plistlib.load rejects XML dictionary keys when dict_type is collections.UserDict

[augusto-rehfeldt]

Bug report

Bug description:

Documented behaviour: Python plistlib documentation, load(): "The dict_type is the type of the dictionaries that are returned (by default, dict)."

Expected: Return a collections.UserDict containing {'a': 'b'}.

Actual: Raises ValueError: unexpected key at line 5.

import plistlib
from io import BytesIO
from collections import UserDict

data = {'a': 'b'}
try:
    if not data or not all(isinstance(k, str) and isinstance(v, str)
                           for k, v in data.items()):
        raise ValueError("input is not a nonempty string-to-string dictionary")
    xml = plistlib.dumps(data, fmt=plistlib.FMT_XML)
    if plistlib.load(BytesIO(xml)) != data:
        raise ValueError("XML does not represent the reported input")
except Exception as e:
    print("REFUTATION REJECTED:", "invalid input:", repr(e))
else:
    expected = UserDict(data)
    try:
        actual = plistlib.load(BytesIO(xml), dict_type=UserDict)
        broken = not isinstance(actual, UserDict) or actual != expected
    except Exception as e:
        actual = ("exception", type(e).__name__, str(e))
        broken = True
    if broken:
        print("REFUTATION CONFIRMED:", "input:", data,
              "actual:", repr(actual), "expected:", repr(expected))
    else:
        print("REFUTATION REJECTED:", "returned the expected UserDict")

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

REFUTATION CONFIRMED: input: {'a': 'b'} actual: ('exception', 'ValueError', 'unexpected key at line 5') expected: {'a': 'b'}

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/plistlib-20261003-042133-c2

CPython versions tested on:

3.14

Operating systems tested on:

Windows

[ZeroIntensity]

See #158748 (comment).