How would I constrain a method to only return one of a set of specific values in python?

Question

Is there a pythonic way to constrain the output of a method so that it can only be one of a set? Sort of like typing but for specific values only. I hope you can see what I’m trying to get at with this snippet:

class Rule:
    def evaluate(self, user_id: int) -> {"PASS", "FAIL", "ERROR"}:
        ...

In the above case I would be hoping for evaluate to only return "PASS", "FAIL" or "ERROR"

Asked By: Kit

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Source

Answer 1

You could create a separate class with all the description and add it as arguments whilst coding. I would recommend you to to watch a few youtube videos of how to create classes, and maybe you could try implement it.

Answered By: Joseph Mattouk

Answer 2

In python 3.8 or higher, you can use Literal types:

from typing import Literal

class Rule:
    def evaluate(self, user_id: int) -> Literal["PASS", "FAIL", "ERROR"]:
        ...

Answered By: jprebys

Answer 3

Yes there is. Commonly Enum is used for this. (https://docs.python.org/3/library/enum.html)

from enum import Enum


class Evaluate(Enum):
    PASS = "value1"
    FAIL = "value2"
    ERROR = "value3"


class Rule:
    def evaluate(self, user_id: int) -> Evaluate:
        ...

Answered By: João Bonfim

How would I constrain a method to only return one of a set of specific values in python?

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