SP0), standard library configparser:

REFUTATION CONFIRMED: {'section': 's', 'option': 'key', 'value': 'a\n#b'} actual: ('write succeeded', '[s]\nkey = a\n\t#b\n\n', 'a') expected: ('exception', 'InvalidWriteError')

This report was found and written by an automated property-testing tool I run (bugforge). The reproducer above was executed and its output is pasted unedited; no person reviewed the report before it was filed. The search script is in https://github.com/augusto-rehfeldt/bugforge-results/tree/main/configparser-20261003-195537-c3

CPython versions tested on:

3.14

Operating systems tested on:

Windows

[ZeroIntensity]

See #158748 (comment).