None alias for NoneType doesn't work on generic bound when using type of the generic

[DetachHead]

from typing import TypeVar
from types import NoneType

T = TypeVar("T", bound=None)
def foo(value: type[T]) -> None: ...

U = TypeVar("U", bound=NoneType)
def bar(value: type[U]) -> None: ...

foo(NoneType) #error: Value of type variable "T" of "foo" cannot be "NoneType"
bar(NoneType) #no error

https://mypy-play.net/?mypy=latest&python=3.10&gist=ce1d20fa9232f609b36053e10a117a8b

[KotlinIsland]

from typing import TypeVar, Generic
from types import NoneType

T = TypeVar("T", bound=None)
def foo(a: T, b: type[T]) -> None: ...

U = TypeVar("U", bound=NoneType)
def bar(a: U, b: type[U]) -> None: ...

foo(None, NoneType) #Value of type variable "T" of "foo" cannot be "Optional[NoneType]"
bar(None, NoneType) #Value of type variable "U" of "bar" cannot be "Optional[NoneType]"

This is absolute whack

[KotlinIsland]

I think this might be related to the fact that the stubs define NoneType as a subtype of the type of None

[KotlinIsland]

wait, not it's not, it's completely unrelated to None

@final
class NoneType:
    def __bool__(self) -> Literal[False]: ...

LOOOOOOL!!

[erictraut]

I wouldn't expect this to work. The types.NoneType symbol has no type relationship to builtins.None, just as types.UnionType has no type relationship to typing.Union. If you want to refer to the type of the object builtins.None, you can use the expression type(None).

[KotlinIsland]

at runtime NoneType is type(None) but typing.Union and types.UnionType are completely different constructs.

[KotlinIsland]

T = TypeVar("T", bound=type(None)) # error: TypeVar "bound" must be a type

[erictraut]

from typing import TypeVar

T = TypeVar("T", bound=None)

def foo(value: type[T]) -> None:
    ...

U = TypeVar("U", bound=type[None])

def bar(value: type[U]) -> None:
    ...

foo(type(None))
bar(type(None)) # Error

[KotlinIsland]

type[None], that's a good one :)

[KotlinIsland]

What a mess lmao.

in a type context, None really means type(None). So when you say type[None] I think in the type realm it is type[type(None)].

[sobolevn]

I think that we need better docs for import types usages.
Today I got another question in our user-group about this case:

from types import FunctionType

def a(): pass

b: FunctionType = a

It also does not work. It should be using typing.Callable instead.

So, I will send a PR with some docs on why it is not support and what to do instead.

[JelleZijlstra]

NoneType should never be used in types. I'd welcome PRs improving documentation for this.

The original example doesn't make much sense to me: None cannot be subclassed, so bounding on None or NoneType isn't useful.

[DetachHead]

The original example doesn't make much sense to me

@JelleZijlstra it's just a minimal example. My actual use case is a class with generics bounded to a union of some class and None, where the generics need to be reified at runtime. The only way I could figure out how to do it was like this

T = TypeVar("T", bound=Foo | NoneType)

class Bar(Generic[T]):
    t: type[T]

bar = Bar(NoneType)

While trying to do this was when I found out that NoneType doesn't work how I thought it does. I agree with you and @KotlinIsland that it's extremely misleading

[KotlinIsland]

Pretty sure this would fix things: python/typeshed#6125