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

Raymond Hettinger python at rcn.com
Wed Jan 21 10:15:46 EST 2004

* In Definition and Introduction', a parenthetical note "(ones_ _that subclass object' or `type')" qualifies the expression "new style objects or classes". Not being an English speaker, I'm a little mixed up with the plural "ones" and the fact I was expecting "subclasses" instead of "subclass", maybe replacing "ones" by "those" would be clearer.

Clarified.

* Another point is that the phrasing suggests that the parenthetical note is a definition for what precedes it. In my understanding, the parenthetical note implies what precedes it, but is not implied by it.

Sounds like an obscure grammar nit (one that doesn't appear in any of my grammar books). I think it reads well enough as is.

A new-style instance may well have no base class at all, only given its meta-class is a subtype of type'. Would it be more precise to state:_ _"... new style objects or classes (those for which the meta-class is a_ _subtype of type')"? Being sub-classed from object or type is just a way, among others, for identifying `type' as the meta-class; but being sub-classed from object is not really required.

Nope, new-style is taken to mean objects/classes inheriting from object/type. Meta-class objects are neither new-style nor old-style. While there is room to argue with this arbitrary distinction, it is consistent with Guido's essay and especially relevant to my article because most of the rules don't necessarily apply when meta-classes are used. 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.

* 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.

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

I considered this and other explanations when writing the article. The subject is already complex enough that additional explanations can make it harder to understand rather than easier. Sidetrips can add more confusion than they cure. I see no reason to delve into what hasattr() doesn't do.

For symmetry, it would also be nice having two boxes defining `getattribute, one for objects, another for classes, instead of only the second one. I was not sure if the assymetry had a purpose, like maybe telling us that the first case may not be expressed using Python.

It did have a purpose. I put in pure python code only when it served as an aid to understanding. In this case, the operation of getattribute is more readily described with prose. I do include a C code reference for those wanting all the gory details. If you look at the reference, you'll see that a pure python equivalent would not be succinct or provide any deep insights not covered the text.

Once again, it was a question of whether to break-up the flow of the argument/explanation. I like it the way it is.

* A little below is a quick description of super()'. It says that_ "The _call ... searches ... mro ... for the base class ... immediately_ _preceding ...". Should not it be following' instead of `preceding'?

It is a matter of perspective. A is defined before its derived class B and can be said to precede it in the inheritance graph; however, the MRO list would have B ... A ... O so there is a case for the word "following". To keep consistent with Guido's essay, I'll make the change and adopt your convention.

In case yes, than maybe A' and B' might be exchanged for clarity.

No. A is higher than B on the inheritance hierarchy. Also, the lettering is consistent with Guido's essay.

* In Properties', the box starting with "class Property(object):"_ _raises a few questions. First is the doc or ""' to defined self._doc_', it does not seem that the or ""' exists in Python 2.2.1 nor 2.3.2, the doc really defaults to None.

Fixed.

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.

Third is that on the last line, "self.fdel(obj, value)" was probably meant to be "self.fdel(obj)".

Fixed.

Raymond Hettinger

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