python what is the type of typing.Optional [duplicate]

Question

I would like to use typing's get_type_hints method to get argument annotations. However I encountered this problem in Python3.6.8

a = typing.Optional[int]
type(a)
Out[13]: typing.Union
type(a) == typing.Union
Out[14]: False
type(a) == type(typing.Optional)
Out[23]: False
type(a) == type(typing.Optional[int])
Out[24]: True
repr(type(a))
Out[25]: 'typing.Union'
repr(typing.Union)
Out[26]: 'typing.Union'

It seems that there's no generic way to judge whether a type is typing.Optional or not beside comparing the repr which is not very pythonic. Is there a hack to it?

P.S. in 3.7 there's typing._GenericAlias and it works perfectly fine.

Answer 1

I believe this is being answered in this post Check if a field is typing.Optional.

I also pasted it below:

Optional[X] is equivalent to Union[X, None]. So you could do,

import re
from typing import Optional

from dataclasses import dataclass, fields


@dataclass(frozen=True)
class TestClass:
    required_field_1: str
    required_field_2: int
    optional_field: Optional[str]


def get_optional_fields(klass):
    class_fields = fields(klass)
    for field in class_fields:
        if (
            hasattr(field.type, "__args__")
            and len(field.type.__args__) == 2
            and field.type.__args__[-1] is type(None)
        ):
            # Check if exactly two arguments exists and one of them are None type
            yield field.name


print(list(get_optional_fields(TestClass)))