Dataclasses derived from empty frozen bases skip immutability checks

[hbq1]

BPO	43176
Nosy	@ericvsmith, @miss-islington, @hbq1
PRs	bpo-43176: Fix processing of empty dataclasses #24484 [3.9] bpo-43176: Fix processing of empty dataclasses (GH-24484) #25205 [3.8] bpo-43176: Fix processing of empty dataclasses (GH-24484) #25206 bpo-43176: Fix blurb #25215 [3.9] Fix blurb for bpo-43176. (GH-25215) #25217 [3.8] Fix blurb for bpo-43176. (GH-25215) #25218

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

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GitHub fields:

assignee = 'https://github.com/ericvsmith'
closed_at = None
created_at = <Date 2021-02-09.07:12:39.343>
labels = ['3.8', 'type-bug', 'library', '3.9', '3.10']
title = 'Dataclasses derived from empty frozen bases skip immutability checks'
updated_at = <Date 2021-05-13.05:17:57.697>
user = 'https://github.com/hbq1'

bugs.python.org fields:

activity = <Date 2021-05-13.05:17:57.697>
actor = 'eric.smith'
assignee = 'eric.smith'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2021-02-09.07:12:39.343>
creator = 'hbq1'
dependencies = []
files = []
hgrepos = []
issue_num = 43176
keywords = ['3.8regression', '3.9regression']
message_count = 12.0
messages = ['386695', '390277', '390278', '390280', '390332', '390336', '390339', '390344', '393101', '393103', '393151', '393152']
nosy_count = 4.0
nosy_names = ['eric.smith', 'ulope', 'miss-islington', 'hbq1']
pr_nums = ['24484', '25205', '25206', '25215', '25217', '25218']
priority = 'normal'
resolution = None
stage = 'needs patch'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue43176'
versions = ['Python 3.8', 'Python 3.9', 'Python 3.10']

[hbq1]

Dataclasses derived from empty frozen bases skip immutability checks.

Repro snippet:

import dataclasses

@dataclasses.dataclass(frozen=True)
class Base:
  pass

@dataclasses.dataclass
class Derived(Base):
  a: int

d = Derived(2)
# OK

Usecase: sometimes developers define an empty frozen base dataclass with detailed docs and require other dataclasses to inherit from it as a code contract to avoid accidental in-place modifications.
This bug makes this strategy ineffective.

Affects all versions of Python that support dataclasses (including the backport for py3.6).

[miss-islington]

New changeset 376ffc6 by Iurii Kemaev in branch 'master':
bpo-43176: Fix processing of empty dataclasses (GH-24484)
376ffc6

[miss-islington]

New changeset b132be8 by Miss Islington (bot) in branch '3.8':
bpo-43176: Fix processing of empty dataclasses (GH-24484)
b132be8

[ericvsmith]

Thanks!

[ericvsmith]

New changeset 8a34a07 by Miss Islington (bot) in branch '3.9':
bpo-43176: Fix processing of empty dataclasses (GH-24484) (GH-25205)
8a34a07

[ericvsmith]

New changeset 1744c96 by Eric V. Smith in branch 'master':
Fix blurb for bpo-43176. (GH-25215)
1744c96