Datatypes (attr), sum types (sumtypes) and pattern matching (pampy) in Python 3.7

maybe.py

from attr import attrs, attrib, validators
from sumtypes import sumtype, constructor
from pampy import match
from typing import Any, Dict


@attrs
class Valid:
    valid_id: int = attrib()
    valid_description: str = attrib()
    valid_content: Dict[str, Any] = attrib()


@sumtype
class Maybe:
    Just = constructor(valid=attrib(validator=validators.instance_of(Valid)))
    Nothing = constructor()


def useless(i: int) -> Maybe:
    if i > 3:
        return Maybe.Just(Valid(i, 'test', {'a': i, 'b': 'nope'}))
        # throws a TypeError: return Maybe.Just(1)
    return Maybe.Nothing()


def what_is(x: Maybe) -> str:
    return match(x,
                 Maybe.Just, 'Just ' + repr(x),
                 Maybe.Nothing, 'Nothing')


if __name__ == '__main__':
    just_i = useless(4)
    nothing = useless(1)
    print(what_is(just_i))
    print(what_is((nothing)))

Even more idiomatic:

from dataclasses import dataclass
from pampy import match
from typing import Any, Dict


@dataclass
class MaybeValid:
    pass


@dataclass
class Just(MaybeValid):
    valid_id: int
    valid_description: str
    valid_content: Dict[str, Any]


@dataclass
class Nothing(MaybeValid):
    pass


def useless(i: int) -> MaybeValid:
    if i > 3:
        return Just(i, 'test', {'a': i, 'b': 'nope'})
    return Nothing()


def what_is(x: MaybeValid) -> str:
    return match(x,
                 Just, repr(x),
                 Nothing, 'Nothing')


if __name__ == '__main__':
    just_i = useless(4)
    nothing = useless(1)
    print(what_is(just_i))
    print(what_is((nothing)))