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isinstance(x, Type) not accepted #3060
Description
isinstance(x, Type) generates in an error, even though it should probably be considered equivalent to isinstance(x, type). Example:
from typing import Type
def f(x: Type[int]) -> None:
if isinstance(x, Type): # Argument 2 to "isinstance" has incompatible type "object";
# expected "Union[type, Tuple[Union[type, Tuple[Any, ...]], ...]]"
pass
Activity
pkch commentedon Mar 28, 2017
I almost fixed it inside #3070, but then I realized that the required change to
typeshed(fromUnion[type, Tuple[Union[type, Tuple[Any, ...]], ...]]toUnion[object, Tuple[Union[object, Tuple[Any, ...]], ...]]) effectively eliminates type checking of the 2nd argument toisinstance. That would be quite disappointing.What else can we do?
Map
Typeto be something other thanobjectduring type inference? Perhaps a special singleton classTypeObject?gvanrossum commentedon Mar 28, 2017
Yeah, that would be equivalent to making the type
object. But there's special-casing forisinstance()calls in mypy -- maybe we can expand that to manually check that the recursive tuple is valid? Or maybe we can implement recursive type aliases, at last (#731).pkch commentedon Mar 29, 2017
I guess a narrow fix could be to redefine
Typeintyping.pyiasclass Type: ...instead ofType = object(). This would pass the type check forisinstance(x, Type).We would still have to add some code elsewhere to make @JukkaL example pass, since at present
Type[int]is not recognized as a subtype ofType, somypythinks that everything afterisinstance(x, Type)is unreachable.gvanrossum commentedon Mar 29, 2017
Is this about mypy or runtime? Making the behaviors of isinstance()/issubclass() match between static checks and runtime can be tricky.
pkch commentedon Mar 29, 2017
I think mypy. It seems Python runtime returns
Trueonisinstance(c, Type)whenevercis of typetype. This is precisely what mypy should do as well (of course, assuming it can statically infer thatcof typetype).ilevkivskyi commentedon Jun 15, 2019
On current master the error is even worse:
Add support for Type[T] typehints when arbitrary_types_allowed==True. (…
ignoring mypy _SpecialForm error
Use `callable()` builtin function over `isinstance(..., Callable)`
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