regression with Generic/ParamSpec between 1.4.1 and 1.5.0 #15844
Description
Bug Report
With 1.5.0, some of our software has started to surface a var-annotated ("Need type annotation for") error that it did not surface with 1.4.1; I could not find anything in the 1.5.0 release notes that suggested that this was intentional.
To Reproduce
Our actual software is a bit domain-specific, but I've tried to extract the details; the concept involves a data descriptor that attaches a callable to a class and generates a new callable that inverts how the callable's constructor and runtime arguments are supplied. The solution depends on ParamSpec to preserve the argument signatures and uses Generic so that the same internals can be used for different functions.
#!/usr/bin/env python3 from dataclasses import dataclass from typing import Generic, ParamSpec, Protocol, Self, TypeVar, overload P = ParamSpec("P") T = TypeVar("T") class Transform(Protocol[P, T]): """Generic protocol to transform a value using inputs.""" def __init__(self, *args: P.args, **kwargs: P.kwargs) -> None: raise NotImplementedError def __call__(self, value: T) -> T: raise NotImplementedError class BoundTransform(Generic[P, T]): """Wrapper around a transform function that performs late-binding.""" def __init__( self, value: T, transform_cls: type[Transform[P, T]], ) -> None: self.value: T = value self.transform_cls: type[Transform[P, T]] = transform_cls def __call__( self, *args: P.args, **kwargs: P.kwargs, ) -> T: return self.transform_cls(*args, **kwargs)(self.value) class HasValue(Protocol[T]): value: T class Transformer(Generic[P, T]): """Data descriptor that applies the bound transformer to something with a value.""" def __init__( self, transform_cls: type[Transform[P, T]], ) -> None: self.transform_cls: type[Transform[P, T]] = transform_cls @overload def __get__(self, obj: None, objtype: type[HasValue[T]]) -> Self: ... @overload def __get__(self, obj: HasValue[T], objtype: type[HasValue[T]]) -> BoundTransform[P, T]: ... def __get__(self, obj: HasValue[T] | None, objtype: type[HasValue[T]]) -> Self | BoundTransform[P, T]: if obj is None: return self return BoundTransform(obj.value, self.transform_cls) @dataclass(frozen=True) class Add: """Transform implementation that adds two integers.""" value: int def __call__(self, value: int) -> int: return value + self.value @dataclass(frozen=True) class HasNumber(HasValue[int]): """HasValue implementation that holds an integer.""" value: int add = Transformer(Add) if __name__ == "__main__": # Usage example foo = HasNumber(42) bar = foo.add(-42) assert bar == 0Expected Behavior
No errors from mypy. This is true for 1.4.1.
Actual Behavior
Errors. This is true for 1.5.0
main.py:81: error: Need type annotation for "add" [var-annotated] Found 1 error in 1 file (checked 1 source file)Your Environment
- Mypy version used:
1.4.1and1.5.0 - Mypy command-line flags: n/a
- Mypy configuration options from
mypy.ini(and other config files): n/a - Python version used: 3.11.4