Use __new__ instead of __init__ in serializers

[Viicos]

Attempt to fix #260. This is starting to be really tricky to handle, so I don't know if this is a viable option. Regarding ModelSerializer, we might want to define the same overload, and only allow specific types for instance with respect to many (e.g. if many=False, only allow _MT, but I'm unsure there's other types that could be allowed as well).

Note: __init__ has been omitted, otherwise mypy will not infer the correct return type from __new__

[intgr]

This has been in the back of my mind also. It would be great if this works.

AFAIK def __new__(...) -> ListSerializer is not allowed: mypy requires __new__ return type to be a subclass of the class being defined. There's a discussion about this somewhere in mypy issues or PRs, I couldn't find it right now.

But I think it's possible to convince mypy developers to change this requirement.

Please add a test case demonstrating whether this works or not.

[intgr]

The CI errors will be fixed in #316.

[Viicos]

AFAIK def __new__(...) -> ListSerializer is not allowed: mypy requires __new__ return type to be a subclass of the class being defined. There's a discussion about this somewhere in mypy issues or PRs, I couldn't find it right now.

Well ListSerializer is a subclass of BaseSerializer, so this should be alright I guess. Will add tests

EDIT: This is not the case for ModelSerializer, so mypy indeed complains about this:

error: Incompatible return type for "__new__" (returns "ListSerializer[_MT]", but must return a subtype of "ModelSerializer[Any]")

So I've added a type: ignore in the meanwhile

[Viicos]

Ok this seems to work fine when using BaseSerializer:

IntBaseSerializer = serializers.BaseSerializer[int]
test_serializer = IntBaseSerializer([1, 2], many=True)
reveal_type(test_serializer) # N: Revealed type is "rest_framework.serializers.ListSerializer[builtins.int]"

But when defining a custom serializer:

class TestSerializer(serializers.Serializer[int]):
    pass

test_serializer = TestSerializer(instance=[1, 2], many=True)
reveal_type(test_serializer) # N: Revealed type is "main.TestSerializer"

We get the wrong type, possibly because mypy isn't using the __new__ overload defined in BaseSerializer. But I suspect this might be because ListSerializer isn't a subclass of TestSerializer, so mypy isn't giving the correct type.

I did some tests locally, and it seems that this issue is specific to mypy:

from __future__ import annotations

class A:
    def __new__(cls) -> B:
        return B()

class B(A):
    pass

class C(A):
    pass

a = C()
reveal_type(a) # Revealed type is "C", but got B with pyright (Pylance)

I'll open a ticket @mypy, and I think this could be an opportunity to discuss what you said @intgr about the requirement that the return type must be a subclass of the class being defined

[intgr]

I found there's already an open mypy issue about this: python/mypy#8330