[RFC] A shortcut to define a classvar with the same name as an attribute

Hello!

Thanks again for the great library! :)

After answering in a couple of issues regarding default values on class-level and debugging another mess with self-using factory which was fixed by migration to @cached_property, I thought that it may be good to have a simple way to set a class-level attribute with the same name as an attr.ib on attrs class.

The issue can be illustrated by a following example:

import attr

@attr.dataclass
class BaseClass:
    value: int
    magic: str

@attr.s
class FixedMagicClass(BaseClass):
    magic = attr.ib(init=False, default='alohomora')

@attr.s
class _ReprMagicClass(BaseClass):
    # A proxy class to exclude 'magic' from init
    magic = attr.ib(init=False)

@attr.s
class ReprMagicClass(_ReprMagicClass):
    @cached_property
    def magic(self):
        return repr(self.value)

I'd like to add classvar parameter in attr.ib and decorator syntactic sugar like this:

@attr.s
class FixedMagicClass(BaseClass):
    magic = attr.ib(init=False, classvar='alohomora')  # Almost no difference, but it can be accessed on the class object

@attr.s
class ReprMagicClass(BaseClass):
    @attr.ib(init=False).classvar
    @cached_property  # Property is a descriptor and is set on class object itself
    def magic(self):
        return repr(self.value)

Alternative approach may be implementing it via metadata + field_transformer, but syntax becomes clumsy. From my perspective this seems more like a core feature than like an extension.

Though, it may be a non-desired feature as it wouldn't work with slotted classes - they use their own slot descriptors - and therefore may complicate migration from dict-classes to slot-classes.

Other possible related feature, which may blend in perfectly with property and class-level constants, is a way to make an attr.ib skippable during __init__ - e.g. if an attribute is init=True and has default=attr.UNSET the generated __init__ code will be like following:

def __init__(self, required_attribute, attribute=attr.UNSET):
    self.required_attribute = required_attribute  #  This attribute had `attr.NOTHING` default in `attr.ib`
    if attribute is not attr.UNSET:  # If it wasn't set, we'll take the classvar instead
        self.attribute = attribute

I'll be happy to implement this proposal (actually - both of them) if it would be considered useful.

Start with the examples in the issue and review the existing attr.ib and decorator behavior; no source files or tests are named. The first milestone is maintainer agreement on whether the classvar proposal, skippable-init proposal, or neither should be pursued, after which the chosen scope can be implemented and validated.