Nick Coghlan added the comment: I'm not clear on what discrepancy you're referring to, as I get the same (expected) exception for both the class statement and the dynamic type creation:
>>> class MyDerived(MyClass, metaclass=metaclass_callable): ... pass ... Traceback (most recent call last): File "<stdin>", line 1, in <module> File "<stdin>", line 2, in metaclass_callable TypeError: metaclass conflict: the metaclass of a derived class must be a (non-strict) subclass of the metaclasses of all its bases >>> MyDerivedDynamic = new_class("MyDerivedDynamic", (MyClass,), >>> dict(metaclass=metaclass_callable)) Traceback (most recent call last): File "<stdin>", line 1, in <module> File "/usr/lib64/python3.5/types.py", line 57, in new_class return meta(name, bases, ns, **kwds) File "<stdin>", line 2, in metaclass_callable TypeError: metaclass conflict: the metaclass of a derived class must be a (non-strict) subclass of the metaclasses of all its bases This is due to the fact that your custom metaclass function returns an instance of a subclass of type, so we end up in type_new to actually create the type, which fails the metaclass consistency check. One of the subtle intricacies here is that, for class statements, the logic that corresponds to types.prepare_class in the Python implementation is actually in the __build_class__ builtin for the C implementation - when there's a custom metaclass that *doesn't* return a subclass of type, we don't end up running type_new at all. As a result of this, *both* implementations include a conditional check for a more derived metaclass in their namespace preparation logic, as well as an unconditional call to that metaclass derivation logic from type_new if the calculated metaclass is either type itself, or a subclass that calls up to super().__new__. Most relevant issues and commit history: - last update to C implementation - http://bugs.python.org/issue1294232 - https://hg.python.org/cpython/rev/c2a89b509be4 - addition of pure Python implementation - http://bugs.python.org/issue14588 - https://hg.python.org/cpython/rev/befd56673c80 The test cases in those commits (particularly the first one) should help make it clear what is and isn't supported behaviour. ---------- _______________________________________ Python tracker <rep...@bugs.python.org> <http://bugs.python.org/issue28437> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com