Object-Oriented Design

Jon Reades

Three Key Concepts for Classes

1. Encapsulation
2. Inheritance
3. Polymorphism

Encapsulation: the bundling of the data and the methods that work on that data in one unit; data is often accessed via methods and not directly. I don’t need to care how something works, only that it works.

Inheritance: classes can inherit functionality from ‘parents’ and then extend this in useful ways. So geopandas extends pandas, and movingpandas extends geopandas!

Polymorphism: a ‘child’ class can ideally be used anywhere its parent class can be used because it only extends (i.e. adds) functionality and does not break parent behaviour. A class that implements functionality from multiple parents can be used anywhere those parents would be used.

Tree of Life

Tree of Vehicles

Most people would call this a class hierarchy or diagram.

There is no natural order here: where do e-bikes, unicycles, and rickshaws go?

Indeed, we could map out vehicles based on the number of axles, their source of power, their driver positioning, etc., etc. The point here is that the designer must make choices that are influenced by (and will influence) the design of the application for which this class hierarchy is buing developed. An automobile manufacturer might make different choices from a government trying to implement a tax policy.

Classes vs Packages

Functionally, a class and a package are indistinguishable, but a class produces objects that use methods and instance or class variables, whereas a package is a group of functions and constants that may, or may not, include classes.

Ugh, now try to keep this straight in your head.

Key Takeaways

1. You’ve been using Classes and Methods since you started.
2. You can ‘package up’ useful code into functions, and useful functions into packages.
3. Together, packages and classes will turbo-charge your programming skills.
4. You can stand on the shoulders of giants!

Resources

• What is object-oriented programming?
• Python object-oriented programming
• Object-oriented programming refresher
• Understanding inheritance
• Abstract base classes
• Python - Object-Oriented