Type variable has no meaning in variable type annotation

[injust]

Code sample in pyright playground

from typing import Any, Callable, TypeVar

WrappedFn = TypeVar("WrappedFn", bound=Callable[..., Any])

def foo(func: WrappedFn) -> WrappedFn:
    return func

reveal_type(foo)

bar: Callable[[WrappedFn], WrappedFn] = foo
reveal_type(bar)

Type of "foo" is "(func: WrappedFn@foo) -> WrappedFn@foo"
Type variable "WrappedFn" has no meaning in this context  (reportGeneralTypeIssues)
Type variable "WrappedFn" has no meaning in this context  (reportGeneralTypeIssues)
Type of "bar" is "(func: WrappedFn@foo) -> WrappedFn@foo"

Why does WrappedFn have no meaning when used to type the variable bar? As far as I understand, it has the same meaning as used in the type annotation for foo.

[injust]

For context, this is my use case:

from collections.abc import Callable

from tenacity import WrappedFn, retry, stop_after_attempt

retry_policy: Callable[[WrappedFn], WrappedFn] = retry(stop=stop_after_attempt(3))


@retry_policy
def foo() -> None:
    print("foo!")
    raise Exception


foo()

[erictraut]

This behavior is mandated by the Python typing spec. A type variable is not valid unless it is associated with a generic class, function, or type alias declaration. You're using the type variable here for a variable declaration, which isn't a valid context for a type variable.

Historically, type variable scoping in Python has been very confusing for users, so you're not alone here. PEP 695 (which was introduced in Python 3.12) modernizes the syntax and eliminates most of the confusion around scoping because the scope is explicit with the new syntax.