[Python-Dev] Hot-To Guide for Descriptors (original) (raw)
François Pinard pinard at iro.umontreal.ca
Tue Jan 20 23:15:03 EST 2004
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Hello, Raymond. Cc:ing python-dev, so others can bash me as well! :-)
This is about `http://users.rcn.com/python/download/Descriptor.htm'. Before anything else, let me thank you for having written this essay! Please also take all my remarks with a grain of salt or two, as I'm far for pretending I really understand all the matters here.
In
Definition and Introduction', a parenthetical note "(ones that subclassobject' 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.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. 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 oftype')"? 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.In
Descriptor Protocol', it might help a bit saying immediately a word or two about thetype=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, asobj' corresponds to None, and `type=None' likely corresponds to an instance instead of a type.In
Invoking Descriptors', it might help a bit insisting on the (presumed) fact thathasattr' itself does not look for descriptors. 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.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 befollowing' instead ofpreceding'? In case yes, than maybeA' and `B' might be exchanged for clarity.In
Properties', the box starting with "class Property(object):" raises a few questions. First is thedoc or ""' to definedself.__doc__', it does not seem that theor ""' exists in Python 2.2.1 nor 2.3.2, the doc really defaults to None. Second is that the argumentobjtype=None' toget' 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'. Third is that on the last line, "self.fdel(obj, value)" was probably meant to be "self.fdel(obj)".
-- François Pinard http://www.iro.umontreal.ca/~pinard
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