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Mypy signals error with Protocol and __rpow__ method with three arguments #10786
Description
Okay, this is a head-scratcher. 😕
To Reproduce
# test_case.py
from typing import Protocol, overload, runtime_checkable
@runtime_checkable
class PowableProto(Protocol):
@overload
def __pow__(self, other: "PowableProto") -> "PowableProto":
...
@overload
def __pow__(self, other: "PowableProto", modulo: "PowableProto") -> "PowableProto": # <- fine
...
@overload
def __rpow__(self, other: "PowableProto") -> "PowableProto":
...
@overload
def __rpow__(self, other: "PowableProto", modulo: "PowableProto") -> "PowableProto": # <- this (of all things) breaks
...
@overload
def __rpOw__(self, other: "PowableProto") -> "PowableProto":
...
@overload
def __rpOw__(self, other: "PowableProto", modulo: "PowableProto") -> "PowableProto": # <- fine
...% mypy --version
mypy 0.910
% mypy --config-file=/dev/null test_case.py
/dev/null: No [mypy] section in config file
test_case.py:15: error: Invalid signature "def (test_case.PowableProto, test_case.PowableProto, test_case.PowableProto) -> test_case.PowableProto"
Found 1 error in 1 file (checked 1 source file)
Expected vs. Actual Behavior
I would expect at least consistent behavior among the overridden methods. This only crops up with __rpow__ for some reason. (I have tested many others.)
Your Environment
- Mypy version used: 0.910
- Mypy command-line flags:
--config-file=/dev/null(i.e., force default configuration) - Mypy configuration options from
mypy.ini(and other config files): N/A - Python version used:
Python 3.9.6(frompython --version) - Operating system and version:
Darwin Kernel Version 19.6.0: Thu May 6 00:48:39 PDT 2021; root:xnu-6153.141.33~1/RELEASE_X86_64(fromuname -v)
Activity
posita commentedon Jul 8, 2021
Actually, scratch that. Here's a simpler test case that still breaks on
__rpow__(and only on__rpow__):This (in the style of using ellipses for default arguments in stub files) works, though:
The callout of
__rpow__still seems…weird. 🤨[-]Protocol doesn't like second @overload-ed __rpow__ method—and *only* the second @overload-ed __rpow__ method[/-][+]Mypy signals error with Protocol and __rpow__ method with three arguments[/+]ilevkivskyi commentedon Nov 16, 2022
I don't think it is related to recursive types. Mypy has strong requirements for Python special methods, and FWIW I think it is fine to relax those for protocols, probably an easy fix.
fix python#10786
Cnoor0171 commentedon Dec 5, 2023
I think the error is correct in this case, isn't it? The three argument version of
__pow__is called when the built-inpowfunction is called, but the python docs explicitly mention that__rpow__does not use the three argument version. From https://docs.python.org/3/reference/datamodel.htmljorenham commentedon Jun 29, 2026
Since Python 3.14, the 3-argument
__rpow__for ternarypowis supported: python/cpython#130251. The 3.14 relnotes also mention this: https://docs.python.org/3/whatsnew/3.14.html#built-insposita commentedon Jun 30, 2026
Hmmm...then maybe @Cnoor0171 was right with #10786 (comment) ? My first two test cases still produce errors:
Sounds like this can be closed as either fixed or as-designed?
Allow a three-argument __rpow__ overload item
Allow a three-argument __rpow__ overload item