Design Patterns
Summary

GoF patterns, examples, and use cases. From creational to behavioral patterns.

Creational Patterns

Singleton

Ensure a class has only one instance.

class Singleton:
    _instance = None
    def __new__(cls):
        if cls._instance is None:
            cls._instance = super().__new__(cls)
        return cls._instance

Factory Method

Create objects without specifying exact class.

def create_animal(type):
    if type == "dog":
        return Dog()
    elif type == "cat":
        return Cat()

Builder

Construct complex objects step by step.

builder = ComputerBuilder()
computer = builder.add_cpu("Intel")
                  .add_ram("16GB")
                  .build()

Structural Patterns

Adapter

Convert interface of class into another interface.

class Adapter:
    def __init__(self, obj):
        self.obj = obj
    def adapted_method(self):
        return self.obj.original_method()

Decorator

Add responsibilities to objects dynamically.

@decorator
def function():
    pass

# Or class-based
class Decorator:
    def __init__(self, wrapped):
        self.wrapped = wrapped
    def __call__(self):
        return self.wrapped()

Behavioral Patterns

Observer

Define one-to-many dependency between objects.

class Observer:
    def update(self, subject):
        pass

class Subject:
    def attach(self, observer):
        self.observers.append(observer)
    def notify(self):
        for obs in self.observers:
            obs.update(self)

Strategy

Define family of algorithms, encapsulate each.

class Context:
    def __init__(self, strategy):
        self.strategy = strategy
    def execute(self, data):
        return self.strategy.execute(data)

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Design PatternsSummary