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rustshed

Python 3.10 codecov license

A collection of Rust types in Python with complete type annotations.

Supported Types

  • Option
  • Result

Install

pip install rustshed

Examples

Option

The Option type represents an optional value: every Option[T] is either Some[T] and contains a value of type T, or Null (None in Rust), and does not.

from typing import SupportsIndex, TypeVar

from rustshed import Null, Option, Some

T = TypeVar("T")


class SafeList(list[T]):
    def get(self, index: SupportsIndex) -> Option[T]:
        try:
            return Some(self[index])
        except IndexError:
            return Null

a_list = SafeList([2, 1, 3, 7])
print(a_list.get(1))  # Some(value=1)
print(a_list.get(420))  # Null

Result

The Result is the type used for returning and propagating errors: every Result[T, E] is either Ok[T], representing success and containing a value of type T, or Err[E], representing failure and containing an error of type E.

from rustshed import to_result, Result


@to_result[ValueError]
def parse(x: str) -> int:
    return int(x)


def multiply(a: str, b: str) -> Result[int, str]:
    # try to parse two strings and multiply them
    # map a possible error to str
    return parse(a).and_then(lambda n: parse(b).map(lambda m: n * m)).map_err(str)


print(multiply("21", "2"))  # Ok(value=42)
print(multiply("2!", "2"))  # Err(error="invalid literal for int() with base 10: '2!'")

Rust's question mark (?) operator

The question mark (?) operator in Rust hides some of the boilerplate of propagating errors up the call stack. Implementing this operator in Python would require changes to the language grammar, hence in rustshed it had to be implemented differently.

Q property

The question mark's functionality has been implemented via the Q property (for both Option and Result types) combined with the rustshed.result_shortcut or rustshed.option_shortcut decorator.

from rustshed import Ok, Result, to_result, result_shortcut


@to_result[ValueError]
def parse(x: str) -> int:
    return int(x)


# explicit early error return with match statements
def try_to_parse_early_return(a: str, b: str) -> Result[int, ValueError]:
    match parse(a):
        case Ok(value):
            x = value
        case err:
            return err

    match parse(b):
        case Ok(value):
            y = value
        case err:
            return err

    return Ok(x + y)


# early error return using the Q property
@result_shortcut
def try_to_parse(a: str, b: str) -> Result[int, ValueError]:
    x = parse(a).Q
    y = parse(b).Q
    return Ok(x + y)