Subclassing PrettyPrinter.format doesn't work in all cases

Bug report

Bug description:

See also the earlier fixed https://github.com/python/cpython/issues/73036. This appears to have only been a partial fix, or it has regressed.

import collections
import pprint
import types


SENTINAL = object()


class _hashable:
    def __hash__(self):
        return 1


HASHABLE_SENTINAL = _hashable()


class CustomPrettyPrinter(pprint.PrettyPrinter):
    def format(self, obj, context, maxlevels, level):
        if obj is SENTINAL:
            return "SENTINAL", True, False
        elif obj is HASHABLE_SENTINAL:
            return "HASHABLE_SENTINAL", True, False
        else:
            return super().format(obj, context, maxlevels, level)


if __name__ == "__main__":
    printer = CustomPrettyPrinter(sort_dicts=False)

    test_data = {
        "item": SENTINAL,
        "dict": {"a": SENTINAL},
        "dict_key": {HASHABLE_SENTINAL: 1},
        "OrderedDict": collections.OrderedDict({"a": SENTINAL}),
        "OrderedDict_key": collections.OrderedDict({HASHABLE_SENTINAL: 1}),
        "list": [SENTINAL],
        "tuple": (SENTINAL,),
        "set": {SENTINAL},
        "frozenset": frozenset({SENTINAL}),
        "mappingproxy": types.MappingProxyType({"a": SENTINAL}),
        "mappingproxy_key": types.MappingProxyType({HASHABLE_SENTINAL: 1}),
        "SimpleNamespace": types.SimpleNamespace(a=SENTINAL),
        "defaultdict": collections.defaultdict(list, {"a": SENTINAL}),
        "defaultdict_key": collections.defaultdict(list, {HASHABLE_SENTINAL: 1}),
        "Counter": collections.Counter({"a": SENTINAL}),
        "Counter_key": collections.Counter({HASHABLE_SENTINAL: 1}),
        "deque": collections.deque([SENTINAL]),
    }

    printer.pprint(test_data)

This outputs:

{'item': SENTINAL,
 'dict': {'a': SENTINAL},
 'dict_key': {HASHABLE_SENTINAL: 1},
 'OrderedDict': OrderedDict({'a': <object object at 0x1041f4660>}),
 'OrderedDict_key': OrderedDict([(HASHABLE_SENTINAL, 1)]),
 'list': [SENTINAL],
 'tuple': (SENTINAL,),
 'set': {<object object at 0x1041f4660>},
 'frozenset': frozenset({<object object at 0x1041f4660>}),
 'mappingproxy': mappingproxy({'a': <object object at 0x1041f4660>}),
 'mappingproxy_key': mappingproxy({HASHABLE_SENTINAL: 1}),
 'SimpleNamespace': namespace(a=<object object at 0x1041f4660>),
 'defaultdict': defaultdict(<class 'list'>,
                            {'a': SENTINAL}),
 'defaultdict_key': defaultdict(<class 'list'>,
                                {HASHABLE_SENTINAL: 1}),
 'Counter': Counter({'a': <object object at 0x1041f4660>}),
 'Counter_key': Counter({<__main__._hashable object at 0x1045e4d70>: 1}),
 'deque': deque([<object object at 0x1041f4660>])}

Every instance of < in that output is a bug.

CPython versions tested on:

3.13

Operating systems tested on:

macOS

Linked PRs

• gh-133238

Start by running the reproducer and inspecting pprint.PrettyPrinter.format, especially how it handles the listed container types and their contents. Compare each output with the expected custom formatting, then add regression coverage for the cases where the override is bypassed. Done means every shown or repr from the reproducer honors the subclass's format method.