"No mandatory attributes after..." error message

[aleaverfay]

I was getting the

No mandatory attributes allowed after an attribute with a default value or factory.

error message related to issues #38 and #93. I had been stumped as to what the order of attributes was for my class and which ones had and which lacked defaults/factory methods. It would be super useful if the exception printed more about the offending attributes. In my case, I wanted to know:

"What is the first attribute that has a default that appears ahead of an attribute which lacks one?"

right now, the error message only tells me what attribute lacks a default.

E.g., I edited _make.py around line 331 (as of version 17.4) to this:

had_default = False
which_had_default = None
for a in attrs:
    if had_default is True and a.default is NOTHING and a.init is True:
        raise ValueError(
            "No mandatory attributes allowed after an attribute with a "
            "default value or factory.  Attribute in question: {a!r} appears after {before!r}"
            .format(a=a, before=which_had_default)
        )
    elif had_default is False and \
            a.default is not NOTHING and \
            a.init is not False:
        had_default = True
        which_had_default = a

(Printing only this message was not quite sufficient for my pretty funky case, and so I ended up printing out the full ordered list of attributes to see which were appearing out of reverse-MRO order and causing my problem. It might be even more helpful to print out the full attrs list in the even of an error.)

[micimize]

It might also be good to better document / message that kw_only can be used to address this use case:

# kw default in parent
@attr.s(auto_attribs=True, hash=True, collect_by_mro=True)
class A:
    a: int = attr.field(default=1, kw_only=True)

@attr.s(auto_attribs=True, hash=True, collect_by_mro=True)
class B(A):
    b: int

B(1)
# kw required in subclass
@attr.s(auto_attribs=True, hash=True, collect_by_mro=True)
class A:
    a: int = 1

@attr.s(auto_attribs=True, hash=True, collect_by_mro=True)
class B(A):
    b: int = attr.field(kw_only=True)

B(b=1)

[four43]

Just leaving a note about this - it's a little more explicit now, like @aleaverfay had suggested. My error message (post stack trace):

ValueError: No mandatory attributes allowed after an attribute with a default value or factory.  Attribute in question: Attribute(name='input', default=NOTHING, validator=None, repr=True, eq=True, eq_key=None, order=True, order_key=None, hash=None, init=True, metadata=mappingproxy({}), type=typing.Tuple[pathlib.Path, pathlib.Path], converter=None, kw_only=False, inherited=False, on_setattr=None, alias=None)