[Tutor] design of Point class

[bob gailer]

  On 8/20/2010 11:45 AM, Gregory, Matthew wrote:
> Hi all,
>
> I often struggle with object design and inheritance.  I'd like opinions on how best to design a Point class to be used in multiple circumstances.
>
> I typically deal with geographic (either 2D or 3D) data, yet there are occasions when I need n-dimensional points as well.  My thought was to create a superclass which was an n-dimensional point and then subclass that to 2- and 3-dimensional cases.  The rub to this is that in the n-dimensional case, it probably makes most sense to store the actual coordinates as a list whereas with the 2- and 3-D cases, I would want 'named' variables, such as x, y, z.
>
> Here's a (very rough) first cut at the constructor and a generic distance function for the n-dimensional case:
>
> class PointND(object):
>      def __init__(self, a_list):
>          self.a_list = a_list[:]
>
>      def distance(self, right):
>          assert(len(self.coord) == len(right))
>          squared_diffs = [(i-j)*(i-j) for (i,j) in zip(self.coord, right)]
>          return math.sqrt(sum(squared_diffs))
>
> But how can I subclass this in such a way to be able to:
>
> 1) Have named variables in the 2- and 3-D cases
> 2) Be able to initialize with separate passed values, e.g. 'p = Point2D(3.0, 5.0)' rather than passing in a list

One class fits all:

class PointND(list):
     def __init__(self, *a_list):
         super(PointND, self).__init__(a_list)
         if len(self)<= 2:
             self.x = self[0]
         if len(self) == 2:
             self.y = self[1]


-- 
Bob Gailer
919-636-4239
Chapel Hill NC