Reported by Raymond Hettinger:

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 "<pyshell#49>", 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'

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


Raymond

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'

If this can't be addressed before 3.7, maybe it should be an error to declare B as non-frozen and then we can properly fix it in a future release.

See https://mail.python.org/pipermail/python-dev/2018-February/152320.html for a discussion.

New changeset f199bc655eb50c28e94010714629b376bbbd077b by Eric V. Smith in branch 'master':
bpo-32953: Dataclasses: frozen should not be inherited for non-dataclass derived classes (#6147)
https://github.com/python/cpython/commit/f199bc655eb50c28e94010714629b376bbbd077b

New changeset 45648312e540cda3b10109b6a808cbf6955c84eb by Eric V. Smith (Miss Islington (bot)) in branch '3.7':
bpo-32953: Dataclasses: frozen should not be inherited for non-dataclass derived classes (GH-6147) (GH-6148)
https://github.com/python/cpython/commit/45648312e540cda3b10109b6a808cbf6955c84eb