Add ability to set __slots__ in dataclasses

[ericvsmith]

BPO	42269
Nosy	@ericvsmith, @jstasiak, @MojoVampire, @pbryan, @uriyyo, @Xtrem532, @lcy0321
PRs	bpo-42269: Add slots parameter to dataclass decorator #24171

^{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 = <Date 2021-05-01.02:15:08.358>
created_at = <Date 2020-11-05.15:38:14.065>
labels = ['type-bug', 'library', '3.10']
title = 'Add ability to set __slots__ in dataclasses'
updated_at = <Date 2021-05-01.02:15:08.358>
user = 'https://github.com/ericvsmith'

bugs.python.org fields:

activity = <Date 2021-05-01.02:15:08.358>
actor = 'eric.smith'
assignee = 'eric.smith'
closed = True
closed_date = <Date 2021-05-01.02:15:08.358>
closer = 'eric.smith'
components = ['Library (Lib)']
creation = <Date 2020-11-05.15:38:14.065>
creator = 'eric.smith'
dependencies = []
files = []
hgrepos = []
issue_num = 42269
keywords = ['patch']
message_count = 7.0
messages = ['380416', '380734', '380740', '380759', '384690', '392563', '392564']
nosy_count = 7.0
nosy_names = ['eric.smith', 'jstasiak', 'josh.r', 'pbryan', 'uriyyo', 'Xtrem532', 'lcy0321']
pr_nums = ['24171']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue42269'
versions = ['Python 3.10']

[ericvsmith]

I resisted adding the ability to set __slots__ in the first version of dataclasses, since it requires that instead of modifying an existing class, an entirely new class is returned.

But I think this feature would be useful enough that I'm now willing to add it. I have the code ready, I just need to work on tests and documentation.

[MojoVampire]

Is the plan to allow an argument to auto-generate __slots__, or would this require repeating the names once in __slots__, and once for annotations and the like?

[ericvsmith]

It would figure it out automatically. See https://github.com/ericvsmith/dataclasses/blob/master/dataclass_tools.py for a decorator that already does this.

I'll have a PR ready soon, I hope.

[jstasiak]

As a moderately-heavy dataclass user I support this. :)

[uriyyo]

Hi Eric, I tried to help you with this feature and have opened a PR. I thought that you are too busy to implement this feature, so that's why I decided to help you (It almost two months since your last message in this thread).

[ericvsmith]

New changeset c241991 by Yurii Karabas in branch 'master':
bpo-42269: Add slots parameter to dataclass decorator (GH-24171)
c241991

[ericvsmith]

Thanks for all of your work, Yurii.