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.

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

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

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.

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

> 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)

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

I doubt it's worth opening a separate issue for this, so I'll mention it here:

In the documentation (https://docs.python.org/3.10/library/dataclasses.html#dataclasses.KW_ONLY) nothing documents KW_ONLY as being new in Python 3.10.

New changeset 5a3f97291eea96037cceee097ebc00bba44bc9ed by Henry Schreiner in branch 'main':
bpo-43532: add version added to KW_ONLY (GH-31235)
https://github.com/python/cpython/commit/5a3f97291eea96037cceee097ebc00bba44bc9ed

New changeset 7445949a4399ab19fbdd5a0b0aca53a690c3251b by Miss Islington (bot) in branch '3.10':
bpo-43532: add version added to KW_ONLY (GH-31235)
https://github.com/python/cpython/commit/7445949a4399ab19fbdd5a0b0aca53a690c3251b

Thanks, Henry Schreiner!

messages: + msg412946
stage: patch review -> resolved

pull_requests: + pull_request29405
stage: patch review