EnumHandler

If you're like me, you were disappointed to see that structural pattern matching (PEP 622) landed in Python without any of the proposed exhaustiveness guarantees (PEP 634). Although match and case are still useful, exhaustive matching over an enum is a comfortable idiom in other languages, and I was looking forward to first-class treatment of this in Python. Worse, I ended up writing a number of repetitive exhaustiveness tests:

import pytest

from myproject import Colors, name_of_color

@pytest.parametrize(MyEnum)
def test_name_of_color_exhaustiveness(value):
    assert name_of_color(value)

Tests that don't care about the return value of the function under test are a hallmark of type system failures.

This library fixes that by providing a base class for classes that exhaustively cover an enumeration. Upon class definition, EnumHandler will validate the exhaustiveness and uniqueness of handlers registered on any subclass using the EnumHandler.register decorator.

from enum import Enum, auto

from enumhandler import EnumHandler, handles


class Colors(Enum):
    RED = auto()
    GREEN = auto()
    BLUE = auto()


class NameOfColor(EnumHandler, enum=Colors):
    @handles(Colors.RED)
    def red(self) -> str:
        return "Red"

    @handles(Colors.GREEN)
    def green(self) -> str:
        return "Green"

    @handles(Colors.BLUE)
    def blue(self) -> str:
        return "Blue"

The class is instantiated with a member of the enum, and the resulting instance is callable:

>>> print(NameOfColor(Colors.RED)())
Red

An InvalidEnumHandler exception is raised in three cases:

• Not all enum values are handled.
• One enum value has multiple registered handlers.
• A handler is registered for the incorrect enum.
• A handler is registered on an existing class.

Please note that subclassing is currently not supported.