Multiple inheritance error when using slots

[jacobg]

The following classes definitions causes an error:

    @attr.s(auto_attribs=True, frozen=True, slots=True, collect_by_mro=True)
    class Foo:
        foo: str

    @attr.s(auto_attribs=True, frozen=True, slots=True, collect_by_mro=True)
    class Bar:
        bar: str

    @attr.s(auto_attribs=True, frozen=True, slots=True, collect_by_mro=True)
    class Baz(Foo, Bar):
        baz: str

>       class Baz(Foo, Bar):
E       TypeError: multiple bases have instance lay-out conflict

Note that this error occurs on master with a commit that seems to purport to fix this problem:
#635

Removing the slots decorator kwargs resolves the error.

Is there a workaround that supports slots?

[hynek]

To be clear: #635 has nothing to do with this problem and the problem exists in 19.3 too.

I suspect it's related to the way we create slotted classes in

attrs/src/attr/_make.py

Line 711 in b02335f

cls = type(self._cls)(self._cls.__name__, self._cls.__bases__, cd)

Since slots become True by default in NG, we should have a look if we can fix this.

[Tinche]

An even smaller test case:

import attr


@attr.s(slots=True)
class A:
    a = attr.ib()


@attr.s(slots=True)
class B:
    b = attr.ib()


@attr.s(slots=True)
class C(A, B):
    pass

[Tinche]

Actually, C doesn't even have to be an attrs class:

import attr


@attr.s(slots=True)
class A:
    a = attr.ib()


@attr.s(slots=True)
class B:
    b = attr.ib()


class C(A, B):
    pass

In fact, browsing the Python docs (https://docs.python.org/3/reference/datamodel.html#notes-on-using-slots):

Multiple inheritance with multiple slotted parent classes can be used, but only one parent is allowed to have attributes created by slots (the other bases must have empty slot layouts) - violations raise TypeError.

So this is a Python limitation.

[hynek]

Thank you for the research Tin! I will add an entry to the glossary.

[jacobg]

@Tinche @hynek Thank you for looking into this issue!