Mypy signals error with Protocol and __rpow__ method with three arguments

[posita]

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 (from python --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 (from uname -v)

[posita]

Actually, scratch that. Here's a simpler test case that still breaks on __rpow__ (and only on __rpow__):

# test_case2.py
from typing import Protocol, runtime_checkable
@runtime_checkable
class PowableProto(Protocol):
    def __pow__(self, other: "PowableProto", modulo: "PowableProto") -> "PowableProto":  # <- fine
        ...
    def __rpow__(self, other: "PowableProto", modulo: "PowableProto") -> "PowableProto":  # <- this breaks
        ...

This (in the style of using ellipses for default arguments in stub files) works, though:

# test_case3.py
from typing import Protocol, runtime_checkable
@runtime_checkable
class PowableProto(Protocol):
    def __pow__(self, other: "PowableProto", modulo: "PowableProto" = ...) -> "PowableProto":  # <- this now breaks
        ...
    def __rpow__(self, other: "PowableProto", modulo: "PowableProto" = ...) -> "PowableProto":  # <- this still breaks
        ...

The callout of __rpow__ still seems…weird. 🤨

[ilevkivskyi]

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.

[Cnoor0171]

I think the error is correct in this case, isn't it? The three argument version of __pow__ is called when the built-in pow function 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.html

Note that ternary pow() will not try calling rpow() (the coercion rules would become too complicated).