Either type in Python

either.py

from typing import Generic, TypeVar


VT = TypeVar("VT")
ET = TypeVar("ET")

class Matchable:
    __match_args__ = ("__match_self_prop__",)
    
    @property
    def __match_self_prop__(self):
        return self


class Left(Matchable, Generic[ET]):
    def __init__(self, error: ET) -> None:
        self.error = error

    def __str__(self) -> str:
        return str(self.error)


class Right(Matchable, Generic[VT]):
    def __init__(self, value: VT) -> None:
        self.value = value

    def __str__(self) -> str:
        return str(self.value)

Either = Left[ET] | Right[VT]

def calculate_ok() -> Either[str, int]:
    return Right(42)

def calculate_bad() -> Either[str, int]:
    return Left("Oops, error!")


x = calculate_ok()
y = calculate_bad()

for el in [x, y]:
    match el:
        case Right(n):
            print(n)
        case Left(err):
            print(err)

# 42
# Oops, error!