[SciPy-user] object-oriented help

Benjamin J. Racine bjracine@glosten....
Fri Nov 21 21:42:06 CST 2008

Very nice.  Many thanks Robert!

I need to think about how best to handle the 'model =' line for the data that I actually have, but this sure puts me down the right path.

Ben R.

From: scipy-user-bounces@scipy.org [scipy-user-bounces@scipy.org] On Behalf Of Robert Kern [robert.kern@gmail.com]
Sent: Friday, November 21, 2008 6:59 PM
To: SciPy Users List
Subject: Re: [SciPy-user] object-oriented help

On Fri, Nov 21, 2008 at 20:37, Benjamin J. Racine <bjracine@glosten.com> wrote:
> Hello all,
> Please let me know if I should be posting just general OO stuff somewhere else, but I figure that this might be relevant to a lot of procedural programming types just jumping into python (and the example is straight out of FEA).  Anyways, I have the following code below.  The problem is, I need to be able to instantiate many nodes for a given element as well as many elements for a model.  Tackling this with built-ins such as arrays, lists and dicts seems straightforward, but I can't wrap my head around it in OO for some reason.  Do I just need to make my element and node inherit from a list and then use the ".append()" when I instantiate it?

If the builtin types work well, then go ahead and use them. OO doesn't
solve every problem, but it can certainly create a few. In languages
like C++, where the builtin types are so limited, being able to write
classes helps a lot. But in Python, lists, dicts, and arrays are
relatively awesome (and are already OO objects) so writing your own
classes is sometimes a downgrade. In any case, you will want to make
use of lists, dicts, and arrays if you need collections.

You almost certainly don't want nested class declarations. Try this on for size:

class Model(object):
    """ An FEA model.
    def __init__(self, elements):
        self.elements = elements

class Element(object):
    """ A single element of an FEA model.
    def __init__(self, element_id, pressure, nodes):
        self.element_id = element_id
        self.pressure = pressure
        self.nodes = nodes

class Node(object):
    """ A node in an FEA model.
    def __init__(self, node_id, xyz):
        self.node_id = node_id
        self.xyz = xyz

model = Model(elements=[
  Element(1, 0.0, nodes=[
    Node(1, (1,2,3)),
    Node(2, (3,4,5)),
    Node(3, (6,7,8)),

Robert Kern

"I have come to believe that the whole world is an enigma, a harmless
enigma that is made terrible by our own mad attempt to interpret it as
though it had an underlying truth."
  -- Umberto Eco
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