Confusing error about __attrs_attrs__ when using multiple inheritance

[JukkaL]

This code now generates an error about class AB:

import attr

@attr.s
class A:
    x = attr.ib(type=int)

@attr.s
class B:
    y = attr.ib(type=int)

# Definition of "__attrs_attrs__" in base class "A" is incompatible with definition in base class "B"
class AB(A, B):
    pass

The code is arguably questionable, and the error goes away if we decorate AB with @attr.s, but that's not clear from the error message. It would be better to add a note, such as something like this:

... :note: Should class "AB" use the @attr.s decorator?

This started happening after #11794, and it might be considered a minor regression because of the confusing message.

(Behind the scenes, a __attrs_attrs__ attribute gets generated with a class-specific named tuple as the type. These named tuples aren't compatible with each other.)

cc @Tinche

[Tinche]

Interesting, I can take a look at this.

[Tinche]

In runtime,

>>> AB.__attrs_attrs__
(Attribute(name='x', default=NOTHING, validator=None, repr=True, eq=True, eq_key=None, order=True, order_key=None, hash=None, init=True, metadata=mappingproxy({}), type=<class 'int'>, converter=None, kw_only=False, inherited=False, on_setattr=None),)

So I'm guessing it just follows the MRO and gets the class attribute from A.

Here's the equivalent situation without attrs:

class C:
    _test_: str = "a"


class D:
    _test_: int = 1


class CD(C, D):
    pass

This is a Mypy error (error: Definition of "_test_" in base class "C" is incompatible with definition in base class "D").

So if we want to keep this behavior and still raise an error, the note approach is the way to go?

[JukkaL]

Yeah, the note approach seems fine. If you don't use @attr.s in the derived class, the inherited __init__ doesn't initialize all the attributes, the value of __attrs_attrs__ won't include all attributes, etc.

[JelleZijlstra]

This feels similar to #12008. (Does attrs autocreate __match_args__?)

[Tinche]

This feels similar to #12008. (Does attrs autocreate __match_args__?)

It does.