This week's book giveaway is in the OO, Patterns, UML and Refactoring forum.
We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line!
See this thread for details.
The moose likes Jython/Python and the fly likes Use of Big Moose Saloon
  Search | Java FAQ | Recent Topics | Flagged Topics | Hot Topics | Zero Replies
Register / Login

JavaRanch » Java Forums » Languages » Jython/Python
Bookmark "Use of "self" and "data" in class inheritance in Python" Watch "Use of "self" and "data" in class inheritance in Python" New topic

Use of "self" and "data" in class inheritance in Python

R. Jain
Ranch Hand

Joined: Aug 11, 2012
Posts: 375

Hello All,

I'm a bit confused while using inheritance in Python..
The problem comes when the book introduced the use of 'self' and 'data'

I know that 'self' is just like 'this' in Java.. It refers to the current instance of class..
Also, 'data' is a data attribute of a class..
** Is this data attribute a Dictionary always?? Can't it be anything else??

Also, at some place I got confused over the use of these in combination..

E.g: -

1). In __init__ method of a class extending UserDict class: - They have used self['name'] = filename where filename is an argument to the __init__

2). In UserDict class : - They have used = {}

When I proceeded further, I found that, self['name'] by default calls a special method named __setname__ with the following syntax: -

I want to ask that, how is the statement self['name'] delegated to this method defined in the ancestor??

Steve Luke

Joined: Jan 28, 2003
Posts: 4181

What you are describing is someone's attempt to create a class named UserDict which can be accessed like it was a Dictionary. The self reference is a 'namespace' that you can think of as a dictionary itself, referring to 'variables' stored in that namespace as key (the name of the variable) and value pairs.

I am guessing the class looks something like this:

The above class could be used like this:

The UserDict class itself acts like a dictionary. In order to make the work simplified it internally uses an object which is a Dictionary object - the data object. This data object is an arbitrary variable - it does not have to exist in your classes, it does not have to be a Dictionary. It is just that whoever implemented UserDict chose to create the variable named data, assign a dictionary to it, and use it as a backing store for their own dictionary implementation.

You could just as easily make a List implementation like UserList which does somehting like this:

In this case there is no data variable, there is a storage variable which serves the same function. And this time the type is a List ([]) not a Dictionary ({}).

I did a little search, and it turns out the Python library provides UserDict and UserList as convenience classes to help users implement their own Dictionary and List. See So if you wanted to make your own Dictionary implementation then you would extend UserDict, and in that case the variable will be named data and will be a Dictionary. If you don't want to implement a Dictionary, don't extend UserDict and you won't have the data variable.

R. Jain
Ranch Hand

Joined: Aug 11, 2012
Posts: 375

Thanks Steve for a very nice and thorough explanation.. That cleared all my doubts for now..
Have you checked out Aspose?
subject: Use of "self" and "data" in class inheritance in Python
It's not a secret anymore!