Add keyword-only fields to dataclasses

[ericvsmith]

BPO	43532
Nosy	@rhettinger, @ericvsmith, @ryanhiebert, @henryiii, @miss-islington, @tirkarthi, @EpicWink, @jack1142, @kamilturek, @cdce8p, @finite-state-machine
PRs	bpo-43532: Add kw_only to dataclass() and field(), and add KW_ONLY marker. #24909 bpo-43532: Add keyword-only fields to dataclasses. #25608 bpo-43532: add version added to KW_ONLY #31235 [3.10] bpo-43532: add version added to KW_ONLY (GH-31235) #31236

^{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 2022-02-09.21:49:39.659>
created_at = <Date 2021-03-17.17:39:21.912>
labels = ['type-feature', 'library', '3.10']
title = 'Add keyword-only fields to dataclasses'
updated_at = <Date 2022-02-09.21:49:39.658>
user = 'https://github.com/ericvsmith'

bugs.python.org fields:

activity = <Date 2022-02-09.21:49:39.658>
actor = 'eric.smith'
assignee = 'eric.smith'
closed = True
closed_date = <Date 2022-02-09.21:49:39.659>
closer = 'eric.smith'
components = ['Library (Lib)']
creation = <Date 2021-03-17.17:39:21.912>
creator = 'eric.smith'
dependencies = []
files = []
hgrepos = []
issue_num = 43532
keywords = ['patch']
message_count = 11.0
messages = ['388949', '388951', '388984', '391920', '392018', '392115', '398856', '404609', '412943', '412944', '412946']
nosy_count = 11.0
nosy_names = ['rhettinger', 'eric.smith', 'ryanhiebert', 'Henry Schreiner', 'miss-islington', 'xtreak', 'Epic_Wink', 'jack1142', 'kamilturek', 'cdce8p', 'finite-state-machine']
pr_nums = ['24909', '25608', '31235', '31236']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue43532'
versions = ['Python 3.10']

[ericvsmith]

The idea is that a keyword-only field becomes a keyword-only argument to __init__().

For the proposal and a discussion, see https://mail.python.org/archives/list/python-ideas@python.org/message/FI6KS4O67XDEIDYOFWCXMDLDOSCNSEYG/

The @DataClass decorator will get a new parameter, kw_only, which defaults to False. If kw_only=True, all fields in the dataclass will be by efault keyword-only. In addition, field() will have a new kw_only parameter. If true, the field will be keyword-only. If false, it will not be keyword-only. If unspecified, it will use the value of dataclass's kw_only parameter.

In addition, a module-level variable KW_ONLY will be added. If a field has this type, then all fields after it will default to kw_only=True. The field is otherwise completely ignored.

Examples:

@dataclasses.dataclass
class A:
    a: Any = field(kw_only=True) 

Will have __init__(self, *, a)

@dataclasses.dataclass(kw_only=True)
class B:
    a: Any
    b: Any 

Will have __init__(self, *, a, b)

@dataclasses.dataclass
class C:
    a: Any
    _: dataclasses.KW_ONLY
    b: Any
    c: Any

Will have __init__(self, a, *, b, c)

If any non-keyword-only parameters are present, they will be moved before all keyword-only parameters, only for the generated __init__. All other generated methods (repr, __lt__, etc.) will keep fields in the declared order, which is the case in versions 3.9 and earlier.

@dataclasses.dataclass
class D:
    a: Any
    b: Any = field(kw_only=True)
    c: Any

Will have __init__(self, a, c, *, b)

PR to follow.

[tirkarthi]

This looks like a duplicate of https://bugs.python.org/issue33129 or that ticket can be closed.

[rhettinger]

+1 for this idea. I don't see any downside.

[ericvsmith]

I've checked in the code, tests, and some basic documentation. I didn't finish the documentation in order to make sure I could get this in before beta 1. I'm going to leave this issue open until I have time for the rest of the documentation.

[EpicWink]

Will this close https://bugs.python.org/issue36077 ?

[ericvsmith]

Will this close https://bugs.python.org/issue36077 ?

No, not directly, although I'm going to close that issue as "won't fix".

But you can specify that the child class requires all params as kw_only. Continuing the example:

>>> @dataclass(kw_only=True)
... class Child(Parent):
...   y: int
...
>>> Child(0, y=1)
Child(x=0, y=1)

[jack1142]

Does this change deserve an entry in the What's New in Python document in addition to the changelog entry?