Mixed inheritance between slotted and non slotted classes leads to issues with parallelisation.

Mixed inheritance between slotted and non-slotted classes seems to lead to some issues when object are copied for parallelization.

The following code example does not work and raises: AttributeError: 'Child' object has no attribute 'z'

import attrs
import multiprocessing as mp

@attrs.define(slots=False, kw_only=True)
class Parent:
    name: str 
    
@attrs.define
class Child(Parent):
    x: float
    
    def __attrs_post_init__(self):
        self.z = self.x + 2

    def arg_plus_z(self, arg):
        return self.z + arg

test = Child(3, name = "as")

with mp.Pool(processes=4) as pool:
    result = pool.map(test.arg_plus_z, range(100))

whilst adding slots=False to the child class or registering z using z: float = attrs.field(default = None, init=False) does work:

import attrs
import multiprocessing as mp

@attrs.define(slots=False, kw_only=True)
class Parent:
    name: str 
    
@attrs.define(slots=False)
class Child(Parent):
    x: float
    
    def __attrs_post_init__(self):
        self.z = self.x + 2

    def arg_plus_z(self, arg):
        return self.z + arg

test = Child(3, name = "as")

with mp.Pool(processes=4) as pool:
    result = pool.map(test.arg_plus_z, range(100))

It seems like the object copy of test passed to each one of the processes does not copy the __dict__ (which includes z and name) but only the __slots__.

I know that according to the attrs documentation mixed inheritance between slotted and non slotted classes is considered a bad practice. I stumbled across this by accident and it might also be more of a missing feature than a bug.

Meta:

• attrs-version: '21.4.0'
• python-version: '3.9.13'

Start with the provided mixed-inheritance reproducer and inspect how the Child instance is copied for multiprocessing, particularly the interaction between dict and slots. The work is done when the copied object retains z and name and the pool example succeeds, with regression coverage for the failing case.