[ZPatterns] AttributeError _v_cache

[Steve Spicklemire]

Yes.. in Python I'm used to something like:

class Foo

	myFoo = None

	def getMyFoo(self):
		if self.myFoo is None:
			self.myFoo = MyFoo()

		return self.myFoo

Which seems pretty clear to me! 

What's the main advantage of the ComputedAttribute version? Is it just
sytactical (i.e., it doesn't look like a method call, but rather an
instance varible)?

take care, and thanks!
-steve

Steve Alexander wrote:
> 
> Steve Spicklemire wrote:
> 
> > Interesting! Thanks Steve. In snooping through the code, I see this is
> > somewhat of an idiom (The Keeper class in Transactions pulls the same
> > trick). It's not at all obvious when you first glance at it though!
> > Basically it's a way of effectively calling a class based method the
> > first time an attribute it accessed (which has the side effect of
> > setting a value for the attribute in the instance), but further
> > references to the same attribute get the "simple" attribute from the
> > instance.
> 
> > Have I got that right?
> 
> Sounds good to me.
> 
> I think of this as a lazy factory pattern. In a language like Java,
> you'd have a "getter" which creates its value the first time it is called.
> 
>    private FooType myFoo = null;
> 
>    public FooType getMyFoo() {
>      if (myFoo == null) {
>        myFoo = new myFoo();
>      }
>      return myFoo;
>    }
> 
> (Sorry if the Java isn't quite correct. It's a while since I've used it.)
> 
> --
> Steve Alexander
> Software Engineer
> Cat-Box limited