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pythonpython-typing

What's the correct type hint for a type hint?

I have a use case for accepting type hints as arguments to a function. But what is the right type hint to to accept a type hint?

T = TypeVar('T', bound='TypeHintType')

def register_handler(type_hint: T, handler: Handler[T]) -> None:
    ...

Is there a "TypeHintType" that I can use to annotate that a type hint is expected as an argument?

Solution

  • Currently there is no such thing yet. But, there is one in the making in PEP 747 - Annotating Type Forms by some prominent figures in Python's typing ecosystem.

    An argument that should be a type-hint itself will be called a TypeForm. Or, more concrete

    A [TypeForm is a] special form representing the value of a type expression.

    TypeForm might come with Python 3.14.

    Nevertheless, TypeForm is already supported by pyright with experimental features enabled(*). And a backport is provided by typing_extensions, however, it is not released yet and will come in typing_extensions 4.13 that should arrive soon.

    It can then, for example be used like this:

    from typing_extensions import TypeForm

def register_handler[T](type_hint: TypeForm[T], handler: Handler[T]) -> None:
    ...

register_handler(int | float, yourobject)

    Some nice use cases are (taken from the PEP):

    def is_assignable[T](value: object, typx: TypeForm[T]) -> TypeIs[T]
def is_match[T](value: object, typx: TypeForm[T]) -> TypeGuard[T]
def convert[T](value: object, typx: TypeForm[T]) -> T

    (*) To enable experimental features in pyright use you need the following line in your pyproject.toml file.

    [tool.pyright]
enableExperimentalFeatures = true

    Note: As PEP 747 is still in its draft phase, there might be changes.

    Some discussions about PEP 747: