Re: [Python-Dev] Should the dataclass frozen property apply to subclasses?

On 2/22/2018 1:56 AM, Raymond Hettinger wrote:

When working on the docs for dataclasses, something unexpected came up. If a dataclass is specified to be frozen, that characteristic is inherited by subclasses which prevents them from assigning additional attributes:

>>> @dataclass(frozen=True) class D: x: int = 10

>>> class S(D): pass

>>> s = S() >>> s.cached = True Traceback (most recent call last): File "", line 1, in <module> s.cached = True File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/dataclasses.py", line 448, in _frozen_setattr raise FrozenInstanceError(f'cannot assign to field {name!r}') dataclasses.FrozenInstanceError: cannot assign to field 'cached'

This is because "frozen-ness" is implemented by adding __setattr__ and __delattr__ methods in D, which get inherited by S.

Other immutable classes in Python don't behave the same way:

>>> class T(tuple): pass

>>> t = T([10, 20, 30]) >>> t.cached = True

>>> class F(frozenset): pass

>>> f = F([10, 20, 30]) >>> f.cached = True

>>> class B(bytes): pass

>>> b = B() >>> b.cached = True

The only way I can think of emulating this is checking in __setattr__ to see if the field name is a field of the frozen class, and only raising an error in that case. A related issue is that dataclasses derived from frozen dataclasses are automatically "promoted" to being frozen.

...

...

@dataclass(frozen=True) ... class A: ... i: int ... @dataclass ... class B(A): ... j: int ... b = B(1, 2) b.j = 3 Traceback (most recent call last): File "<stdin>", line 1, in <module> File "C:\home\eric\local\python\cpython\lib\dataclasses.py", line 452, in _frozen_setattr raise FrozenInstanceError(f'cannot assign to field {name!r}') dataclasses.FrozenInstanceError: cannot assign to field 'j'

Maybe it should be an error to declare B as non-frozen? Eric.