How to require the first parameter of a Paramspec is a specific type

[mike-barry-gmo]

Hi,

I'm writing a decorator what has an implicit contract I'd like to enforce but I'm having an issue making the type system do what I would like. Here is what I have working:

TIndex = TypeVar("TIndex")
TReturn = TypeVar("TReturn")
P = ParamSpec("P")

def model(
    reindex_function: Callable[[TIndex], TIndex],
) -> Callable[[Callable[P, TReturn]], Callable[P, TReturn]]:
    ...

def sample_reindex_function(index: str) -> str:
    ...
    
@model(reindex_function=sample_reindex_function)
def decorated_method(index: str, pos_arg2: int, *, kwarg1: int) -> Any:
    ...

Essentially, I'd love for the return type of the model decorator to enforce that the first positional parameter of a decorated method is of type TIndex. In the example above that would be str.

Is there any way to do this?

Thanks for your help!

Mike

[erictraut]

The type system includes a special form called Concatenate. You can use it to prepend positional parameters for a Callable. For documentation, refer to this section of the typing spec.

Here's an updated sample using Concatenate.

from typing import Any, Callable, Concatenate, ParamSpec, TypeVar

TIndex = TypeVar("TIndex")
TReturn = TypeVar("TReturn")
P = ParamSpec("P")

def model(
    reindex_function: Callable[[TIndex], TIndex],
) -> Callable[[Callable[Concatenate[TIndex, P], TReturn]], Callable[Concatenate[TIndex, P], TReturn]]: ...

def sample_reindex_function(index: str) -> str: ...

@model(reindex_function=sample_reindex_function)
def decorated_method(index: str, pos_arg2: int, *, kwarg1: int) -> Any: ...

Or using the more modern Python 3.12 syntax:

from typing import Any, Callable, Concatenate

def model[T, R, **P](
    reindex_function: Callable[[T], T],
) -> Callable[[Callable[Concatenate[T, P], R]], Callable[Concatenate[T, P], R]]: ...

def sample_reindex_function(index: str) -> str: ...

@model(reindex_function=sample_reindex_function)
def decorated_method(index: str, pos_arg2: int, *, kwarg1: int) -> Any: ...

If you have other general questions about the Python type system, the Python typing forum is a good place to get answers.

[mike-barry-gmo]

Thank you!