Objects with different data views

[George Sakkis]

"Steven D'Aprano" <steve at REMOVETHIScyber.com.au> wrote:

> I'm not sure how to do this, or where to start looking for the right
> information, so any advice would be appreciated.
>
> I want to implement a class with two (or more) different ways of looking
> at its attributes.
>
> One example of this might be complex numbers, which can be written in
> Cartesian form (x+yi) or polar form (r cis theta).
>
> (Yes, I know Python already has complex numbers. That was just an example.)
>
> Another might be 3D vectors, which can be written in Cartesian form
> [x, y, z], polar form [r, theta, z] or spherical polar [r, theta, phi].
>
> It is important that there are no privileged attributes, e.g. in the
> above example, I can set any of x, y, z, r, theta or phi and all the
> others will automatically reflect the changes. A concrete, if simple,
> example will make it clear.
>
> Suppose I have a transformation (a,b) <-> (x,y) where:
>
> x = a+b
> y = a+2*b
>
> I create an instance spam, and set a and b:
>
> spam.a = 1
> spam.b = 2
>
> Now I should be able to read x and y:
>
> print spam.x, spam.y
> # prints 3 5
>
> If I set attribute y:
>
> spam.y = 0
>
> a and b automatically change to match:
>
> print spam.a, spam.b
> # prints 6, -3
>
>
> Anyone have any good ideas for how I should implement this?

As others have replied, properties is the way to go. There have been a few recipes in the Cookbook
that avoid cluttering the class namespace with temporary get/set/del methods, e.g.
http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/410698.

HTH,
George