TypeError: with a diamond-like multiple inheritance

[roynico]

Hello,

I encounter a problem when trying to use attrs class with a diamond-like inheritance:
The error :

...  class D(B, C):
TypeError: multiple bases have instance lay-out conflict

Minimal Example:

from attrs import define, field

@define
class A:
    a: str = field()

@define
class B(A):
    b: str = field()

@define
class C(A):
    c: str = field()

@define
class D(B, C):
    d: str = field()

I do not understand why there should be any conflict in this example.

[hynek]

This is because @define uses slots by default and it's not possible to inherit from more than one slotted class.

See also our glossary: https://www.attrs.org/en/stable/glossary.html#term-slotted-classes

Usually that's not a problem, because diamond-like inheritance is generally considered bad design. If you want to force it, pass (slots=False) to the last define.

[roynico]

Many thanks for the quick answer !