[Python-3000] __special__ method lookup [was Re: Confused about getattr() and special methods]

[Nick Coghlan]

Georg Brandl wrote:
> Thomas Heller schrieb:
> 
>> IIUC, in py3k, classic classes do not exist any longer, so the __metaclass__ line
>> has no effect anyway.  Is this behaviour intended?
> 
> It is another incarnation of special methods being looked up on the class,
> not the instance. This was always the behavior with new-style classes, see
> the thread at
> 
> http://mail.python.org/pipermail/python-3000/2007-March/006261.html
> 
> for a previous discussion.
> 
> I think we should tackle this issue now and make sure the decided resolution
> is consistently applied throughout Python.

This issue came up when implementing PEP 343 as well - because the with 
statement is just syntactic sugar without any dedicated opcodes, 
__enter__/__exit__ are accessed via a conventional attribute lookup 
opcode. So unlike the special methods that use a C-level slot in the 
type object, these two operations *can* be affected by instance 
attributes and __getattr__.

However, Guido did say at the time that he was OK with the effect of 
instance attributes on special method lookups being formally undefined 
and implementation dependent. I wasn't too worried either way - mucking 
with special methods outside the scope of 'provide this on your class to 
support operation X' has long been a pretty dubious exercise.

Cheers,
Nick.

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