Michal Kwiatkowski <[EMAIL PROTECTED]> wrote: ... > I'm trying to understand attributes lookups made by Python, having > properties and special methods in mind. So far I've come up with kind of > reasoning I've coded below. I would appreciate any comments and > suggestions.
First, let's forget legacy-style classes, existing only for backwards compatibility, and focus on new-style ones exclusively -- never use legacy classes if you can avoid that. Descriptors come in two varieties: overriding and non-overriding. Both kinds have a __get__ method (that's what makes them descriptors). Overriding ones also have a __set__, non-overriding ones don't. (Until pretty recently, the two kinds were known as data and non-data, but the new terminology of overriding and non-overriding is much clearer). I covered descriptors in one of my talks at Pycon '05 and elsewhere; you can find just about all of my talks by starting at www.aleax.it (mostly just PDF forms of the slides in my presentations). I cover them in quite a central role in the forthcoming 2nd edition of Python in a Nutshell, too. But, summarizing: overriding descriptors are FIRST looked up in the class (class overrides instance); non-overriding descriptors, first in the instance (class does not override). Functions are NON-overriding descriptors. Special methods IMPLICITLY looked up by Python ALWAYS go to the class ONLY -- adding an x.__getitem__ per-class attribute that happens to be callable doesn't mean that attribute is gonna handle z=x[y] and the like (in legacy-style classes the rule was different, but that led to a host of complications that we're much better off without!). Alex -- http://mail.python.org/mailman/listinfo/python-list
