[New-bugs-announce] [issue33493] dataclasses: allow keyword-only arguments

[Eric V. Smith]

New submission from Eric V. Smith <eric at trueblade.com>:

I've had several requests for keyword-only arguments. This is a placeholder to remind me to work on it. I have not decided if it's a good idea or not.

I propose adding a keyword_only argument to field(), defaulting to False.

I'm thinking that the basic idea would be to put all keyword-only fields at the end of the arguments to __init__, but for all other uses, leave them where they appear in the class definition. That way comparison operations, in particular, would use the fields as they appear in the class definition (which is the current behavior). Since they'd be at the end of __init__, and since order doesn't matter (they're keyword-only, after all), then this would work as expected for base classes.

That is, given:

@dataclass
class B:
    a: field(type=int, keyword_only=True)
    b: int

@dataclass
class C(B):
    c: int
    d: field(type=int, keyword_only=True)

Then B's __init__ would take (b, c, *, a, d) as its arguments, but its comparison functions would compare the tuples as (a, b, c, d).

It would be an error for a ClassVar field to be keyword-only. I think it would be okay if an InitVar field were keyword-only, but I haven't given it a lot of thought.

----------
assignee: eric.smith
components: Library (Lib)
messages: 316498
nosy: eric.smith
priority: normal
severity: normal
status: open
title: dataclasses: allow keyword-only arguments
type: enhancement
versions: Python 3.8

_______________________________________
Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue33493>
_______________________________________