How to annotate that a classmethod returns an instance of that class

Question

Using PEP 484, is there a way to annotate that a classmethod returns an instance of that class?

e.g.

@dataclass
class Bar:

    foo: str

    @classmethod
    def new_from_foo(cls, foo) -> Bar
        ...

or

    @classmethod
    def new_from_foo(cls, foo) -> cls

Doesn't it work? It should work in Python3.7: https://docs.python.org/3/whatsnew/3.7.html#pep-563-postponed-evaluation-of-annotations — Sraw, Aug 02 '18 at 07:31
I'm using the backport. https://pypi.org/project/dataclasses/ — Gary van der Merwe, Aug 02 '18 at 07:44
Don't use `Bar` as the return annotation. A proper classmethod should also support subclassing. You don't want `BarSubclass.new_from_foo()` to return a `Bar` instance, do you? — Aran-Fey, Aug 02 '18 at 07:47
Yip - was just saying I agreed. Trying out your solution atm. — Gary van der Merwe, Aug 02 '18 at 07:54
Does this answer your question? [How do I specify that the return type of a method is the same as the class itself?](https://stackoverflow.com/questions/33533148/how-do-i-specify-that-the-return-type-of-a-method-is-the-same-as-the-class-itsel) — suvayu, Mar 22 '20 at 19:54

score 12 · Accepted Answer · answered Aug 02 '18 at 07:33

12

The trick is to use a TypeVar to connect the cls parameter to the return annotation:

from typing import TypeVar, Type

T = TypeVar('T')

class Bar:
    @classmethod
    def new_from_foo(cls: Type[T], foo) -> T:
        ...

answered Aug 02 '18 at 07:33

Aran-Fey

This works, but somehow dose not work with dataclasses. I will investigate to try understand why. – Gary van der Merwe Aug 02 '18 at 08:08
@GaryvanderMerwe Huh, you're right. I didn't realize. I'll investigate that. – Aran-Fey Aug 02 '18 at 08:11
3

Looks like a bug in mypy to me, honestly. As far as I can tell, it only happens in classmethods - and only in dataclasses. Edit: Found the relevant [github issue](https://github.com/python/mypy/issues/5263). – Aran-Fey Aug 02 '18 at 08:24

1 Answers1