[Python-Dev] Issue 643841: Including a new-style proxy base class in 2.6/3.0

[Nick Coghlan]

One of the tasks where classic classes are currently far superior to 
new-style classes is in writing proxy classes like weakref.proxy - cases 
where nearly all operations need to be delegated to some other object, 
with only a few being handled via the proxy type object itself.

With classic classes, this is trivial, since __getattr__ is always 
consulted, even for retrieval of special methods.

With new-style classes, however, the __getattribute__ machinery can be 
bypassed, meaning the only way to proxy an arbitrary instance is to 
define all of the special methods that have C-level slots.

This issue was actually first raised five and a half years ago [1], but 
has never been a particularly pressing problem, as anyone with any sense 
that needed to write a proxy object just used a classic class instead of 
a new-style one. In 3.0, with the demise of classic classes, that 
workaround goes away.

So what do people think of including a ProxyBase implementation in 2.6 
and 3.0 that explicitly delegates all of the C-level slots to a 
designated target instance? For some proxy class implementers, it would 
be possible to use this class as a base-class to get the special method 
delegation 'for free', and for others with more esoteric needs, it would 
at least provide a reference for which special methods needed to be 
provided explicitly by the proxy type.

I attached a sample implementation to [1] which is essentially 
equivalent to weakref.proxy, but with a strong reference to the target, 
and written in Python rather than C.

I expect the target audience for such a feature to be quite small, but 
without better support for it in the standard library, I also suspect it 
could prove to be a show-stopper for the affected developers as far as 
Py3k migration is concerned.

Cheers,
Nick.

[1] http://bugs.python.org/issue643841

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
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