[Python-Dev] RFC: readproperty (original) (raw)

Jim Fulton jim at zope.com
Wed Sep 28 16:16:12 CEST 2005

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?

Jim

-- Jim Fulton mailto:jim at zope.com Python Powered! CTO (540) 361-1714 http://www.python.org Zope Corporation http://www.zope.com http://www.zope.org

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