[Python-Dev] Submitting PEP 422 (Simple class initialization hook) for pronouncement (original) (raw)
Nick Coghlan ncoghlan at gmail.com
Sun Feb 10 15:09:55 CET 2013
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On Sun, Feb 10, 2013 at 11:34 PM, Antoine Pitrou <solipsis at pitrou.net> wrote:
Nobody can claim this is simple and easy to wrap your head around. It is a maintenance burden, and it discourages understanding of the underlying model by anyone but language experts.
You think anyone but language experts fully understands the metaclass mechanism now? I got the description of some details wrong in the PEP, and I'm probably in the top half dozen people on the planet when it comes to understanding how Python 3's class model works.
Plenty of people understand decorators though, and that's all init_class is: a special class decorator that is applied before any other decorators, and is automatically inherited by subclasses. It massively lowers the barrier to entry for inherited post-modification of classes.
Will it mean people will only bother to understand metaclasses if they actually need metaclass.prepare or to control the identity of the object returned by metaclass.call? Absolutely. However, I don't see that as any different from the fact that vastly more people understand how to use instance init methods correctly than understand the ins and outs of using new to control the creation of the object, rather than merely initialising it after it is already created. In some respects, cleanly separating out the two steps of controlling initialisation and controlling creation can actually make the two easier to grasp (since you can tackle them individually as distinct concepts, rather than having to wrap your head around both of them at the same time).
As far as the maintenance burden goes, the patch to enable PEP 422 for types.new_class() is trivial:
- return meta(name, bases, ns, **kwds)
- cls = meta(name, bases, ns, **kwds)
- try:
initcl = cls.__init_class__- except AttributeError:
pass- else:
initcl()- return cls
The change in the builtin.build_class implementation is similarly trivial (and has the same semantics), it's just more verbose due to its being written in C.
The documentation changes are quite straightforward (moving init_class and decorator invocation out to their own subsection), and the tests Daniel has provided are extensive.
Cheers, Nick.
-- Nick Coghlan | ncoghlan at gmail.com | Brisbane, Australia
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