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is_dataclass(Any) return type incorrect, differs from is_dataclass(object) #12401
Description
I found this surprising and arguably incorrect. According to mypy:
from typing import Any
from dataclasses import is_dataclass
def example1(arg: Any) -> None:
if is_dataclass(arg):
reveal_type(arg) # ❌ Revealed type is "type[_typeshed.DataclassInstance]"
def example2(arg: object) -> None:
if is_dataclass(arg):
reveal_type(arg) # ✅ Revealed type is "Union[_typeshed.DataclassInstance, type[_typeshed.DataclassInstance]]"This is because Any matches the 1st overload, even if 2nd overload with object argument is more appropriate for Any:
@overload
def is_dataclass(obj: type) -> TypeIs[type[DataclassInstance]]: ...
@overload
def is_dataclass(obj: object) -> TypeIs[DataclassInstance | type[DataclassInstance]]: ...I couldn't think of an elegant fix for this.
Activity
srittau commentedon Jul 22, 2024
I don't think there is a fix, as
Anyis compatible with everything, so it will always match the first overload. Reordering is also not possible, at least not until we get a "Not" type (python/typing#801).intgr commentedon Jul 22, 2024
As a hack, we could add another overload case, with argument type that no real type ever satisfies but only
Anycan satisfy. Could we use theNevertype?sobolevn commentedon Jul 27, 2024
Refs python/mypy#17579
max-muoto commentedon Aug 6, 2024
For reference Pyright is handling this just fine: https://pyright-play.net/?pyrightVersion=1.1.374&pythonVersion=3.14&strict=true&code=GYJw9gtgBALgngBwJYDsDmUkQWEMoCCKcAUKJFACYCGM1AxgDbUDOLApi5trvkiwH0adJqxYkJldsCjsAHtWyN2ARgAU1EGgBchYgEooAWgB8UAHJgU7bSSj3MM-kNoNmbDVv22HvqCHYAN3ZqRgF4BHZPNEMoCRIpGXlFBGUAJmjdMAAjACt2ehhDUwsrGzsHJCdBYTcxaO8Kv38gkLCIqKbmh000Lt9DEiA
srittau commentedon Aug 7, 2024
@intgr We could try your suggestion in an experimental PR. (We should definitely add tests, though.) Otherwise I would close this issue as unfixable.
intgr commentedon Aug 12, 2024
Opened PR:
is_dataclass(Any)#12517sobolevn commentedon Aug 14, 2024
MyPy PR is here: python/mypy#17678