Here's something I've thought about off and on for a while.

Occasionally it would be useful to me to have a class attribute I can use to represent an attribute that will exist on *instances* of the class. Properties provide that to an extent, but they are data descriptors which means they will not defer to like-named instance attributes. However, a similar non-data descriptor would fit the bill.

For the sake of clarity, here is a simple implementation that demonstrates what I mean. I know it's asking a lot <wink>, but try to focus on the idea rather than the code. I've posted a more complete (and feature-rich) implementation online [1].

class Attr: """A non-data descriptor specifying an instance attribute.""" def __init__(self, name, doc=None): self.__name__ = name self.__doc__ = doc def __get__(self, obj, cls): if obj is None: return self else: # The attribute wasn't found on the instance. raise AttributeError(self.__name__)

def attribute(f=None): """A decorator that converts a function into an attribute spec.""" return Attr(f.__name__, f.__doc__)

def attrs(names): """A class decorator that adds the requested attribute specs.""" def decorator(cls): for name in names: attr = Attr(name) setattr(cls, name, attr) return cls return decorator

Other features not shown here (see [1]):

* an optional "default" Attr value * an optional "type" Attr (derived from f.__annotations__['return']) * __qualname__ * auto-setting self.__name__ during the first Attr.__get__() call * a nice repr * Attr.from_func() * proper ABC handling in attrs() (not an obvious implementation) * optionally inheriting docstrings

Such a descriptor is particularly useful for at least 2 things:

1. indicating that an abstractproperty is "implemented" on *instances* of a class 2. introspecting (on the class) all the attributes of instances of a class

Alternatives:

* "just use a property". As already noted, a property would work, but is somewhat cumbersome in the case of writable attributes. A non-data descriptor is a more natural fit in that case. * for #1, "just use a normal class attribute". This would mostly work. However, doing so effectively sets a default value, which you may not want. Furthermore, it may not be clear to readers of the code (or of help()) what the point of the class attr is.

Thoughts?

-eric

[1] https://bitbucket.org/ericsnowcurrently/odds_and_ends/src/default/attribute.... [2] Where would Attr/attribute/attrs live in the stdlib? inspect? types?