🏛️ Design Patterns — The Definitive Python 3.14 Reference

One-stop. Evergreen. For all levels — from curious beginners to battle-hardened architects.

This repository is a comprehensive, notebook-style guide to software design patterns. Every pattern includes vivid real-world analogies, UML diagrams (Mermaid), Python 3.14 code examples, and real-world framework sightings from Django, FastAPI, SQLAlchemy, Celery, and beyond.

---

📚 Table of Contents

🏗️ Foundations

• 00 - Foundations — SOLID, DRY, KISS, YAGNI, Composition vs Inheritance, UML Primer

---

🔨 Part I: Creational Patterns

How objects are created — controlling instantiation to keep systems flexible and decoupled.

• 01 - Singleton
• 02 - Factory Method
• 03 - Abstract Factory
• 04 - Builder
• 05 - Prototype

---

🧱 Part II: Structural Patterns

How objects are composed — building larger structures from individual parts.

• 06 - Adapter
• 07 - Bridge
• 08 - Composite
• 09 - Decorator
• 10 - Facade
• 11 - Flyweight
• 12 - Proxy

---

💬 Part III: Behavioral Patterns

How objects communicate — defining interaction protocols and responsibility distribution.

• 13 - Chain of Responsibility
• 14 - Command
• 15 - Iterator
• 16 - Mediator
• 17 - Memento
• 18 - Observer
• 19 - State
• 20 - Strategy
• 21 - Template Method
• 22 - Visitor

---

🚀 Part IV: Advanced & Architectural

Beyond GoF — patterns that shape real-world systems at scale.

• 23 - Repository Pattern
• 24 - Dependency Injection
• 25 - Event Sourcing & CQRS
• 26 - Patterns in the Wild (Capstone)

---

🗺️ Pattern Decision Matrix

"I have a problem. Which pattern do I reach for?"

Problem	Reach For	Chapter
Need exactly one instance globally	Singleton	01
Create objects without specifying exact class	Factory Method	02
Create families of related objects	Abstract Factory	03
Construct complex objects step-by-step	Builder	04
Clone existing objects instead of creating new	Prototype	05
Make incompatible interfaces work together	Adapter	06
Vary abstraction AND implementation independently	Bridge	07
Treat individual objects and compositions uniformly	Composite	08
Add behavior to objects dynamically	Decorator	09
Simplify a complex subsystem	Facade	10
Share state to support large numbers of fine-grained objects	Flyweight	11
Control access to an object	Proxy	12
Pass a request along a chain of handlers	Chain of Responsibility	13
Encapsulate a request as an object (undo, queue)	Command	14
Access elements sequentially without exposing internals	Iterator	15
Reduce chaotic dependencies between objects	Mediator	16
Save and restore object state	Memento	17
Notify multiple objects about state changes	Observer	18
Object behaves differently based on internal state	State	19
Swap algorithms at runtime	Strategy	20
Define a skeleton algorithm with customizable steps	Template Method	21
Add operations to objects without modifying them	Visitor	22
Abstract data persistence layer	Repository	23
Decouple construction from usage of dependencies	Dependency Injection	24
Track state changes as a sequence of events	Event Sourcing & CQRS	25

---

🔗 Master Pattern Relationships

graph TB
    subgraph "Creational"
        S["01 Singleton"]
        FM["02 Factory Method"]
        AF["03 Abstract Factory"]
        B["04 Builder"]
        P["05 Prototype"]
    end

    subgraph "Structural"
        AD["06 Adapter"]
        BR["07 Bridge"]
        CO["08 Composite"]
        DE["09 Decorator"]
        FA["10 Facade"]
        FW["11 Flyweight"]
        PR["12 Proxy"]
    end

    subgraph "Behavioral"
        COR["13 Chain of Resp."]
        CMD["14 Command"]
        IT["15 Iterator"]
        MED["16 Mediator"]
        MEM["17 Memento"]
        OBS["18 Observer"]
        ST["19 State"]
        STR["20 Strategy"]
        TM["21 Template Method"]
        VI["22 Visitor"]
    end

    subgraph "Architectural"
        RP["23 Repository"]
        DI["24 Dependency Injection"]
        ES["25 Event Sourcing"]
    end

    FM --> AF
    AF --> FM
    B --> S
    AF --> B
    P --> S

    AD --> BR
    DE --> CO
    PR --> DE
    FA --> AD
    FW --> FA

    COR --> CO
    CMD --> MEM
    IT --> CO
    MED --> OBS
    OBS --> MED
    ST --> STR
    STR --> TM
    VI --> CO
    VI --> IT

    RP --> FM
    DI --> STR
    DI --> FM
    ES --> OBS
    ES --> CMD

Loading

---

🎯 How to Use This Repository

1. Start with Chapter 00 — Foundations if you need a refresher on SOLID, UML, or composition vs inheritance
2. Go chapter-by-chapter for a structured learning path, or jump directly to a pattern you need
3. Each chapter has a theory.md — read this first for the conceptual deep-dive
4. Then explore example_*.py files — runnable Python 3.14 code you can execute immediately
5. End with Chapter 26 — Capstone for the decision matrix, anti-patterns, and interview playbook

---

Built with ❤️ using Python 3.14 • Diagrams in Mermaid • Real-world examples from Django, FastAPI, SQLAlchemy & more