# Classes

Larry Hudson orgnut at yahoo.com
Sun Nov 2 23:03:19 CET 2014

```On 11/02/2014 01:50 AM, Denis McMahon wrote:
[snip]
> from math import sqrt
>
> class SquareGeometryError(Exception):
>      """The parameters create an illegal geometry for a square"""
>      pass
>
> class Rectangle:
>
>      def __init__(self,length,width):
>          self.length=length
>          self.width=width
>
>      def area(self):
>          return self.length*self.width
>
>      def perimeter(self):
>          return 2*self.length+2*self.width
>
>      def diagonal(self):
>          return sqrt(self.length*self.length+self.width*self.width)
>
>      def get_width(self):
>          return self.width
>
>      def get_length(self):
>          return self.length
>
>      def set_width(self, width):
>          self.width = width
>
>      def set_length(self, length):
>          self.length = length
>
> class Square(Rectangle):
>
>      _def _init__(self, length, width):
>          if not length == width:
>              raise SquareGeometryError("Length must equal width")
>          self.length = length # or width
>          self.width = length # or width
>
>      def set_width(self, width):
>          self.length = width
>          self.width = width
>
>      def set_length(self, length):
>          self.length = length
>          self.width = length
>
> Note that to make my square, I only need to over-ride those rectangle
> methods which allow the setting of length and width to enforce squareness
> upon the square. All the other methods of rectangle will work equally
> well for the square.
>

Why do you want to instantiate a Square with two parameters?  You only need one -- side.  I
agree that since you are inheriting from Rectangle you need to keep the length and width
attributes to allow the get/set of length/width, so you do need to override the Rectangle set
methods.  Consider this version:

#   ------ <code> ---------
class Square(Rectangle):
def __init__(self, side):
self.set_side(side)

def get_side(self):
return self.length     #  or width

def set_side(self, side):
self.length = self.width = side

def set_width(self, width):
self.set_side(width)

def set_length(self, length):
self.set_side(length)
#   ------ </code> --------

This also eliminates the need for your exception.  Of course, it still allows someone to
directly change either the length or width -- but since "we are all consenting adults here",
anyone who does this should know the consequences.   ;-)

-=- Larry -=-

```