Inheritance and Overriding __init__ in python

Question

I was reading 'Dive Into Python' and in the chapter on classes it gives this example:

class FileInfo(UserDict):
    "store file metadata"
    def __init__(self, filename=None):
        UserDict.__init__(self)
        self["name"] = filename

The author then says that if you want to override the __init__ method, you must explicitly call the parent __init__ with the correct parameters. What if that FileInfo class had more than one ancestor class? Do I have to explicitly call all of the ancestor classes' __init__ methods? Also, do I have to do this to any other method I want to override?

Answer 1

If the FileInfo class has more than one ancestor class then you should definitely call all of their __init__() functions. You should also do the same for the __del__() function, which is a destructor.

Answer 2

The book is a bit dated with respect to subclass-superclass calling. It's also a little dated with respect to subclass built-in classes.

It looks like this nowadays.

class FileInfo(dict):
    """store file metadata"""
    def __init__(self, filename=None):
        super( FileInfo, self ).__init__()
        self["name"] = filename

Note the following.

1. We can directly subclass built-in classes, like dict, list, tuple, etc.

2. The super function handles tracking down this class's superclasses and calling functions in them appropriately.

Answer 3

Yes, you must call __init__ for each parent class. The same goes for functions, if you are overriding a function that exists in both parents.

Answer 4

You don't really have to call the __init__ methods of the base class(es), but you usually want to do it because the base classes will do some important initializations there that are needed for rest of the classes methods to work.

For other methods it depends on your intentions. If you just want to add something to the base classes behavior you will want to call the base classes method additionally to your own code. If you want to fundamentally change the behavior, you might not call the base class' method and implement all the functionality directly in the derived class.