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

Michael Foord fuzzyman at voidspace.org.uk
Tue May 20 16:38:02 CEST 2008

Nick Coghlan wrote:

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. I've needed to do this a few times when wrapping libraries.

Michael Foord

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

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