neverraise

A mix of Rust's Result type and the neverthrow typescript library.

from neverraise import Result, Ok, Err


def divide(a: int, b: int) -> Result[float, ZeroDivisionError]:
    if b == 0:
        return Err(ZeroDivisionError())
    return Ok(a / b)

match divide(1, 0):
    case Ok(res): print(f"Got {res}")
    case Err(): print("Couldn't divide")

def get_todos() -> ResultAsync[list[Todo], HTTPError | JsonParseError | DecodeError]:
    client = httpx.AsyncClient()
    return (
        ResultAsync.from_coro(
            client.get("https://jsonplaceholder.typicode.com/todos/"),
            lambda e: HTTPError(e),
        )
        .try_catch(lambda response: response.json(), lambda e: JsonParseError(e))
        .try_catch(
            lambda json: msgspec.convert(json, type=list[Todo]),
            lambda e: DecodeError(e),
        )
    )

Important

Functions that return neverraise.Result or neverraise.ResultAsync do not throw exceptions, apart from

• UnwrapError caused by calling
  • map when the mapping function throws (use .try_catch for map operations that can fail)
  • unwrap, expect, unwrap_err, or expect_err when called on a non-matching variant of Result (e.g. Err("oh no").unwrap())
• Any subclass of BaseException such as asyncio.CancelledError.

Do not catch UnwrapError, as they are not expected to be raised. If one is raised, it means that the code is incorrect. It should be treated as a failed assertion.