Can you annotate return type when value is instance of cls?

Question

Given a class with a helper method for initialization:

class TrivialClass:
    def __init__(self, str_arg: str):
        self.string_attribute = str_arg

    @classmethod
    def from_int(cls, int_arg: int) -> ?:
        str_arg = str(int_arg)
        return cls(str_arg)

Is it possible to annotate the return type of the from_int method?

I'v tried both cls and TrivialClass but PyCharm flags them as unresolved references which sounds reasonable at that point in time.

Answer 1

In Python 3.11 there is a nicer way to do this using the new Self type:

from typing import Self

class TrivialClass:
    def __init__(self, str_arg: str):
        self.string_attribute = str_arg

    @classmethod
    def from_int(cls, int_arg: int) -> Self:
        str_arg = str(int_arg)
        return cls(str_arg)

This also works correctly with sub classes as well.

class TrivialSubClass(TrivialClasss):
    ...

TrivialSubclass.from_int(42)

The IDE shows return type TrivialSubClass and not TrivialClass.

This is described in PEP 673.