[Python-Dev] Things to Know About Super (original) (raw)

Michele Simionato michele.simionato at gmail.com
Fri Aug 29 06:07:27 CEST 2008

On Thu, Aug 28, 2008 at 8:54 PM, Phillip J. Eby <pje at telecommunity.com> wrote:

I created a "universal metaclass" in DecoratorTools whose sole function is to delegate metaclass new, init, and call to class-level methods (e.g. classnew, classcall, etc.), thereby eliminating the need to have custom metaclasses for most use cases in the first place. Now, wherever possible, I use that single metaclass in my frameworks, so that there's no need to mix them.

easy_installed DecoratorTools and found it: classy_class.

From the point of view of the code, this is a beautiful and elegant snippet. However, suppose that from tomorrow everybody starts using it. Since metaclasses would become so easy to use, possibly a lot of people would take advantage of them. Then we would have potentially complex (multiple) inheritance hierarchies with chains of methods (class__new/class__init) calling themselves cooperatively in the MRO. Would the resulting code be readable? How easy would be for an average framework user to understand what is happening to his class? I think class decorators would be a much better solution than classy_class for most use cases and we should push that way, not the cooperative inheritance way.

Generally speaking I like more solutions bases on functional composition (as in WSGI that you know very well) than on method cooperation. Rather than improve the support for inheritance, I would like (in an ideal world) to reduce it, to make easier the choice for people between inheritance and alternatives (object composition, delegation, functional composition). In the real world, I am content in documenting the pitfalls of super, warn people about the dangers of complex design involving multiple inheritance and cooperation, and suggest alternatives.

       Michele Simionato

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