Feature

Reproduction

pow(1, None, 0)

RustPython:

ValueError: pow() 3rd argument cannot be 0

CPython:

TypeError: unsupported operand type(s) for ** or pow(): 'int', 'NoneType', 'int'

Root cause

PyInt::modpow downcasts modulus to PyInt and immediately raises ValueError("pow() 3rd argument cannot be 0") when it is zero, without first validating that other is an integer. CPython's long_pow instead runs CHECK_BINOP(v, w) and the x type-check before the zero-modulus branch, so a non-integer exponent (or non-int/non-None modulus) returns NotImplemented and the dispatch falls through to a TypeError.

The same observation applies to the related variant pow(True, -3939.40..., False): once PyInt::modpow correctly returns NotImplemented for a non-int exponent, the ternary_op fallback dispatches to PyFloat::AS_NUMBER.power, which raises the expected TypeError("pow() 3rd argument not allowed unless all arguments are integers").

Fix

In PyInt::modpow, downcast other to PyInt first and return NotImplemented if the cast fails; only then perform the zero-modulus check.

Environment

• RustPython d248a04 (Python 3.14.0)
• CPython v3.14.3
• OS: Debian 12

Python Documentation or reference to CPython source code

• CPython long_pow: Objects/longobject.c#L4888-L4941 — type-check (CHECK_BINOP, PyLong_Check(x)) runs before the zero-check:

  CHECK_BINOP(v, w);
  a = (PyLongObject*)Py_NewRef(v);
  b = (PyLongObject*)Py_NewRef(w);
  if (PyLong_Check(x)) {
      c = (PyLongObject *)Py_NewRef(x);
  }
  else if (x == Py_None)
      c = NULL;
  else {
      Py_DECREF(a); Py_DECREF(b);
      Py_RETURN_NOTIMPLEMENTED;
  }
  ...
  if (c) {
      if (_PyLong_IsZero(c)) {
          PyErr_SetString(PyExc_ValueError,
                          "pow() 3rd argument cannot be 0");
          goto Error;
      }

• Documentation: builtins.pow