Get variable from parent class for use in method of child class [duplicate]

Question

I'm trying to understand how parent and child classes in Python work and I ran into this seemingly simple problem:

class parent(object):

    def __init__(self):
        self.data = 42


class child(parent):

    def __init__(self):
        self.string = 'is the answer!'

    def printDataAndString(self):
        print( str(self.data) + ' ' + self.string )


c = child()
c.printDataAndString()

I'm expecting the string 42 is the answer! but I get

AttributeError: 'child' object has no attribute 'data'

What am I missing?

I experimentated with pass and also super(parent,...) but couldn't get it right.

Answer 1

Since your child has its own __init__() function, you need to call the parent class' __init__() , otherwise it does not get called. Example -

def __init__(self):
    super(child,self).__init__()
    self.string = 'is the answer!'

super() from documentation -

super(type[, object-or-type])

Return a proxy object that delegates method calls to a parent or sibling class of type. This is useful for accessing inherited methods that have been overridden in a class. The search order is same as that used by getattr() except that the type itself is skipped.

So the first argument to super() should be the child class (whose' parent class' method you want to call) , and the second argument should be the object itself , that is self. Hence , super(child, self) .

In Python 3.x , you can simply call -

super().__init__()

And it would call the __init__() method from the correct parent class.