Question

I’m trying to wrap my brain around something here… Let’s say that I have a simple setup of class inheritance:

class SuperClass(object):
    def __init__(self):
        self.var = 1

class SubClass(SuperClass):
    def getVar(self):
        return self.var

If I have an existing instance of SuperClass, is there a way that I can ‘cast’ it as SubClass, without creating a new instance?

Answer 1

In Python you don’t have a cast operator – you can get around this by assigning the type to the instance’s __class__ attribute:

>>> super_instance = SuperClass()
>>> super_instance.__class__ = SubClass
>>> print super_instance.getVar()
1

However, this is more error prone than cast in many other languages as the validity and safety of your “cast” is not verified by the compiler.

For example, if SubClass had a method that accessed an attribute that was not available on SuperClass then attempting to call that method on super_instance would result in an error at run time, even though it appears to be a valid instance of SubClass.