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

Martin Teichmann lkb.teichmann at gmail.com
Sat Feb 14 16:16:26 CET 2015

Hi Petr, Hi all,

Well said, and yes, it needs ironing out. I just realized it doesn't work since I'm calling initclass before its class is set. But this is a solvable problem: Let's rewrite PEP 422 so that the initialization is done on subclasses, not the class itself. This is actually a very important usecase anyways, and if you need your method to be called on yourself, just call it after the class definition! One line of code should not be such a big deal. -1, that would be quite surprising behavior.

Honestly, the opposite would be (actually was to me) surprising behavior. One standard usage for PEP 422 are registry classes that register their subclasses. From my experience I can tell you it's just annoying having to assure not to register the registry class itself...

It also makes a lot of sense: what should init_class do? Initializing the class? But that's what the class body is already doing! The point is that we want to initialize the subclasses of a class, why should we initialize the class?

I also simplified the code, requiring now that decorates initsubclass with @classmethod. You're writing library code. A bit more complexity here i worth it. See again the Rejected Design Options in the PEP.

I'm fine with or without explicit classmethod, I can put it back in if people prefer it. I had just realized that at least the test code for the currently proposed C implementation still uses classmethod.

I think it's time you put this in a repository; e-mailing successive versions is inefficient.

Done. It's at https://github.com/tecki/metaclasses

Also, run the tests on it -- that might uncover more problems.

Done. For sure I had to modify the tests so that my approach that subclasses get initialized is tested - which makes the tests ugly, but they run.

I also took out the part where other metaclasses may block init_class, that doesn't really make sense in an approach like mine, it's much more easy to simply circumvent my metaclass.

If it is really desired it should be no problem to put it in as well.

Greetings

Martin

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