Haha, sorry to disappoint, but after some experimentation, it's quite clear that this API doesn't make sense. Have a look at this test:

def test_wraps_wrappers():
    call_list = []

    def call_list_appender(func):
        @wraps(func)
        def appender(*a, **kw):
            call_list.append((a, kw))
            return func(*a, **kw)
        return appender

    class Num(object):
        def __init__(self, num):
            self.num = num

        @call_list_appender
        @classmethod
        def added(cls, x, y=1):
            return cls(x + y)

    assert Num(1).num == 1
    assert Num.added(0, 2).num == 2

    assert len(call_list) == 1

All the details aside, the func that is passed to call_list_appender is a classmethod instance. Classmethods are not callable objects. Done deal. Pretty sure this is mentioned in Graham's gigantic decorator posts, too.

The solve is to simply reverse the order of the decoration in your code, and add better exception messaging to boltons, of course:

...
        @classmethod
        @call_list_appender
        def added(cls, x, y=1):
            return cls(x + y)
...

The above works fine for me. This is one of those corner cases that makes wrapt so complex, and another reason why I only call out wraps as a replacement for decorator.py, not wrapt. I'll leave this issue open until I have those boltons exceptions in place.

Well, you could simplify this to the fact that it’s about descriptors in general. :) I only used classmethods as a concrete example. And yeah, shit’s complicated but descriptors happen.

(please ping me when you release this fix so I can update the error message in the post :))