ARTICLE AD BOX
I would like to make __slots__-based classes, without having to repeat the attribute names that I've already listed in type annotations.
Before Python 3.14, I could do this:
class C: foo: int bar: int __slots__ = (*__annotations__,) not_a_slot: ClassVar[str] = 'some class variable'Python 3.14, however, switched to lazily-evaluated annotations, which means that when it comes time to assign __slots__, the __annotations__ dict does not yet exist. Python's own best practices say to not use __annotations__ directly after 3.10 anyway.
@dataclass(slots=True) works for python 3.10 and up (with many side-effects). Unfortunately, my code targets a lot of legacy systems, and so has to work both on python 3.8 and on the 3.14 I have on my laptop.
Is there a 3.8-compatible, but future-proof way to achieve a __slots__ based on some subset of annotated attributes, but without repeating the names?
The only workaround I've found myself is to make a class decorator that creates a class with __slots__ set, thus:
from typing import ClassVar, get_origin try: from inspect import get_annotations except ImportError: def get_annotations(x): return x.__annotations__ def annotations_to_slots(klass): class Out(klass): __slots__ = tuple( name for name, declared_type in get_annotations(klass).items() if get_origin(declared_type) is not ClassVar ) Out.__name__ = klass.__name__ try: Out.__doc__ = klass.__doc__ except AttributeError: pass return Out @annotations_to_slots class C: __slots__ = () # will be filled in by @annotations_to_slots foo: int bar: int not_a_slot: ClassVar[str] = 'not a slot'This seems to work, but is ugly for a couple of reasons. The __slots__ end up on a machine-generated class rather than on the class that defined the field, the defining class has to include __slots__ = () which is misleading even with the comment, and the extra class in the MRO makes inheritance less-obvious.
Is there a better approach?
