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 = TrueRaymond

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 = 3Traceback (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.

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

New changesetf199bc655eb50c28e94010714629b376bbbd077b 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 changeset45648312e540cda3b10109b6a808cbf6955c84eb 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