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Cannot specify weakref_slot=True twice in dataclass MRO #93521
Description
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.
Activity
ericvsmith commented on Jun 5, 2022
Here's an example of the same thing without dataclasses:
Traceback (most recent call last): File "gh-93521.py", line 4, in <module> class B(A): TypeError: __weakref__ slot disallowed: either we already got one, or __itemsize__ != 0ericvsmith commented on Jun 5, 2022
I'm not sure this is a bug, just the way things work.
JelleZijlstra commented on Jun 6, 2022
It would be nice if dataclasses took care of this issue automatically though.
Example scenario: I have a library in which I expose a dataclass. A user subclasses my class in another dataclass, and wants to use weakrefs, so they use weakref_slots. Then I change my library to allow weakrefs on the base class, but now the user's child class is broken.
Bluenix2 commented on Jun 6, 2022
Locally I fixed this already, and I am planning on opening a PR with an added test. Should that PR point to main or the 3.11 branch (seeing as both branches should get the patch)?
ericvsmith commented on Jun 6, 2022
main
pythonGH-93521: Filter out `__weakref__` slot if present in bases
GH-93521: For dataclasses, filter out `__weakref__` slot if present i…
pythonGH-93521: For dataclasses, filter out `__weakref__` slot if pre…
GH-93521: For dataclasses, filter out `__weakref__` slot if present i…
Revert 30935 defer bpo45162 to 312 (#94285)