How to create Haskell-like sum types in Python?

Question

If I have some simple sum type in Haskell, like

data Owner = Me | You

How do I express that in Python in a convenient way?

Does this answer your question? How to specify multiple return types using type-hints — Georgy
– Georgy, Commented Feb 22, 2021 at 9:04
@Georgy Unfortunately no. That answer shows how to express classes of values, i.e., int or str. I want to limit it to values, i.e., 1 and 2, or the strings me and you. — dmvianna
– dmvianna, Commented Feb 22, 2021 at 9:18
Oh, I see. Then you are looking for typing.Literal. Here is a fitting duplicate question: Type hint for a function that returns only a specific set of values — Georgy
– Georgy, Commented Feb 22, 2021 at 10:03
Are you looking for static types or runtime types? An enum might be appropriate for the latter. — MisterMiyagi
– MisterMiyagi, Commented Feb 22, 2021 at 18:01
For the other commenters the poster is asking about a tagged union or sum types en.wikipedia.org/wiki/Tagged_union — Fuji
– Fuji, Commented Apr 21, 2021 at 8:32

Ry- · Accepted Answer · 2025-01-28 07:42:10Z

4

An Enum or a Union is the closest thing to a tagged union or sum type in Python.

Enum

from enum import Enum

class Owner(Enum):
    Me = 1
    You = 2

Union

https://docs.python.org/3/library/typing.html#typing.Union

import typing

class Me:
    pass
class You:
    pass

owner: typing.Union[Me, You] = Me

edited Jan 28 at 7:42

Ry-♦

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answered Apr 21, 2021 at 8:39

Fuji

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1 Comment

MisterMiyagi Over a year ago

It would be helpful if you could try to translate the sample given in the question. While these constructs are theoretically close, it's not entirely clear to me that these two actually match the desired effect. Since Python values are natively "tagged" with their type, the effect of a tagged union may be achieved otherwise – for example, by doing nothing.

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