Summary

If a child dataclass of a parent dataclass who specified weakref_slot, also decides to set weakref_slot to True, Python will raise a TypeError as seen below:

TypeError: __weakref__ slot disallowed: either we already got one, or __itemsize__ != 0

Description

I am porting dataclasses' slots and weakref slots kwargs into a decorator I can use. While doing so, I decided to change the default of the weakref_slot because I would like all of my classes to be weakref:able. This is when I ran into the issue which can be reproduced by the code below:

from dataclasses import dataclass


@dataclass(slots=True, weakref_slot=True)
class A:
    field: str


@dataclass(slots=True, weakref_slot=True)
class B(A):
    ...

Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
  File "C:\Users\%username%\Projects\testing\copyclasses.py", line 1207, in wrap
    return _process_class(cls, init, repr, eq, order, unsafe_hash,
  File "C:\Users\%username%\Projects\testing\copyclasses.py", line 1108, in _process_class
    cls = _add_slots(cls, frozen, weakref_slot)
  File "C:\Users\%username%\Projects\testing\copyclasses.py", line 1176, in _add_slots    
    cls = type(cls)(cls.__name__, cls.__bases__, cls_dict)
TypeError: __weakref__ slot disallowed: either we already got one, or __itemsize__ != 0

Environment

This was tested by creating a file named copyclasses.py with the current contents of Lib/dataclasses.py ran using a 3.10 CPython interpreter. Note that I've restored the naming of the module in the codeblock above assuming those who will be running it will have a built 3.11 CPython interpreter.

Footnotes: I will mention @ericvsmith as the code owner of the dataclasses file.