Item assignment and deletion on a tuple or str raise TypeError before calling the key's __index__ #9510

Feature

Assigning to or deleting an item of a tuple, str, bytes or range raises TypeError in RustPython without calling the key's __index__, but CPython calls __index__ first, so an exception from it propagates.

Reproduction

class RaisingIndex:
    def __index__(self):
        raise ZeroDivisionError

t = ()
t[RaisingIndex()] = 0

RustPython:

TypeError: 'tuple' object does not support item assignment

CPython:

ZeroDivisionError

del t[RaisingIndex()], operator.setitem(), operator.delitem() and instances of a class without __setitem__ or __delitem__ diverge the same way.

Root cause

PyObject::set_item and PyObject::del_item convert the key only when the type has an ass_item slot and otherwise raise TypeError straight away. CPython's PyObject_SetItem and PyObject_DelItem convert an __index__ key whenever the type has a sequence table, and only then does PySequence_SetItem or PySequence_DelItem refuse the operation.

Fix

When the type has a sequence table (the has_any() || heaptype_ext() test del_item already uses for its message) and the key has __index__, convert the key to isize before raising the TypeError, raising IndexError for a value that does not fit, as PyNumber_AsSsize_t(key, PyExc_IndexError) does.

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 PyObject_SetItem: Objects/abstract.c#L250-L265 converts an index key before the assignment is refused:

  if (Py_TYPE(o)->tp_as_sequence) {
      if (_PyIndex_Check(key)) {
          Py_ssize_t key_value;
          key_value = PyNumber_AsSsize_t(key, PyExc_IndexError);
          if (key_value == -1 && PyErr_Occurred())
              return -1;
          return PySequence_SetItem(o, key_value, value);
      }

• CPython PyObject_DelItem: Objects/abstract.c#L284-L299 does the same before PySequence_DelItem.

• Documentation: object.__index__

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.