[Python-Dev] Re: Hot-To Guide for Descriptors (original) (raw)

François Pinard pinard at iro.umontreal.ca
Wed Jan 21 20:55:43 EST 2004

[Raymond Hettinger]

Clarified. [...] Fixed. [...] (etc.)

Thanks, Raymond, for all those.

This is because the machinery for descriptors is embedded in type.getattribute and object.getattribute. Override or fail to inherit either of these and all bets are off.

I thinks I understand better what you are saying. Maybe part of my problem is that I'm not sure how __getattribute__' is itself fetched. While I know you do not want to speak about metatypes, it might be that whenever you write type.getattribute', you really mean the __getattribute__' found in the dict of the metatype (or is it?); while when you write object.getattribute', you really mean the __getattribute__' found by scanning the base classes of the current class, and object' always when there is no base class for a "new-style" class. These are sensible hypotheses, yet I'm not sure, of course.

Maybe that failing to say the full truth, if this is the case, might raise questions more than calm them! :-)

> * In Descriptor Protocol', it might help a bit saying immediately a_ _> word or two about the type=None' argument. Looking down for some > hint, the box holding "def getattribute(self, key): ..." uses > the statement "return v.get(None, self)", which I find confusing > respective to the _get_' prototype, as obj' corresponds to None, > and `type=None' likely corresponds to an instance instead of a type.

I'll keep an open mind on this one but think it is probably fine as it stands. Though the calling pattern may not be beautiful and could be confusing to some, my description accurately represents the signature and cases where are argument is potentially ignored. That is just the way it is.

Just adding your own quote above might help the How-To. About:

descr.__get__(self, obj, type=None) --> value

I really think it would help readers like me saying that the pattern is not beautiful, that obj' may be None, and that type' may be something else than a type, and this is the way it is. It would invite the reader to swallow the snake, instead of fighting it.

> * In Invoking Descriptors', it might help a bit insisting on the_ _> (presumed) fact that hasattr' itself does not look for descriptors.

I see no reason to delve into what hasattr() doesn't do.

One has to memorise a few rules about the order in which the lookups are done for finding an attribute, and unless hasattr' is more used as pseudo-code than Python code, my fear is that hasattr' does not look at the same place, nor in the same order, as the few rules above. Maybe there is no danger of any confusion, maybe there is, I surely wondered.

> Second is that the argument objtype=None' to get' is unused, > and once more, this does not help understanding its purpose, maybe > some explanation would be welcome about why it exists and is > unneeded in the context of `property'.

That's the way it is.

I would not doubt it, but my suggestion was that the How-To could offer more help for readers to understand or find some logic in the way it is, enough to get a grasp and assimilate the notion. If there is logic to present, merely saying that it should be assimilated without understanding might help readers like me at not stumbling on the matter for too long.

I hope you read my criticism or requests as constructive...

-- François Pinard http://www.iro.umontreal.ca/~pinard

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