Python: parameter type hint during run time

Question

Is it possible to check the parameter type hint at run time? I would like to use the Python dataclasses in a similar fashion to a Scala case class with pureconfig and HOCON config file. That is, I want to have some optional parameters but would need to check them in another class constructor. However, I dont see how I can get if a param is optional.

from dataclasses import dataclass
from typing import Optional

@dataclass
class Params:
    x: int
    y: int
    z: Optional[int] = None


params = Params(x=2, y=3)

Let's say I want to use the dataclass Params somewhere. If I check z , I will get a NoneType . Maybe I can by pass any errors if I know this by design.

# just some example code 
if z is None and z is not Optional (not sure if or how to check this):
    raise ValueError("z must be specified as an integer")

if z is None and z is optional:
    return x ** 2 + y ** 2
elif z is not None:
    return x ** 2 + y ** 2 + z ** 2

Answer 1

See below. The idea is to use __post_init__ and check the annotations of z . Uncomment #z: int and see how it works.

from dataclasses import dataclass
from typing import Optional


@dataclass
class Params:
    x: int
    y: int
    z: Optional[int] = None
    #z: int

    def __post_init__(self):
        print('post init')
        if self.z is None:
            z_annotation = self.__annotations__['z']
            none_is_ok = False
            args = z_annotation.__dict__.get('__args__')
            if args is not None:
                for entry in args:
                    none_is_ok = entry == type(None)
                    if none_is_ok:
                        break
            if not none_is_ok:
                raise ValueError('z can not be None')


p: Params = Params(3, 5, None)
print(p)