Inheritable custom class constructor in python

Question

How can I implement a custom constructor (class method) that is inheritable in python?

The following minimized example might give an idea:

from dataclasses import dataclass
from typing import Type, TypeVar

T = TypeVar("T")


@dataclass
class Parent:
    something: int = 2

    @classmethod
    def from_float(cls: Type[T], something_as_float: float) -> T:
        return Type[T](something=int(something_as_float))


@dataclass
class Child(Parent):
    """ Should also be constructible via from_float
    """


assert isinstance(Parent.from_float(1.0), Parent)
assert isinstance(Child.from_float(1.0), Child)

mypy does not like the constructor I call when returning from from_float. I don't know how to refer to class (Parent or Child) from the class method.

not sure... adding a C++ pattern so that I can implement a Rust pattern in python feels like I would just be adding fuel to the garbage fire though :) — Andrew Wagner
– Andrew Wagner, Commented Sep 28, 2021 at 13:06
The @dataclass implicitly adds the constructor (and __new__too?) that takes the fields, and I can't disturb that. I'm trying to add rust inspired conversion methods onto some existing dataclasses that inherit from each other, and I can't change the dataclasses-with-inheritance part of the design. — Andrew Wagner
– Andrew Wagner, Commented Sep 28, 2021 at 13:11

Iain Shelvington · Accepted Answer · 2021-09-28 13:15:46Z

3

Pass the bound argument to TypeVar to specify that the type is a subclass of Parent. This lets mypy know that the type has the something attribute

When you create the instance use cls not Type[T]

from dataclasses import dataclass
from typing import Type, TypeVar

T = TypeVar("T", bound='Parent')


@dataclass
class Parent:
    something: int = 2

    @classmethod
    def from_float(cls: Type[T], something_as_float: float) -> T:
        return cls(something=int(something_as_float))


@dataclass
class Child(Parent):
    pass


assert isinstance(Parent.from_float(1.0), Parent)
assert isinstance(Child.from_float(1.0), Child)

answered Sep 28, 2021 at 13:15

Iain Shelvington

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2 Comments

Andrew Wagner Over a year ago

Whoooo that did it. Thanks! I can confirm locally that it runs with out asserting, and also passes mypy!

Andrew Wagner Over a year ago

You can't refer a class within its definition... and escaping as cls: "Parent" and -> "Parent" didn't help either. I hit "The erased type of self "minimized_typing_puzzle.Parent" is not a supertype of its class "Type[minimized_typing_puzzle.Parent]"" and then error: "Parent" not callable. Iain's solution works, but feel free to keep the thread going if you enjoy typing puzzles. Thanks!

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