Check if a field is typing.Optional

Question

What is the best way to check if a field from a class is typing.Optional?

Example code:

from typing import Optional
import re
from dataclasses import         


        

Answer 1

Another approach (That works on both python 3.7 & and 3.8) is to relay on how the set Union operation works:

union([x,y],[y])= union([x],[y]) = union(union([x],[y]),[x,y])

The logic is that Optional type can't be Optionaler. While you can't directly know if a type is nullable/optional, Optional[type] would be the same as type is the type is optional and other (Union[type,None] to be exact) otherwise.

So, in our case:

Union[SomeType,None] == Union[Union[SomeType,None]]

(the first is eqivalent to Optional[SomeType] and the second to Optional[Optional[SomeType]]

This allows very easy check for Optional values:

from dataclasses import dataclass, fields
from typing import Optional


@dataclass()
class DC:
    x: Optional[str] = None
    y: str = "s"


def get_optional_fields(cls):
    fields_list = fields(cls)
    return [
        field.name 
        for field in fields_list if 
        field.type == Optional[field.type]
    ]



if __name__ == '__main__':
    print(get_optional_fields(DC())) # ['x']