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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