[python] How do I initialize the base (super) class?

How do I initialize the base (super) class?

class SuperClass(object):
    def __init__(self, x):
        self.x = x

class SubClass(SuperClass):
    def __init__(self, y):
        self.y = y

Use a super object to ensure you get the next method (as a bound method) in the method resolution order. In Python 2, you need to pass the class name and self to super to lookup the bound __init__ method:

 class SubClass(SuperClass):
      def __init__(self, y):
          super(SubClass, self).__init__('x')
          self.y = y

In Python 3, there's a little magic that makes the arguments to super unnecessary - and as a side benefit it works a little faster:

 class SubClass(SuperClass):
      def __init__(self, y):
          super().__init__('x')
          self.y = y

Hardcoding the parent like this below prevents you from using cooperative multiple inheritance:

 class SubClass(SuperClass):
      def __init__(self, y):
          SuperClass.__init__(self, 'x') # don't do this
          self.y = y

Note that __init__ may only return None - it is intended to modify the object in-place.

Something __new__

There's another way to initialize instances - and it's the only way for subclasses of immutable types in Python. So it's required if you want to subclass str or tuple or another immutable object.

You might think it's a classmethod because it gets an implicit class argument. But it's actually a staticmethod. So you need to call __new__ with cls explicitly.

We usually return the instance from __new__, so if you do, you also need to call your base's __new__ via super as well in your base class. So if you use both methods:

class SuperClass(object):
    def __new__(cls, x):
        return super(SuperClass, cls).__new__(cls)
    def __init__(self, x):
        self.x = x

class SubClass(object):
    def __new__(cls, y):
        return super(SubClass, cls).__new__(cls)

    def __init__(self, y):
        self.y = y
        super(SubClass, self).__init__('x')

Python 3 sidesteps a little of the weirdness of the super calls caused by __new__ being a static method, but you still need to pass cls to the non-bound __new__ method:

class SuperClass(object):
    def __new__(cls, x):
        return super().__new__(cls)
    def __init__(self, x):
        self.x = x

class SubClass(object):
    def __new__(cls, y):
        return super().__new__(cls)
    def __init__(self, y):
        self.y = y
        super().__init__('x')