How to check if a variables fits a custom type

Question

I have this code:

from typing import Tuple, Dict, List

CoordinatesType = List[Dict[str, Tuple[int, int]]]

coordinates: CoordinatesType = [
    {"coord_one": (1, 2), "coord_two": (3, 5)},
    {"coord_one": (0, 1), "coord_two": (2, 5)},
]

I would like to check at runtime if my variable fits my custom type definition. I was thinking on something like:

def check_type(instance, type_definition) -> bool:
    return isinstance(instance, type_definition)

But obviously isinstance is not working. I need to check this at runtime, what would be the correct way to implement it?

Answer 1

• Try to use Typeguard:https://typeguard.readthedocs.io/en/latest/userguide.html#using-type-checker-functions

Example:

code:

from typeguard import check_type
from typing import Tuple, Dict, List
coordinates = [
    {"coord_one": (1, 2), "coord_two": (3, 5)},
    {"coord_one": (0, 1), "coord_two": (2, 5)},
]
try:
    check_type('coordinates', coordinates, List[Dict[str, Tuple[int, int]]])
    print("type is correct")
except TypeError as e:
    print(e)

coordinates = [
    {"coord_one": (1, 2), "coord_two": ("3", 5)},
    {"coord_one": (0, 1), "coord_two": (2, 5)},
]
try:
    check_type('coordinates', coordinates, List[Dict[str, Tuple[int, int]]])
    print("type is correct")
except TypeError as e:
    print(e)

result:

type is correct
type of coordinates[0]['coord_two'][0] must be int; got str instead