dancergraham / HeadFirstDesignPatterns_python

Example code from Head First Design Patterns translated to python

Head First Design Patterns python code

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 mostly 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
• Builder (Bonus Pattern)
• Singleton
• Command
• Adapter
• Façade
• Template Method
• Iterator
• Composite
• State
• Proxy
• Model View Controller (MVC)
• Manager (Bonus Pattern)

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.