pprint.saferepr raises RecursionError when sorting self-referential list-subclass dictionary keys

[augusto-rehfeldt]

Bug report

Bug description:

Documented behaviour: The pprint.saferepr documentation states: "Return a string representation of object, protected against recursive data structures." It further specifies: "This only handles recursive instances of dict, list and tuple or subclasses whose repr is not overridden."

Expected: Return a string representing the dictionary, with recursion markers for its self-referential keys.

Actual: RecursionError: Stack overflow (used 2912 kB) in comparison

import pprint

class Key(list):
    __hash__ = object.__hash__

a, b = Key(), Key()
a.append(a)
b.append(b)
d = {a: 1, b: 2}

if not (len(d) == 2 and a is not b and all(
    isinstance(k, list) and type(k).__repr__ is list.__repr__
    and type(k).__hash__ is object.__hash__ and k[0] is k
    for k in d
)):
    print("REFUTATION REJECTED: input does not meet the documented conditions")
else:
    expected = "{" + ", ".join(
        f"[<Recursion on Key with id={id(k)}>]: {v}" for k, v in d.items()
    ) + "}"
    try:
        actual = pprint.saferepr(d)
        broken = not isinstance(actual, str)
    except Exception as e:
        actual = f"{type(e).__name__}: {e}"
        broken = True
    if broken:
        print("REFUTATION CONFIRMED:", "input:", repr(d),
              "actual:", repr(actual), "expected:", repr(expected))
    else:
        print("REFUTATION REJECTED: saferepr returned a string without raising")

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

REFUTATION CONFIRMED: input: {[[...]]: 1, [[...]]: 2} actual: 'RecursionError: Stack overflow (used 2912 kB) in comparison' expected: '{[<Recursion on Key with id=1278655537488>]: 1, [<Recursion on Key with id=1278655537136>]: 2}'

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/pprint-20261003-073352-c1

CPython versions tested on:

3.14

Operating systems tested on:

Windows

[ZeroIntensity]

See #158748 (comment).