[Python-ideas] A (meta)class algebra (original) (raw)

Thomas Kluyver thomas at kluyver.me.uk
Fri Feb 13 19:27:15 CET 2015

On 12 February 2015 at 20:43, Martin Teichmann <lkb.teichmann at gmail.com> wrote:

class ConsoleWidget(MetaQObjectHasTraits('NewBase', (LoggingConfigurable, QtGui.QWidget), {})):

which, in my opinion, is hard to read and grasp. Everyone has to be taught on how to use those metaclass mixers.

Yes, it's hard to read, but it indicates that there's something complex going on. Your proposal is cleaner, but to my mind it's just sweeping under the carpet the fact that multiple metaclasses are in use. To fully understand the code, you would still need to get a headache understanding how multiple metaclasses interact, but you would first have to notice that one metaclass is able to combine itself with another, and that there is another one to combine with.

I'm completely with you arguing that inheritance is overrated and composition should be used more often. Yet, I actually think that the above example is actually not such a bad idea, why should a QWidget not be LoggingConfigurable, and have traits?

It's not in itself a ridiculous idea, but it's bad practice precisely because multiple inheritance is tricky. If inheritance means 'is a', it's hard to be two completely different kinds of thing at the same time - especially when both of those things rely on advanced features of Python. If I was writing that code from scratch, I would try very hard to make it such that Configurable objects have QObjects, rather than being QObjects.

Moreover, I don't think you can overcome this difficulty by adding any new features to inheritance or metaclasses in Python. I doubt you could overcome it even if you could redesign multiple inheritance with a blank slate, but perhaps that is possible.

Given that I think the intersection of multiple inheritance and metaclasses will always be horrible to understand, I think anything that makes it easier to get into that situation is a bad idea. I'd support something that makes it easier to understand such situations when they do exist, but I think your proposal would make it harder, not easier.

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