Cast base class to derived class python (or more pythonic way of extending classes)

Question

I need to extend the Networkx python package and add a few methods to the Graph class for my particular need

The way I thought about doing this is simplying deriving a new class say NewGraph, and adding the required methods.

However there are several other functions in networkx which create and return Graph objects (e.g. generate a random graph). I now need to turn these Graph objects into NewGraph objects so that I can use my new methods.

What is the best way of doing this? Or should I be tackling the problem in a completely different manner?

Answer 1

If you are just adding behavior, and not depending on additional instance values, you can assign to the object's __class__:

from math import pi  class Circle(object):     def __init__(self, radius):         self.radius = radius      def area(self):         return pi * self.radius**2  class CirclePlus(Circle):     def diameter(self):         return self.radius*2      def circumference(self):         return self.radius*2*pi  c = Circle(10) print c.radius print c.area() print repr(c)  c.__class__ = CirclePlus print c.diameter() print c.circumference() print repr(c) 

Prints:

10 314.159265359 <__main__.Circle object at 0x00A0E270> 20 62.8318530718 <__main__.CirclePlus object at 0x00A0E270> 

This is as close to a "cast" as you can get in Python, and like casting in C, it is not to be done without giving the matter some thought. I've posted a fairly limited example, but if you can stay within the constraints (just add behavior, no new instance vars), then this might help address your problem.