[Python-Dev] Non-string keys in namespace dicts

[Phillip J. Eby]

At 12:27 PM 12/3/2007 -0700, Neil Toronto wrote:
>Guido van Rossum wrote:
> > On Dec 2, 2007 12:49 PM, Neil Toronto <ntoronto at cs.byu.edu> wrote:
> >> It turned out not *that* hard to code around for attribute caching, and
> >> the extra cruft only gets invoked on a cache miss. The biggest problem
> >> isn't speed - it's that it's possible (though extremely unlikely), while
> >> testing keys for equality, that a rich compare alters the underlying
> >> dict. This causes the caching lookup to have to try to get an entry
> >> pointer again, which could invoke the rich compare, which might alter
> >> the underlying dict..
> >
> > How about subclasses of str? These have all the same issues...
>
>Yeah. I ended up having it, per class, permanently revert to uncached
>lookups when it detects that a class dict in the MRO has non-string
>keys. That's flagged by lookdict_string, which uses PyString_CheckExact.

I'm a bit confused here.  Isn't the simplest way to cache attribute 
lookups to just have a cache dictionary in the type, and update that 
dictionary whenever a change is made to a superclass?  That's 
essentially how __slotted__ attribute changes on base classes work 
now, isn't it?  Why do we need to mess around with the dictionary 
entries themselves in order to do that?