Allow keyword-only attributes before positional attributes

[oakkitten]

I want to be able to write both Message("message", timestamp=time.time()) and Message("message"), where timestamp is an attribute of a base class. Currently the following code:

@attrs
class Base:
    timestamp = attrib(kw_only=True, default=None)


@attrs
class Message(Base):
    message = attrib()

produces the following error:

ValueError: Non keyword-only attributes are not allowed after a keyword-only attribute.

As timestamp is keyword-only, it, as far as I understand, does not pose an ordering issue.

(In my particular use-case, I have a bunch of Messages that are convenient to create using positional arguments, but the timestamp field is not always meaningful, e.g. for outgoing messages)

[hynek]

it’s indeed weird that we don’t allow kw_only attributes anywhere. Someone should research why we chose that route. they are a lot more useful if you can sprinkle them anywhere.

[wsanchez]

I suspect that "Non keyword-only attributes are not allowed after a keyword-only attribute" because that's an existing constraint in function argument ordering.

One problem is allowing them anywhere is that the ordering of arguments to __init__ becomes a bit less obvious, since you need to mentally sort the attributes to account for the kw_only attributes. That's not horrible, but also not nothing. It could be helped by doc generators that figured that out for you, though.

[trianta2]

Agreed this type of functionality would be useful. I'm curious what a current best-practice workaround would look like.

[hynek]

This has been fixed by #559