TypeForm + TypeVar

[superlopuh]

I have a question about passing TypeVars to functions that expect TypeForms. Is my understanding of the proposal correct that this is one of the planned supported use-cases? Pyright currently seems not to support it, so I thought I'd ask here whether this is intentional:

from typing_extensions import TypeForm, TypeVar
class A: ...
B = TypeVar("B", bound=A)
def func(tf: TypeForm[A]) -> TypeForm[A]: ...
def this_works(val: B) -> B:
    # B is a valid spelling of a type in this context
    val0: B = val
    # Argument of type "TypeVar" cannot be assigned to parameter "tf" of type "TypeForm[A]" in function "func" "TypeVar" is not assignable to "TypeForm[A]" (reportArgumentType)
    val1 = func(B)
    return val0

[erictraut]

You need to set "enableExperimentalFeatures" to true to enable TypeForm support. Once you do this, the above code type checks without error.

Code sample in pyright playground

[superlopuh]

Amazing, thank you

[superlopuh]

Am I right in understanding that the current shipping version of typing_extensions does not export TypeForm, and that the way to use it via if TYPE_CHECKING: and in quotes?

[erictraut]

I'm not sure if the latest version of typing_extensions has support for this. The PEP is still in draft form, so the design may still change. I recommend that you don't use it in any real code. It's implemented in pyright for experimentation so people can provide feedback on the PEP.