[Python-Dev] RFC: readproperty
Phillip J. Eby
pje at telecommunity.com
Wed Sep 28 17:00:18 CEST 2005
At 10:16 AM 9/28/2005 -0400, Jim Fulton wrote:
>I do this often enough that I think it would be useful to include it
>in python, either as a builtin (like property) or in the library. (Or
>possibly by adding an option to property to generate a read
>descriptor.) I'd be happy to add this for 2.5.
You mean something like defaultproperty(func), where func(ob) is called at
most once when there is no dictionary entry for the attribute?
I started using such properties with PEAK a few years ago, and found some
corner cases that made me decide to stick with ones that have both __get__
and __set__. Mostly those cases have to do with creating class-level
properties, which can end up being inherited by subclasses if you don't
include __set__. Also of course you can't hook the setting of attributes
without a __set__.
Of course, most people aren't likely to be creating metaclass properties,
so it's probably not a big issue for the stdlib. But I thought *you* might
want to know about it, in case you hadn't already encountered the issue. :)
The other issue I found with such properties is that I really wanted to be
able to use functions that didn't rebind the attribute value directly,
i.e., I wanted to be able to use lambdas for short computed property
descriptions. However, to make this work you have to know what attribute
name(s) the property is stored under in the class, which then leads to
other interesting complications. So now I use a custom C type that knows
its name and takes two functions (a filter for values set, and a function
to compute the default).
Unfortunately, finding out a descriptor's name is non-trivial; it'd be nice
if there were a descriptor hook __bind__(cls,name) that was called by
classes during cls.__new__ or assignment to a class attribute, and which
you could define to return a replacement descriptor. It seems like one of
the first metaclasses I end up writing in any new project is something to
do this, and I believe Ian Bicking has encountered the same thing in e.g.
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