Perl vs. Python question...

[Lloyd Kvam]

On Mon, 2009-07-13 at 22:59 -0400, Paul Lussier wrote:
> Lloyd Kvam <python at venix.com> writes:
> 
> > You've already gotten two useful responses.  I'd just like to add that
> > typically, the object attributes are referenced directly:
> >         rect.length * rect.width
> 
> Lloyd, thanks. But what if the attribute isn't set yet?  If I have
> self.foo, and self.foo hasn't yet been set, I want it to go and get set,
> then return the correct value.

If the initial set value is a simple constant, make it a class attribute
with the default value.  The object can override the attribute with a
different value when that value is determined.
> 
> I get the impression the __getattr__() method helps here, 

If the value will be computed on demand, __getattr__ is one way to go.
        def __getattr__(self, attr):
            if attr == 'foo':
                return self.compute_foo()
            elif ....
            else:
                raise AttributeError( attr + ' is invalid name')

This is the 'traditional' approach.  __getattr__ is *only* called when
an attribute is not found.  If you wanted to save the computed value, so
that __getattr__ was no longer used:
	self.__dict__['foo'] = self.compute_foo()
	return self.foo

The simplistic self.foo = self.compute_foo() will trigger a call to
__getattr__, so you can't use that within __getattr__.


The new-style python classes allow for cleaner dispatching of your
attribute computations using properties, but for a small number of
names, __getattr__ will do just fine.  If there is no setter method and
foo is always computed, your class could be coded:

	foo = property( compute_foo)

> I just don't
> quite get it yet.

HTH
> 
> --
> Thanks,
> Paul
-- 
Lloyd Kvam
Venix Corp
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