Lib/dataclasses.py (part 3)

Source:

cpython 3.14 @ ab2d84fe1023/Lib/dataclasses.py

This annotation covers advanced dataclasses features. See lib_dataclasses_detail2 for the basic @dataclass, field(), and asdict/astuple.

Map

Lines	Symbol	Role
1-100	`InitVar`	Marker: init-only field, not stored
101-250	`KW_ONLY`	Sentinel: all following fields become keyword-only
251-500	`__post_init__` call	Hook after `__init__`
501-750	Frozen dataclass	`__setattr__`/`__delattr__` raising `FrozenInstanceError`
751-1000	Slots dataclass (`slots=True`)	Dynamic `__slots__` via class re-creation
1001-1500	Inheritance rules	Field ordering, frozen + non-frozen mixing

Reading

`InitVar`

# CPython: Lib/dataclasses.py:306 InitVar
class InitVar:
    __slots__ = ('type', )
    def __init__(self, type):
        self.type = type
    def __class_getitem__(cls, type):
        return cls(type)

InitVar[T] fields appear in __init__ but are not stored as instance attributes. They are passed to __post_init__ as positional arguments.

# CPython: Lib/dataclasses.py usage
@dataclass
class C:
    x: int
    y: InitVar[int]   # passed to __post_init__, not stored

    def __post_init__(self, y):
        self.x += y

`KW_ONLY`

# CPython: Lib/dataclasses.py:350 KW_ONLY
KW_ONLY = KW_ONLY()   # sentinel

@dataclass
class C:
    x: int
    _: KW_ONLY
    y: int   # keyword-only from here

After KW_ONLY, all fields are keyword-only in __init__. This was added in Python 3.10.

Frozen dataclass

# CPython: Lib/dataclasses.py:820 _frozen_setattr
def _frozen_setattr(self, name, value):
    if type(self).__dataclass_fields__ and name in type(self).__dataclass_fields__:
        raise FrozenInstanceError('cannot assign to field {!r}'.format(name))
    object.__setattr__(self, name, value)

@dataclass(frozen=True) generates __setattr__ and __delattr__ that raise FrozenInstanceError for dataclass fields but allow setting non-field attributes.

`slots=True` (3.10+)

# CPython: Lib/dataclasses.py:1020 _process_class (slots path)
if slots:
    # Re-create the class with __slots__
    cls_dict = {k: v for k, v in cls.__dict__.items() if k not in field_names}
    cls_dict['__slots__'] = tuple(field_names)
    new_cls = type(cls)(cls.__name__, cls.__bases__, cls_dict)
    # Copy attributes
    ...
    return new_cls

Because __slots__ must be declared at class creation time, @dataclass(slots=True) creates a brand-new class, copying over methods and non-field attributes.

gopy notes

InitVar, KW_ONLY, and __post_init__ are pure Python. The frozen feature requires __setattr__ and __delattr__ to be settable on instances (gopy supports this via the __setattr__ slot on objects.Type). slots=True requires type() to accept a __slots__ dict, which in gopy maps to pre-allocating fixed attribute slots rather than using a __dict__.

Map​

Reading​

InitVar​

KW_ONLY​

Frozen dataclass​

slots=True (3.10+)​

gopy notes​

Map