Polymorphism - adding to existing methods while overwriting them

Question

I want to be able to subclass a class, and define __init__ but still run the old __init__ as well.

To illustrate, say I have the following classes:

class A(object):
    def __init__(self):
        self.var1 = 1

class B(A):
    def __init__(self)
        self.var2 = 2
        doInitForA()

And I want to be able to do this:

instB = B()
print (instB.var1) #1
print (instB.var2) #2

Edited as Ignacio Vazquez-Abrams suggested. (Is it possible to edit without bumping?)

Answer 1

Either call a.__init__(self) or derive a from object and use super().

Answer 2

class a:
    def __init__(self):
        self.var1 = 1

class b(a):
    def __init__(self)
        self.var2 = 2
        a.__init__(self)

You can even write super().__init__() if you are using python 3.
See this question about the use of super().

Answer 3

Call your father's c'tor from within your c'tor: a.__init__(self). Note that you need to pass self as first parameter. If the parent c'tor takes more parameters, pass them after self.

Answer 4

replace

doInitForA()

with

super(b, self).__init__()

Answer 5

You might want to look at this question: http://stackoverflow.com/questions/904036/chain-calling-parent-constructors-in-python, specifically use the super(b, self).__init__() method.