Reclassing an instance in Python

Question

I have a class that is provided to me by an external library. I have created a subclass of this class. I also have an instance of the original class.

I now want to t

Answer 1

"The State Pattern allows an object to alter its behavior when its internal state changes. The object will appear to change it's class." - Head First Design Pattern. Something very similar write Gamma et.al. in their Design Patterns book. (I have it at my other place, so no quote). I think that's the whole point of this design pattern. But if I can change the class of an object at runtime, most of the time i don't need the pattern (there are cases when State Pattern does more than simulate a class change).

Also, changing class at runtime doesn't always work:

class A(object):
    def __init__(self, val):
        self.val = val
    def get_val(self):
        return self.val

class B(A):
    def __init__(self, val1, val2):
        A.__init__(self, val1)
        self.val2 = val2
    def get_val(self):
        return self.val + self.val2


a = A(3)
b = B(4, 6)

print a.get_val()
print b.get_val()

a.__class__ = B

print a.get_val() # oops!

Apart from that, I consider changing class at runtime Pythonic and use it from time to time.