[Python-ideas] A way out of Meta-hell (was: A (meta)class algebra) (original) (raw)

Martin Teichmann lkb.teichmann at gmail.com
Wed Feb 18 09:50:43 CET 2015

Zope already implements classinit via a metaclass, so I'm not clear on how this addition would help with a backport of the initclass spelling to Python 2.

Well, there is just a small issue in the zope implementation. They don't support super(). So you cannot write:

from ExtensionClass import Base
class Spam(Base):
    def __class_init__(self):
        super().__class_init__(self)  # this line raises RuntimeError
        # do something here

The precise error is: RuntimeError: super(): empty class cell

My pure python implementation with init_subclass support super() without problems.

The problem is that at the point they call class_init (at the end of ExtensionClass.new) the class in the methods is not initialized yet. Interestingly, it also doesn't help to move that call into init, which is still a mystery to me, I was under the assumtion that at the point of init the class should be fully initialized. Interestingly, type manages to set the class of the methods AFTER init has finished. How it manages to do so I don't know, but probably that's special-cased in C.

I agree a clean idiom for detecting whether you're in the base class or not might be useful, but even without specific support in the PEP you'll at least be able to do that either by inspecting the MRO, or else by assuming that the class name not being bound yet means you're still in the process of creating it.

That sounds very complicated to me. Could you please give me an example how I am supposed to do that?

Greetings

Martin

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