Repository navigation
Compatibility of return type with subclass upon overload #19952
Description
Hi, we have the following minimal code snippet that reproduces our concern from pandas-dev/pandas-stubs#1385. In words, when the return types from different overloads are parametrised by a type variable, type checkers can work. When return types are subclasses, however, mypy does not seem to work.
from typing import Any, Generic, Never, TypeVar, overload, reveal_type
T = TypeVar("T")
class Arg(Generic[T]): ...
class Gen(Generic[T]): ...
# part 1: return type is Generic
class D0:
@overload
def m(self, p: Arg[Never]) -> Gen: ...
@overload
def m(self, p: Arg[int]) -> Gen[int]: ...
reveal_type(D0().m(Arg[Any]())) # pyright gives Gen[Unknown], mypy gives Gen[Any], good
reveal_type(D0().m(Arg[int]())) # Gen[int], good
reveal_type(D0().m(Arg[float]())) # error, good
# part 2: return types are subclasses
class Base: ...
class Sub(Base): ...
class D1:
@overload
def m(self, p: Arg[Never]) -> Base: ... # we want to get Base for Arg[Any]; mypy warns overload-overlap here, whereas pyright does not warn (at least without strict)
@overload
def m(self, p: Arg[int]) -> Sub: ...
reveal_type(D1().m(Arg[Any]())) # pyright gives Base, but mypy gives Any
reveal_type(D1().m(Arg[int]())) # Sub, good
reveal_type(D1().m(Arg[float]())) # error, goodPart 1 is understood from #19525. Could someone explain part 2? Thank you 🙂
Activity
[-]Compatibility of return type upon overload[/-][+]Compatibility of return type with subclass upon overload[/+]ilevkivskyi commentedon Sep 29, 2025
When multiple overloads can be matched because of an
Anyargument(s), and the corresponding return types are not equivalent moduloAny, mypy intentionally infersAnyas the result of the call. This is done to guarantee a fundamental rule of gradual typing: replacing a precise type withAnyshould not cause new errors. Consider this code:this code would generate an error, while it shouldn't.
cmp0xff commentedon Oct 20, 2025
Hi @ilevkivskyi , thank you for triaging.
The complaint in the OP is that the return types should be considered as equivalent in the covariant sense, so that returning
Anyshould not happen.DOC: record confirmed mechanism and two failed fix attempts for the a…