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

Guido van Rossum guido at python.org
Tue Sep 4 16:31:53 CEST 2007

I only care about getting this right when there is a reasonable chance that a class is being used as an object. For example, at the sprint we ran into this with the format special method, when someone discovered that format(object, "") raised a weird error rather than returning str(object), which was due to the default format method defined on the object class. It's important that you can format anything, so we fixed this right away.

OTOH for the with-statement, the object passed to it is always specially constructed to work in this context, and passing something random like a type object just isn't a reasonable use case. As long as you get some kind of error (and you do, usually complaining about the arg count) I'm okay.

--Guido

On 9/4/07, Nick Coghlan <ncoghlan at gmail.com> wrote:

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 --------------------------------------------------------------- http://www.boredomandlaziness.org

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-- --Guido van Rossum (home page: http://www.python.org/~guido/)

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