Converting an object into a subclass in Python?

This can be done if the initializer of the subclass can handle it, or you write an explicit upgrader. Here is an example:

class A(object):
    def __init__(self):
        self.x = 1

class B(A):
    def __init__(self):
        super(B, self).__init__()
        self._init_B()
    def _init_B(self):
        self.x += 1

a = A()
b = a
b.__class__ = B
b._init_B()

assert b.x == 2