Easy patterns in Python: The Composite Pattern — Hacking With Python

Sometimes you want to handle part of an object in the same way as the whole object. This whole-part relationship is elegantly represented by the Composite pattern. As you will see, the implementation in Python is simple and elegant.

Introduction

The composite pattern allows you treat a group of objects like a single object. The objects are composed into some form of a tree-structure to make this possible.

This patterns solves two problems:

• Sometimes it is practical to treat part and whole objects the same way
• An object hierarchy should be represented as a tree structure

We do this by doing the following:

• Define a unified interface for both the part objects (Leaf) and the whole object (Composite)
• Composite delegate calls to that interface to their children, Leaf objects deal with them directly.

This all sounds rather cryptic, so let us have a look at the diagram:

This is basically a graphical representation of the last two points: Composites delegate and Leafs perform the actual operation.

Implementation in Python