Dataclasses: frozen should not be inherited for non-dataclass derived classes

[ericvsmith]

BPO	32953
Nosy	@rhettinger, @ericvsmith
PRs	bpo-32953: Dataclasses: frozen should not be inherited for non-dataclass derived classes #6147 [3.7] bpo-32953: Dataclasses: frozen should not be inherited for non-dataclass derived classes (GH-6147) #6148

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

Show more details

GitHub fields:

assignee = 'https://github.com/ericvsmith'
closed_at = <Date 2018-03-19.01:04:16.336>
created_at = <Date 2018-02-25.22:23:51.445>
labels = ['3.7', '3.8', 'type-bug', 'library']
title = 'Dataclasses: frozen should not be inherited for non-dataclass derived classes'
updated_at = <Date 2018-03-19.01:04:16.312>
user = 'https://github.com/ericvsmith'

bugs.python.org fields:

activity = <Date 2018-03-19.01:04:16.312>
actor = 'eric.smith'
assignee = 'eric.smith'
closed = True
closed_date = <Date 2018-03-19.01:04:16.336>
closer = 'eric.smith'
components = ['Library (Lib)']
creation = <Date 2018-02-25.22:23:51.445>
creator = 'eric.smith'
dependencies = []
files = []
hgrepos = []
issue_num = 32953
keywords = ['patch']
message_count = 5.0
messages = ['312866', '312867', '313075', '314070', '314071']
nosy_count = 2.0
nosy_names = ['rhettinger', 'eric.smith']
pr_nums = ['6147', '6148']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue32953'
versions = ['Python 3.7', 'Python 3.8']

[ericvsmith]

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

[ericvsmith]

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.

[ericvsmith]

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

[ericvsmith]

New changeset f199bc6 by Eric V. Smith in branch 'master':
bpo-32953: Dataclasses: frozen should not be inherited for non-dataclass derived classes (bpo-6147)
f199bc6

[ericvsmith]

New changeset 4564831 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)
4564831