Python Result type simulating Rust's Result enum.

py_result.py

from typing import Generic, Optional, TypeVar
from abc import ABC, abstractmethod
from dataclasses import dataclass

T = TypeVar('T')
E = TypeVar('E')


class Result(Generic[T, E], ABC):
    @abstractmethod
    def ok(self) -> Optional[T]:
        pass

    @abstractmethod
    def err(self) -> Optional[E]:
        pass


@dataclass
class Ok(Result[T, E]):
    value: T

    def ok(self) -> Optional[T]:
        return self.value

    def err(self) -> Optional[E]:
        return None


@dataclass
class Err(Result[T, E]):
    error: E

    def ok(self) -> Optional[T]:
        return None

    def err(self) -> Optional[E]:
        return self.error


def test(x: int) -> Result[int, str]:
    if x > 10:
        return Ok(x)
    else:
        return Err("error")


def main():
    if result := test(20).ok():  # same as `if let Ok(result) = test(20)` in Rust
        print(result + 42)  # the type checker knows result is `int` here

    # New in 3.10
    match test(8):
        case Ok(result):
            print(result + 42)
        case Err(e):
            print(e + " :(")


if __name__ == "__main__":
    main()