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

Steven Bethard steven.bethard at gmail.com
Wed Sep 28 18:04:42 CEST 2005

Jim Fulton wrote:

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

I've also needed behavior like this a few times, but I use a variant of Scott David Daniel's recipe[1]:

class _LazyAttribute(object): def init(self, calculate_function): self._calculate = calculate_function

def __get__(self, obj, _=None):
    if obj is None:
        return self
    try:
        value = self._calculate(obj)
    except AttributeError, e:
        # I don't like this, but if _calculate raises an
        # AttributeError and I don't catch it, the descriptor
        # machinery hides it and I can't debug my code
        raise Exception(e)
    setattr(obj, self._calculate.func_name, value)
    return value

It uses the .func_name attribute to put the "self.eggs = result" into the property. I like that I don't have to do the set at the end of every function, and I'm never doing anything complicated enough that I don't want the attribute named the same as the function that I passed in.

[1] http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/363602

STeVe

You can wordify anything if you just verb it. --- Bucky Katt, Get Fuzzy

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