How does "TypeError: class set to <class 'A'> defining 'A' as <class 'A'>" occur?

blhsing · March 13, 2024, 3:22pm

Since dataclass._add_slots gets its namespace from the __dict__ of the current class:

python/cpython/blob/8332e85b2f079e8b9334666084d1f8495cff25c1/Lib/dataclasses.py#L1183


      
          
          def _add_slots(cls, is_frozen, weakref_slot):
              # Need to create a new class, since we can't set __slots__
              #  after a class has been created.
          
              # Make sure __slots__ isn't already set.
              if '__slots__' in cls.__dict__:
                  raise TypeError(f'{cls.__name__} already specifies __slots__')
          
              # Create a new dict for our new class.
              cls_dict = dict(cls.__dict__)
              field_names = tuple(f.name for f in fields(cls))
              # Make sure slots don't overlap with those in base classes.
              inherited_slots = set(
                  itertools.chain.from_iterable(map(_get_slots, cls.__mro__[1:-1]))
              )
              # The slots for our class.  Remove slots from our base classes.  Add
              # '__weakref__' if weakref_slot was given, unless it is already present.
              cls_dict["__slots__"] = tuple(
                  itertools.filterfalse(
                      inherited_slots.__contains__,

I solved the problem by propagating __classcell__ as a class attribute for dataclass to create a namespace with, and deleting it afterwards:

_dataclass_keywords = set((code := dataclass.__code__).co_varnames[
    code.co_argcount:code.co_argcount + code.co_kwonlyargcount])

@dataclass_transform()
class DataclassMeta(type):
    def __new__(metacls, name, bases, namespace, **kwargs):
        class InitSubclassBlocker:
            def __init_subclass__(cls, **kwargs):
                pass
        configured_dataclass = dataclass(**{key: kwargs.pop(key)
            for key in kwargs.keys() & _dataclass_keywords})
        cls = super().__new__(
            metacls, name, (InitSubclassBlocker,) + bases, namespace, **kwargs)
        if classcell := namespace.get('__classcell__'):
            cls.__classcell__ = classcell
        cls = configured_dataclass(cls)
        if hasattr(cls, '__classcell__'):
            del cls.__classcell__
        del InitSubclassBlocker.__init_subclass__
        super(cls, cls).__init_subclass__(**kwargs)
        return cls

Demo here

How does "TypeError: __class__ set to <class 'A'> defining 'A' as <class 'A'>" occur?

How does "TypeError: class set to <class 'A'> defining 'A' as <class 'A'>" occur?