json.dump silently drops entries from dict subclasses with false truthiness

[augusto-rehfeldt]

Bug report

Bug description:

Documented behaviour: json.dump documentation: "Serialize obj as a JSON formatted stream to fp (a .write()-supporting file-like object)." JSONEncoder documentation lists "dict" as supported and maps it to JSON "object".

Expected: {"a": 1}

Actual: {}

import io, json

class D(dict):
    def __bool__(self):
        return False

d = D(a=1)
if not (D.__iter__ is dict.__iter__ and D.items is dict.items
        and all(type(k) is str and type(v) is int for k, v in d.items())
        and not bool(d) and dict(d) == {"a": 1}):
    print("REFUTATION REJECTED: invalid input")
else:
    expected = json.dumps(dict(d))
    fp = io.StringIO()
    json.dump(d, fp)
    actual = fp.getvalue()
    if actual != expected:
        print("REFUTATION CONFIRMED:", repr(d),
              "actual:", repr(actual), "expected:", repr(expected))
    else:
        print("REFUTATION REJECTED: dump preserves all entries")

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

REFUTATION CONFIRMED: {'a': 1} actual: '{}' expected: '{"a": 1}'

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/json-20261003-130022-c1

CPython versions tested on:

3.14

Operating systems tested on:

Windows

[ZeroIntensity]

See #158748 (comment).