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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