Feature

Found by differential testing against CPython 3.12. All 7 items reproduce on RustPython 0.6.1 and on main at 3c3ffc5 (the outputs below are from main), and CPython 3.12.13, 3.14.3 and 3.15.2 give the same output for each. Every item has a reproduction, both outputs and the cause, with a suggested fix folded underneath.

1. tuple has a __bool__ slot

class T(tuple):
    def __len__(self):
        return 0

impl AsNumber for PyTuple (tuple.rs#L661-L672) defines a boolean slot, so a subclass's __len__ is never consulted. CPython's PyTuple_Type has no tp_as_number (tupleobject.c#L855), so truth testing falls back to __len__.

2. staticmethod and classmethod objects accept weak references

Both types are declared with HAS_WEAKREF (staticmethod.rs#L99, classmethod.rs#L93). In CPython both have tp_weaklistoffset == 0 (funcobject.c#L1149, #L1342).

3. int / int loses the sign of a zero result

true_div (int.rs#L7-L13) rounds a malachite Rational, which has no signed zero. CPython's long_true_divide returns -0.0 when a zero numerator meets a negative denominator (longobject.c#L4342-L4349, #L4486-L4487). The literal 0 / -5 is constant-folded to -0.0, so only the runtime path differs.

4. itertools.count() keeps an int subclass start

PyItertoolsCount::py_new (itertools.rs#L211-L228) stores start as passed. CPython's fast mode (itertoolsmodule.c#L4028-L4067) keeps an int start with step 1 as a C Py_ssize_t, so it produces exact ints. count(True, 2) keeps True on both.

5. unicodedata.normalize() validates form before returning an empty string

form is bound as NormalizeFormArg (unicodedata.rs#L15-L30), which rejects an unknown form during argument binding, before normalize and is_normalized run. CPython returns early for an empty string before it looks at form (unicodedata.c#L935-L943, #L863-L873).

6. types.TracebackType() rejects keyword arguments

import sys
import types

PyTraceback's constructor takes type Args = (Option<PyRef<Self>>, FrameObjectRef, i32, usize) (traceback.rs#L103), and a tuple binds every element positionally. CPython's tb_new has no /, so all four parameters accept keywords (traceback.c#L65-L74).

7. Three-argument pow() calls __pow__ on the wrong operand when only the modulus defines it

class C:
    def __pow__(self, other, mod=None):
        return "C.__pow__"

ternary_op calls the modulus's slot (vm_ops.rs#L383-L388), and number_ternary_op_wrapper! (slot.rs#L577-L587) then looks up __pow__ on the first operand, None. CPython's slot_nb_power (typeobject.c#L8600-L8613) calls __pow__ only when the first operand's type uses that slot, and returns NotImplemented otherwise.

Python Documentation or reference to CPython source code

The CPython source for each item is linked in the item above (CPython v3.12.13).

AI disclosure: this report was drafted with Claude Code (claude-opus-5-5) and reviewed by me before submission. The divergences come from a differential-testing run against CPython 3.12.13, and every source reference was checked against the linked revisions.