Attrs: Most elegant way to avoid None objects in class?

[madig]

(continuing from https://github.com/python-attrs/attrs/issues/170 which I don't want to derail)

I have a class like this:

@attr.s(frozen=True, auto_attribs=True)
class Status:
    weight: int = attr.ib(cmp=True)
    message: str = attr.ib(
        cmp=False,
        converter=lambda value: str() if value is None else value,
    )
    code: str = attr.ib(
        cmp=False,
        converter=lambda value: str() if value is None else value,
    )

    @classmethod
    def Debug(cls, message=None, code=None):
        return cls(weight=0, message=message, code=code)

    [...]

    @classmethod
    def Error(cls, message=None, code=None):
        return cls(weight=6, message=message, code=code)

It can be instantiated like this: Status.Error("This and that is wrong etc.", code="mismatched-something"). However, instantiating it like Status.Error() should result in Status(weight=6, message="", code=""), i.e. I want to avoid None objects. Python unfortunately makes it more tedious than it should be to give out non-None default arguments.

@hynek noted that I may want to use default="" and use attr.NOTHING as the default argument in the constructors, however:

In [1]: import attr
   ...: @attr.s
   ...: class Status:
   ...:     a = attr.ib(default="")
   ...:     @classmethod
   ...:     def aaa(cls, a=attr.NOTHING):
   ...:         return cls(a)
   ...:

In [2]: Status.aaa()
Out[2]: Status(a=NOTHING)

In [3]: Status.aaa("")
Out[3]: Status(a='')

In [4]: import attr
   ...: @attr.s
   ...: class Status:
   ...:     a = attr.ib(default="")
   ...:     @classmethod
   ...:     def aaa(cls, a=None):
   ...:         return cls(a=a or attr.NOTHING)

In [5]: Status.aaa()
Out[5]: Status(a=NOTHING)

[madig]

(I suppose I can use def aaa(cls, a=""): here because strings are immutable, but I'd like to know about a more general approach.)

[hynek]

Ugh right the NOTHING way doesn’t work because attrs uses literal "" as default parameters if possible.

[hynek]

I guess what could be done is some kind of attr.converters.default_if_none(default=NOTHING, factory=None) so you could write attr.converters.default_if_none(default="") or attr.converters.default_if_none(factory=list). I think I had uses for that too before. 🤔