In my experience, a dataclass with more than a few attributes makes using positional arguments unwieldy and error-prone.

I would think something like @dataclass(kwonly=bool) with default of False would be reasonably clean to implement and understand.

While I appreciate supporting existing behavior for backward compatibility, I'm not so clear on the value of supporting a hybrid of positional and keyword __init__ arguments. Could you shed some light on your reasoning for supporting it?

The same as any function or class. From PEP 3102:

def compare(a, b, *, key=None):

This seems like a reasonable thing to want a dataclass to represent. Using my off-the-cuff proposal from below:

@dataclasses.dataclass
class Comparator:
    a: Any
    b: Any
    _: dataclasses.KEYWORD_ONLY
    key: Optional[Callable[whatever]] = None

I don't want to restrict dataclasses: I'd like the full range of argument types to be available. This is especially true as dataclasses are used for more and more things (at least that's what happens in my code).

Eric

On Thu, 2021-03-11 at 00:47 -0500, Eric V. Smith wrote:

As I've said before, I'm not opposed to the feature of keyword-only arguments. I think it would be a great addition.

However, the proposal from Laurie O is only for changing fields without default values following fields with default values to be keyword-only. At least that's how I understand it.

So, that means that:

@dataclasses.dataclass
class Point:
    x: int = 0
    y: int
    z: int
    t: int = 0

Would generate a __init__ with this signature:

def __init__(self, x=0, *, y, z, t=0):

While it's an interesting application, I think that's just too limiting. Among other things, I can't define a dataclass where all fields are keyword-only, or a class where there's only a single field and it's keyword-only. I also have to have at least one keyword-only field (here, y) that has no default value. z and t can have defaults, but not y.

What I'd like to see is some way of having keyword-only arguments, with or without defaults. And I'd also like to see if we could get support for positional-only arguments at the same time.

I'm not sure of the best way to achieve this. Using flags to field() doesn't sound awesome, but could be made to work. Or maybe special field names or types? I'm not crazy about that, but using special types would let you do something like:

@dataclasses.dataclass
class Point:
    x: int = 0
    _: dataclasses.KEYWORD_ONLY
    y: int
    z: int
    t: int = 0

And the dataclasses machinery would ignore the "_" field except for making everything after it keyword-only. Here the name "_" isn't special: any field (of any name) that's of type dataclasses.KEYWORD_ONLY would be ignored except for the keyword-only changing behavior. In some ways, it's like dataclasses.ClassVar, where the type is treated specially and the field doesn't become a __init__ argument.

There are also issues with inheritance that would need to be thought through. This idea could also be extended for positional-only.

I'm open to other suggestions.

Eric

On 3/10/2021 10:22 AM, Guido van Rossum wrote:

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