Exhaustiveness Research

[johnthagen]

typing.assert_never()

• Exhaustiveness checking util python/typing#735 (comment)
  • bpo-46475: Add typing.Never and typing.assert_never python/cpython#30842
• https://docs.python.org/3.11/library/typing.html#typing.assert_never

Mypy Exhaustiveness Checking

• Literal: https://mypy.readthedocs.io/en/stable/literal_types.html#exhaustiveness-checking
• Enum: https://mypy.readthedocs.io/en/stable/literal_types.html#id3
• Exhaustive Matching on Enums / Literals python/mypy#6366

[johnthagen]

I ran into an issue testing mypy. assert_never() is quite young (only a month or so old) so there could be some bugs:

• assert_never() throws error on Python 3.10 python/mypy#12613

[johnthagen]

Here is an example that works in the Python 3.10 and current mypy that can be used to play around with exhaustiveness checking.

from enum import Enum, auto

from typing_extensions import NoReturn


# TODO: Remove the need for this when mypy > 0.942 is released.
#     At that point, simply import this from `typing_extensions`
def assert_never(__arg: NoReturn) -> NoReturn:
    raise AssertionError("Expected code to be unreachable")


class Color(Enum):
    Red = auto()
    Green = auto()
    Blue = auto()


def c(color: Color) -> int:
    match color:
        case Color.Red:
            pass
        case Color.Blue:
            pass
        case _ as u:
            assert_never(u)


def int_or_str(arg: int | str) -> None:
    match arg:
        case int():
            print("It's an int")
        case str():
            print("It's a str")
        case _ as unreachable:
            assert_never(unreachable)

[johnthagen]

While exhaustiveness checking does not work currently on the proposed base-class method without @sealed, it does work with the existing Union method:

from dataclasses import dataclass
from enum import Enum, auto

# TODO: Remove the need for this when mypy > 0.942 is released.
#     At that point, simply import this from `typing_extensions`
def assert_never(__arg: NoReturn) -> NoReturn:
    raise AssertionError("Expected code to be unreachable")


class UsState(Enum):
    Alabama = auto()
    Alaska = auto()
    # --snip--


@dataclass
class Nickle:
    ...


@dataclass
class Dime:
    ...


@dataclass
class Quarter:
    us_state: UsState


Coin = Nickle | Dime | Quarter


def value_in_cents(coin: Coin) -> int:
    match coin:
        case Nickle():
            return 5
        # try removing one of the `case`s
        case Dime():
            return 10
        case Quarter(state):
            print(f"State quarter from {state}")
            return 25
        case _ as u:
            assert_never(u)