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

Note: typing.Optional[x] is an alias for typing.Union[x, None]

Now, one could inspect the attributes of your input field annotation to check if it is defined like Union[x, None]:
You can read its attributes __module__, __args__ and __origin__:

from typing import *

def print_meta_info(x):
      print(x.__module__, x.__args__, x.__origin__)

x = Optional[int]
print_meta_info(x) # 'typing', (class Int,), typing.Union

x = Union[int, float]
print_meta_info(x) # 'typing', (class int, class float), typing.Union

x = Iterable[str]
print_meta_info(x) # 'typing', (class int,), typing.Iterable

You need to take this steps to define your checker:

1. Make sure that the annotation has the keys __module__, __args__ and __origin__
2. __module__ must be set to 'typing'. If not, the annotation is not an object defined by the typing module
3. __origin__ value is equal to typing.Union
4. __args__ must be a tuple with 2 items where the second one is the class NoneType (type(None))

If all conditions are evaluated to true, you have typing.Optional[x]

You may also need to know what is the optional class in the annotation:

x = Optional[int].__args__[0]
print(x) # class int