Example of type-level validation in python

val.py

from dataclasses import dataclass
from typing import Generic, Optional, TypeVar


class Raw:
    pass


class Valid:
    pass


V = TypeVar("V", Raw, Valid)
T = TypeVar("T")


class Validable(Generic[V, T]):
    def __init__(self, a: T):
        self.a = a


def validateAge(x: Validable[Raw, int]) -> Optional[Validable[Valid, int]]:
    if x.a > 0:
        return Validable(x.a)
    else:
        return None


@dataclass
class Input(Generic[V]):
    name: str
    age: Validable[V, int]

    @staticmethod
    def from_json(json: dict) -> "Input[Raw]":
        pass


def validateInput(x: Input[Raw]) -> Optional[Input[Valid]]:
    vAge = validateAge(x.age)
    if vAge is None:
        return None
    else:
        return Input(x.name, vAge)


def business(x: Input[Valid]) -> None:
    pass


data = {"name": "foo", "age": -5}
inp = Input.from_json(data)

# reveal_type(inp)
# note: Revealed type is 'val.Input[val.Raw]'

# business(inp)
# error: Argument 1 to "business" has incompatible type "Input[Raw]"; expected "Input[Valid]"

vInp = validateInput(inp)
if vInp is not None:
    business(vInp)