CODEWITHSUNDEEP
pythonpython-typingliterals
Lingster
Lingster

Reputation: 1087

in python 3.8 how to test a field annotated as Literal in a dataclass is valid at run

Given the following example:

from typing import Literal
from dataclasses import dataclass
@dataclass
Class Example:
    answer: Literal['Y', 'N']

x = Example('N')
field = fields(x)[0]

How can I check that the variable field is of type Literal? issubclass(field.type, Literal) doesn't seem to work.

Secondly how can I then get the list ['Y', 'N'] from field.type, so that I could check the value at run time and raise an error when fail = Example('invalid')

pydantic does this but you would have to use their drop in dataclass...

Upvotes: 0

Views: 2959

Answers (3)

tamuhey
tamuhey

Reputation: 3535

I created a tiny Python library for this purpose: https://github.com/tamuhey/dataclass_utils

This library can be applied for such dataclass that holds another dataclass (nested dataclass), and nested container type (like Tuple[List[Dict...). Of course, it can test Literal at runtime.

Upvotes: 0

Evgeniy_Burdin
Evgeniy_Burdin

Reputation: 703

from dataclasses import dataclass
from typing import Literal

from validated_dc import ValidatedDC


@dataclass
class Example(ValidatedDC):
    answer: Literal['Y', 'N']


instance = Example('N')
assert instance.is_valid()


instance = Example('X')
assert not instance.is_valid()

Upvotes: 1

Iain Shelvington
Iain Shelvington

Reputation: 32294

Literal is not a normal type that a python object would have at runtime, it doesn't make sense to check that an object is a Literal

You can access the annotations for a class using __annotations__, following on from your example:

>>> Example.__annotations__['answer'].__args__
('Y', 'N')

Upvotes: 1

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