New submission from João Bernardo: This is actually a proposition for a behavior change caused by a bugfix.
I have a project dealing with a metaclass factory and the next thing in the TODO list was to add multiple inheritance support. For my surprise, I tried and there was no metaclass conflict on Python 2.7... I've been developing on Py3.2 and now on Py3.3 an it couldn't obviously choose one of the metaclass to use. I found this(http://hg.python.org/cpython/rev/c2a89b509be4) commit saying the metaclass calculation was fixed on some special cases (I think it is because of the __new__ method on the metaclass). Py3.1 and inferior are not affected. ---- The thing is: My metaclasses are smart enough to produce the right behavior so the "bug" doesn't bother me as long as this bugfix never gets backported. For Python 3.2+, the solution requires some more code/knowledge for the user: If I have this class diagram for "Obj", "A" and "B": Obj[type=M] M / \ -> / \ A[type=MA] B[type=MB] MA MB then, I want to create C: class C(A, B, metaclass=something): pass Where "something" cannot be neither "MA", "MB" or "M" because of metaclass conflict. but if something = lambda *args: M(*args) I can call finally call my "smart" metaclass to solve the problem. ---- Now the proposition: It is possible for the metaclass to have some special attribute (e.g. __call_me_maybe__ = True) so Python knows it can deal with these special cases and avoid the user to explicitly use the `metaclass` parameter? So class C(A, B): pass Would call "MA" because of the special attribute inherited from "M" and it would know how to deal with the B class. If it can't do it with some X class, just return NotImplemented or other thing so the `TypeError: metaclass conflict...` will finally be shown. ---------- components: Interpreter Core messages: 179140 nosy: JBernardo priority: normal severity: normal status: open title: Metaclass conflict proposition type: behavior versions: Python 3.4 _______________________________________ Python tracker <rep...@bugs.python.org> <http://bugs.python.org/issue16872> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
