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

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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.

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