gh-130104: Call __rpow__ in ternary pow() if necessary by serhiy-storchaka · Pull Request #130251 · python/cpython

serhiy-storchaka · 2025-02-18T09:54:54Z

Previously it was only called in binary pow() and the binary power operator.

Issue: Inconsistent behavior of ternary pow() for Python classes and C extension types #130104

📚 Documentation preview 📚: https://cpython-previews--130251.org.readthedocs.build/

Previously it was only called in binary pow() and the binary power operator.

skirpichev

LGTM (few nitpicks)

Maybe it's possible to refactor code to avoid a copy of SLOT1BINFULL, but I doubt it worth efforts.

skirpichev · 2025-02-19T00:21:19Z

   is a subclass of ``type(x)``. [#]_

-   .. index:: pair: built-in function; pow
+   Note that :meth:`__rpow__` should be defined to accept an optional third


Maybe you should use object.__rpow__(self, other, modulo=None) signature?

I do not understand you. Were and how can it be used?

This sentence is a copy of the corresponding sentence for __pow__.

Sorry, I meant L3329-3342 above.

I see. There are other similar cases (for example __round__), so I will left this for other issue. Actually, __pow__() and __round__() are never called with None, so it is not necessary that they support None.

skirpichev · 2025-02-20T10:36:10Z

CC @picnixz (this is an alternative to #122193)

picnixz

First round of style review only. I'll look at the implementation and implications more in detail either later or tomorrow.

picnixz · 2025-03-02T13:15:39Z

+* Three-argument :func:`pow` now try calling :meth:`~object.__rpow__` if necessary.
+  Previously it was only called in two-argument :func:`!pow` and the binary
+  power operator.
+  (Contributed by Serhiy Storchaka in :gh:`130104`.)
+


Suggested change

* Three-argument :func:`pow` now try calling :meth:`~object.__rpow__` if necessary.

Previously it was only called in two-argument :func:`!pow` and the binary

power operator.

(Contributed by Serhiy Storchaka in :gh:`130104`.)

* Three-argument :func:`pow` now try calling :meth:`~object.__rpow__` if necessary.

Previously it was only called in two-argument :func:`!pow` and the binary

power operator.

(Contributed by Serhiy Storchaka in :gh:`130104`.)

What is the difference?

picnixz · 2025-03-02T13:18:07Z

@@ -9813,13 +9813,46 @@ slot_nb_power(PyObject *self, PyObject *other, PyObject *modulus)
 {
    if (modulus == Py_None)


Let's insert some PEP-7 here if (modulus == Py_None) { ...

picnixz · 2025-03-02T13:18:53Z

+    int do_other = !Py_IS_TYPE(self, Py_TYPE(other)) &&
+        Py_TYPE(other)->tp_as_number != NULL &&
+        Py_TYPE(other)->tp_as_number->nb_power == slot_nb_power;


We use a lot of Py_TYPE(other), so I think we can have a variable holding it just for readability.

This code is a copy of SLOT1BINFULL. I do not want to introduce more difference than necessary.

We can make SLOT1BINFULL supporting the third argument, but I am not sure that it is worth.

picnixz · 2025-03-02T13:19:29Z

+    int do_other = !Py_IS_TYPE(self, Py_TYPE(other)) &&
+        Py_TYPE(other)->tp_as_number != NULL &&
+        Py_TYPE(other)->tp_as_number->nb_power == slot_nb_power;


Suggested change

int do_other = !Py_IS_TYPE(self, Py_TYPE(other)) &&

Py_TYPE(other)->tp_as_number != NULL &&

Py_TYPE(other)->tp_as_number->nb_power == slot_nb_power;

int do_other = !Py_IS_TYPE(self, Py_TYPE(other))

&& Py_TYPE(other)->tp_as_number != NULL

&& Py_TYPE(other)->tp_as_number->nb_power == slot_nb_power;

picnixz · 2025-03-02T13:19:55Z

-       slot_nb_power, so check before calling self.__pow__. */
+
+    /* The following code is a copy of SLOT1BINFULL, but for three arguments. */
+    PyObject* stack[3];


Suggested change

PyObject* stack[3];

PyObject *stack[3];

picnixz · 2025-03-02T13:20:32Z

+                if (r != Py_NotImplemented)
+                    return r;


Suggested change

if (r != Py_NotImplemented)

return r;

if (r != Py_NotImplemented) {

return r;

}

picnixz · 2025-03-02T13:20:44Z

+        if (r != Py_NotImplemented ||
+            Py_IS_TYPE(other, Py_TYPE(self)))
+            return r;


Suggested change

if (r != Py_NotImplemented ||

Py_IS_TYPE(other, Py_TYPE(self)))

return r;

if (r != Py_NotImplemented || Py_IS_TYPE(other, Py_TYPE(self))) {

return r;

}

picnixz · 2025-03-04T22:48:24Z

Sorry I didn't have time. I will leave for 10 days so I won't be able to review it. If you and Serhiy thinks it's good, then go ahead. If the PR is still not merged when I'm back then I'll try to review it.

As for whether I find it useful or not, my answer would be "yes".

Checking a reverse operator method validates it against a two-positional- argument callable, and it runs once per overload item. An overloaded __rpow__ declaring both a two-argument and a three-argument variant was therefore rejected, even though that is a correct signature: binary '**' uses the two-argument variant, and since Python 3.14 ternary pow() may call __rpow__ with a 'modulo' argument (python/cpython#130251). Accept the three-argument form for __rpow__ when the definition is an overload item. A lone __rpow__ that requires 'modulo' is still an error, because binary '**' calls __rpow__ with a single argument and so can never call it successfully. Refs python#10786.

pythongh-130104: Call __rpow__ in ternary pow() if necessary

85192cb

Previously it was only called in binary pow() and the binary power operator.

serhiy-storchaka requested a review from skirpichev February 18, 2025 09:54

serhiy-storchaka requested review from AA-Turner, markshannon and willingc as code owners February 18, 2025 09:54

bedevere-app Bot added the awaiting core review label Feb 18, 2025

bedevere-app Bot mentioned this pull request Feb 18, 2025

Inconsistent behavior of ternary pow() for Python classes and C extension types #130104

Closed

skirpichev reviewed Feb 18, 2025

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Comment thread Objects/typeobject.c Outdated

Get rid of remnants of debugging.

b563d1c

skirpichev self-requested a review February 19, 2025 00:16

skirpichev approved these changes Feb 19, 2025

View reviewed changes

skirpichev mentioned this pull request Feb 19, 2025

gh-122193: simplify object.__rpow__() signature (no modulo) #122260

Closed

serhiy-storchaka added 2 commits February 20, 2025 12:23

"ternary" -> "three-argument"

56daac8

Merge branch 'main' into rpow

55fbb13

picnixz self-requested a review March 2, 2025 13:14

picnixz reviewed Mar 2, 2025

View reviewed changes

Merge branch 'main' into rpow

230a832

serhiy-storchaka merged commit 62ff86f into python:main Apr 16, 2025

bedevere-app Bot removed the awaiting core review label Apr 16, 2025

serhiy-storchaka deleted the rpow branch April 16, 2025 15:32

jorenham mentioned this pull request Jun 29, 2026

Mypy signals error with Protocol and __rpow__ method with three arguments python/mypy#10786

Open

		@@ -9813,13 +9813,46 @@ slot_nb_power(PyObject self, PyObject other, PyObject *modulus)
		{
		if (modulus == Py_None)

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skirpichev commented Feb 20, 2025

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picnixz commented Mar 4, 2025

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serhiy-storchaka commented Feb 18, 2025 •

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