Is it possible to self-reference type in Python attr

Question

I am looking to do something like this:

@attr.s
class A(object):
    a_dict = attr.ib(factory=Dict, type=Dict[str, A], validator=optional(instance_of(Dict)))

It is possible to type it just as type=Dict but I wonder if you can self-reference like type=Dict[str, cls], but it might be limited by possibilities of Python.

Thanks.

Answer 1

If I interpret is correctly, what you want is this:

from typing import Dict, Optional

import attr

from attr.validators import optional, instance_of


@attr.s
class A(object):
    a_dict = attr.ib(
        factory=dict,
        type=Optional[Dict[str, "A"]],
        validator=optional(instance_of(dict))
    )


A({"key": A(None)})

And it passes mypy.

Please note:

• You cannot use typing.Dict for factory because it's really just for type hints and fails if you try to instantiate it.
• instance_of uses isinstance() internally and while isinstance({}, Dict) is True, I don't think it's the intended to be used like that.
• If a_dict is optional as per validator, you also need to declare it optional using type annotations by wrapping it in an Optional[].