enforce type hinting at runtime with a decorator

insist.py

from functools import wraps
from typing import get_type_hints


class InputError(Exception):
    pass


def make_error_message(arg_name, arg_type, expected_type):
    return f"{arg_name} value has type {arg_type} - must be {expected_type}"


def insist(func):
    hints = get_type_hints(func)
    pos_types = tuple(hints.values())
    pos_args = tuple(hints.keys())

    @wraps(func)
    def wrapped(*args, **kwargs):
        num_posargs = len(args)
        posargs_hints = pos_types[:num_posargs]
        for idx, arg in enumerate(args):
            arg_name = pos_args[idx]
            arg_type = type(arg)
            expected_type = posargs_hints[idx]
            type_mismatch = not isinstance(arg, expected_type)
            if type_mismatch:
                cast_as_expected = expected_type(arg)
                if arg == cast_as_expected:
                    continue

                error_message = make_error_message(arg_name, arg_type, expected_type)
                raise InputError(error_message)

        kwargs_keys = pos_args[num_posargs:]
        for arg_name, arg_val in kwargs.items():
            arg_type = type(arg_val)
            expected_type = hints[arg_name]
            type_mismatch = not isinstance(arg_val, expected_type)
            if type_mismatch:
                cast_as_expected = expected_type(arg_val)
                if arg_val == cast_as_expected:
                    continue

                error_message = make_error_message(arg_name, arg_type, expected_type)
                raise InputError(error_message)

        result = func(*args, **kwargs)
        if "return" in hints:
            arg_name = "return"
            arg_type = type(result)
            expected_type = hints["return"]
            type_mismatch = not isinstance(result, expected_type)
            if type_mismatch:
                error_message = make_error_message(arg_name, arg_type, expected_type)
                raise Exception(error_message)

        return result

    return wrapped