operator.length_hint() rejects a negative default and a default that defines __index__ #9509

Feature

operator.length_hint(obj, default) raises in RustPython when default is negative or is not an int instance, but CPython accepts any default with __index__ and returns a negative one as-is.

Reproduction

import operator

print(operator.length_hint(object(), -1))

RustPython:

ValueError: can't convert negative number to unsigned

CPython:

-1

A default with __index__ is refused as well: operator.length_hint(object(), Index()) raises TypeError: 'Index' object cannot be interpreted as an integer where CPython returns the __index__ value, and an exception raised inside that __index__ is replaced by the same TypeError.

Root cause

length_hint accepts default only as an int instance and converts it to usize, and PyObject::length_hint takes a usize default as well. CPython converts default to Py_ssize_t with _PyNumber_Index, so any object with __index__ and any negative value are accepted, and PyObject_LengthHint returns the default unchanged when there is no hint.

Fix

Bind default as isize, whose conversion calls __index__, and return vm.length_hint_opt(obj)?.map_or(default, |hint| hint as isize).

Environment

• RustPython 713e14c (0.6.1, macOS arm64), also reproduced on 0.4.0
• CPython v3.15.2, v3.14.4 and v3.12.10 (macOS arm64)

Python Documentation or reference to CPython source code

• CPython _operator_length_hint: Modules/clinic/_operator.c.h#L1448-L1461 converts default with _PyNumber_Index into a signed Py_ssize_t:

  {
      Py_ssize_t ival = -1;
      PyObject *iobj = _PyNumber_Index(args[1]);
      if (iobj != NULL) {
          ival = PyLong_AsSsize_t(iobj);
          Py_DECREF(iobj);
      }
      if (ival == -1 && PyErr_Occurred()) {
          goto exit;
      }
      default_value = ival;
  }
  skip_optional:
  _return_value = _operator_length_hint_impl(module, obj, default_value);

• Documentation: operator.length_hint

AI disclosure: this report was drafted with Claude Code (claude-opus-5-5) and reviewed by me before submission. The divergences were found by differential testing against CPython, checked against CPython 3.15.2, and every source reference above was checked against the linked revisions.