On 9/28/05, Jim Fulton <[EMAIL PROTECTED]> wrote: > Read descriptors (defining only __get__) and data descripttors have > (defining __get__ and __set__) different semantics. In particular, > read descriptors are overridden by instance data, while data > descriptors are not. A common use of read descriptors is for lazily > computed data: > > class readproperty(object): > "Create a read descriptor from a function" > > def __init__(self, func): > self.func = func > > def __get__(self, inst, class_): > if inst is None: > return self > > return self.func(inst) > > class Spam: > > @readproperty > def eggs(self): > ... expensive computation of eggs > > self.eggs = result > return result > > When we ask for the eggs attribute the first time, we call the > descriptor, which calls the eggs function. The function sets the eggs > attribute, overriding the descriptor. The next time the eggs attribute > is accessed, the saved value will be used without calling the > descriptor. > > 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. > > Thoughts?
I like the idea of adding this functionality somewhere. I do note that the semantics were somewhat of an accident, required by backwards compatibility with pre-2.2 method lookup semantics. I think we need to be real careful with chosing a name -- in Jim's example, *anyone* could assign to Spam().eggs to override the value. The name "readproperty" is too close to "readonlyproperty", but read-only it ain't! "Lazy" also doesn't really describe what's going on. I believe some folks use a concept of "memo functions" which resemble this proposal except the notation is different: IIRC a memo function is always invoked as a function, but stores its result in a private instance variable, which it returns upon subsequent calls. This is a common pattern. Jim's proposal differs because the access looks like an attribute, not a method call. Still, perhaps memoproperty would be a possible name. Another way to look at the naming problem is to recognize that the provided function really computes a default value if the attribute isn't already set. So perhaps defaultproperty? (Sorry to turn this into a naming game, which is bound to produce 100s of competing proposals that are totally unintuitive except to the proposer. But naming is important.) -- --Guido van Rossum (home page: http://www.python.org/~guido/) _______________________________________________ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
