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Inheritance, Composition and Associated objects - when to use which - Python example
• Inheritance is where one object is based on another. • Composition is where one object contains another. Question: Which should I use? Answer: Probably BOTH! On yesterday's Python course, the question came up, and I wrote an illustrative answer - source code [here]. The scenario we took was a restaurant. We have tables of various shapes - rectangular ones, square ones, and circular ones. They're all based on a base class called table as all tables, no matter what shape, have some properties in common. And then we inherit from that base class to give somewhat updated table subclasses for each shape adding in the extra logic to work out how many people can be seated without being on the corners. We also have persons - staff and customers - which have some features in common (such as names) but some differences in their other properties. Again, that's a case of inheritance.  The next step is to say that we have a member of staff responsible for each table, and multiple customers (perhaps) seated at each table. And that's where our composite / associated objects come into play. We don't want to add all the attributes of a staff member to a table - we just want to have a contained or associated object of type staff in our object of type table, and a list of objects of type customer in our object of type table.The diagram, drawn using "class" and "object" tables in a UML-like way, shows these various relationships. Update ... the course carried on the following day, and I added various static methods including a factory, comparators, and overrode the print method for some objects. The final source code is [here]. (written 2012-10-10, updated 2012-10-13) Associated topics are indexed under Y116 - Python - Applying OO design techniques and best practise[4028] Really Simple Class and Inheritance example in Python - (2013-03-04) [2604] Tips for writing a test program (Ruby / Python / Java) - (2010-01-29) [2523] Plan your application before you start - (2009-12-02) [2485] How do I set up a constant in Python? - (2009-10-31) [2407] Testing code in Python - doctest, unittest and others - (2009-09-16) [2363] Alpaca Case or Camel Case - (2009-08-16) [1181] Good Programming practise - where to initialise variables - (2007-05-09) [945] Code quality counts - (2006-11-26) [836] Build on what you already have with OO - (2006-08-17) [668] Python - block insets help with documentation - (2006-04-04) [656] Think about your design even if you don't use full UML - (2006-03-24) [340] Code and code maintainance efficiency - (2005-06-08) Y112 - Python - Objects - Intermediate [4094] Python Properties - how and why - (2013-05-18) [3796] Backquote, backtic, str and repr in Python - conversion object to string - (2012-07-05) [3524] Metaclasses (Python) and Metatables (Lua) - (2011-11-17) [3472] Static variables in functions - and better ways using objects - (2011-10-10) [3442] A demonstration of how many Python facilities work together - (2011-09-16) [3002] A list of special method and attribute names in Python - (2010-10-17) [2994] Python - some common questions answered in code examples - (2010-10-10) [2905] Defining static methods in Python - (2010-08-05) [2889] Should Python classes each be in their own file? 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