[SOLVED] How to inspect return type of Callable

How to inspect return type of Callable

Let's say I have something like the following:

import inspect
from collections.abc import Callable  # Using Python 3.10+
from typing import get_type_hints

def foo(arg: Callable[..., int]) -> None:
    pass

type_hints = get_type_hints(foo)["arg"]
annotation = inspect.signature(foo).parameters["arg"].annotation
# How can one get the return type `int` from either of these?

I can figure out how to get to the Callable type hint. From there, how can one inspect the return type of the Callable?

Solution

To avoid access to dunder attributes, which in general is unsafe and can break between version releases, you should use a method provided by typing module:

type_hint = get_type_hints(foo)["arg"]
arg_types, return_type = get_args(type_hint)

This works for any Callable. arg_types is Ellipsis (...) or a list of args (also may be a typing.ParamSpec or typing.Concatenate, if you use them), return_type is whatever occupies 2nd place.