Postpone creation of attributes until needed

[Frank Millman]

On Jun 11, 11:47 am, Phil Thompson <p... at riverbankcomputing.co.uk>
wrote:
> On Monday 11 June 2007 10:24 am, Frank Millman wrote:
>
> > Hi all
>
> > I have a small problem. I have come up with a solution, but I don't
> > know if it is a) safe, and b) optimal.
>
> > I have a class with a number of attributes, but for various reasons I
> > cannot assign values to all the attributes at __init__ time, as the
> > values depend on attributes of other linked classes which may not have
> > been created yet. I can be sure that by the time any values are
> > requested, all the other classes have been created, so it is then
> > possible to compute the missing values.
>
>
> Properties...
>
>     @property
>     def z(self):
>         return self.x * self.y
>

In my simple example I showed only one missing attribute - 'z'. In
real life I have a number of them, so I would have to set up a
separate property definition for each of them.

With my approach, __getattr__ is called if *any* of the missing
attributes are referenced, which seems easier and requires less
maintenance if I add additional attributes.

Another point - the property definition is called every time the
attribute is referenced, whereas __getattr__ is only called if the
attribute does not exist in the class __dict__, and this only happens
once. Therefore I think my approach should be slightly quicker.

Frank