HeadFirstDesignPatterns_python

Example code from Head First Design Patterns second edition translated to python to help me understand and memorise the patterns.

I have added examples of pattern usage in the Python standard library and pypi - I am starting to see patterns everywhere!

Note I am aiming for a reasonably literal translation whilst trying to make the code a little more pythonic by, e.g. using python conventions for ClassNames and method_names and putting all of the code in a single file where it makes sense to do so.

Patterns Implemented

• Strategy
• Observer
• Decorator
• Factory Method
• Simple Factory
• Abstract Factory
• Singleton
• Command
• Adapter
• Façade
• Template Method
• Iterator
• Composite
• State
• Proxy
• Model View Controller (MVC)

Sample Code : Java

From the book 📖:

package headfirst.designpatterns.strategy;

public abstract class Duck {
    FlyBehavior flyBehavior;
    QuackBehavior quackBehavior;

    public Duck() {
    }

    public void setFlyBehavior(FlyBehavior fb) {
        flyBehavior = fb;
    }

    public void setQuackBehavior(QuackBehavior qb) {
        quackBehavior = qb;
    }

    abstract void display();

    public void performFly() {
        flyBehavior.fly();
    }

    public void performQuack() {
        quackBehavior.quack();
    }

    public void swim() {
        System.out.println("All ducks float, even decoys!");
    }
}

Sample Code : Python 🐍

From this repository :

class Duck():
    _fly_behavior = None
    _quack_behavior = None

    def set_fly_behavior(self, fly_behavior):
        self._fly_behavior = fly_behavior

    def set_quack_behavior(self, quack_behavior):
        self._quack_behavior = quack_behavior

    def display(self):
        raise NotImplementedError

    def perform_fly(self):
        self._fly_behavior.fly()

    def perform_quack(self):
        self._quack_behavior.quack()

    def swim(self):
        print("All ducks float, even decoys! 〰🦆〰")

Object Oriented Design Principles

• Encapsulate what varies
• Open-Closed Principle: Open for extension, closed for modification
• Program to an interface, not to an implementation
• Favour composition over inheritence
• Dependency Inversion Principle
• Depend upon abstractions
• The Hollywood Principle : "Don't call us, we'll call you"
• One Class, One Responsibility Principle
• Single Responsibility Principle
• Principle of Least Knowledge

Model View Controller (MVC)

I started to work on the MVC pattern here but have a small complete MVC implementation in JavaScript in another repo.