On Jun 11, 8:27 am, Frank Millman <[EMAIL PROTECTED]> wrote:
> On Jun 11, 1:56 pm, Steven D'Aprano
>
> <[EMAIL PROTECTED]> wrote:
>
> > Unless you have thousands and thousands of instances, __slots__ is almost
> > certainly not the answer. __slots__ is an optimization to minimize the
> > size of each instance. The fact that it prevents the creation of new
> > attributes is a side-effect.
>
> Understood - I am getting there slowly.
>
> I now have the following -
>
> >>> class A(object):
>
> ... def __init__(self,x,y):
> ... self.x = x
> ... self.y = y
> ... def __getattr__(self,name):
> ... print 'getattr',name
> ... self.compute()
> ... return self.__dict__[name]
> ... def compute(self): # compute all missing attributes
> ... self.__dict__['z'] = self.x * self.y
> [there could be many of these]
>
> >>> a = A(3,4)
> >>> a.x
> 3
> >>> a.y
> 4
> >>> a.z
>
> getattr z
> 12>>> a.z
> 12
> >>> a.q
>
> KeyError: 'q'
>
> The only problem with this is that it raises KeyError instead of the
> expected AttributeError.
>
>
>
> > You haven't told us what the 'compute' method is.
>
> > Or if you have, I missed it.
>
> Sorry - I made it more explicit above. It is the method that sets up
> all the missing attributes. No matter which attribute is referenced
> first, 'compute' sets up all of them, so they are all available for
> any future reference.
>
> To be honest, it feels neater than setting up a property for each
> attribute.
I don't see why this all-or-nothing approach is neater; what if you
have a hundred expensive computed attributes but you just need one ?
Unless you know this never happens in your specific situation because
all missing attributes are tightly coupled, properties are a better
way to go. The boilerplate code can be minimal too with an appropriate
decorator, something like:
class A(object):
def __init__(self,x,y):
self.x = x
self.y = y
@cachedproperty
def z(self):
return self.x * self.y
where cachedproperty is
def cachedproperty(func):
name = '__' + func.__name__
def wrapper(self):
try: return getattr(self, name)
except AttributeError: # raised only the first time
value = func(self)
setattr(self, name, value)
return value
return property(wrapper)
HTH,
George
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