python frozen dataclass immutable with object.__setattr__

Question

I used namedtuples for immutable data structures until I came across dataclasses, which I prefer in my use-cases (not relevant to the question).

Now I learned that they are not immutable! At least not strictly speaking.

setattr(frozen_dc_obj, "prop", "value") raises an exception. ok.
But why does object.__setattr__(frozen_dc_obj,..) work?

Compared to namedtuple, where it raises an exception!

from collections import namedtuple
from dataclasses import dataclass


NTTest = namedtuple("NTTest", "id")

nttest = NTTest(1)
setattr(nttest, "id", 2)  # Exception
object.__setattr__(nttest, "id", 2)  # Exception


@dataclass(frozen=True)
class DCTest:
    id: int

dctest = DCTest(1)
setattr(dctest, "id", 2)  # Exception
object.__setattr__(dctest, "id", 2)  # WORKS

Answer 1

namedtuple defines __slots__ = () and hence you can't set any attribute (it doesn't have a __dict__).

Frozen dataclasses on the other hand perform a manual check in their __setattr__ method and raise an exception if it's a frozen instance.

Compare the following:

>>> class Foo:
...     __slots__ = ()
... 
>>> f = Foo()
>>> f.__dict__  # doesn't exist, so object.__setattr__ won't work
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'Foo' object has no attribute '__dict__'
>>> @dataclass(frozen=True)
... class Bar:
...     pass
... 
>>> b = Bar()
>>> b.__dict__  # this exists, so object.__setattr__ works
{}