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🐍 Python Low Level Design Problems

A comprehensive collection of 24 Low-Level Design (LLD) problems implemented in Python, following SOLID principles and design patterns — curated for 6+ years experienced backend developer interviews.

🆕 Also check out our Java Low-Level Designs and Golang Low-Level Designs!

📋 Project Index

# Project Focus Area Key Design Patterns
1 Parking Lot OOD Basics, Vehicle Management Strategy, Factory, Singleton
2 Chess Game Game State, Complex Rules Factory, State, Memento
3 Tic-Tac-Toe Simple Game, AI (Minimax) Strategy, State, Command
4 Snakes & Ladders Board Game, Dice Strategies Strategy, Observer, State
5 Vending Machine State Machine, Payments State, Strategy, Factory
6 LRU/LFU/TTL Cache Data Structures, Eviction Strategy, Decorator
7 Rate Limiter API Rate Limiting Strategy, Factory, Decorator
8 Pub-Sub System Messaging, Events Observer, Strategy, Decorator
9 Movie Ticket Booking Concurrency, Booking Strategy, Singleton, Observer
10 Splitwise (Expense Sharing) Graphs, Debt Simplification Strategy, Factory
11 Cab Booking (Uber) Real-time Matching, GeoRadius, Kafka, Zone Surge Strategy, State, Observer, Pub-Sub
12 Library Management Catalog, Fines, Circulation Strategy, Observer, Facade
13 Car Rental Platform Fleet, Availability Calendar, Hourly Booking Strategy, State, Decorator
14 ATM/Banking System State Machine, Security State, Strategy, Chain of Responsibility
15 Inventory Management Stock, Reorder, Warehouse Strategy, Observer, Facade
16 Payment Processing Payments, Refunds, Fraud Strategy, Chain of Responsibility
17 Job Scheduling Scheduling, Recurring Jobs Command, Strategy, Observer
18 Search Platform Indexing, Ranking, BM25 Strategy, Facade
19 Notification Service Multi-channel delivery, Retries, Quiet Hours Strategy, Observer
20 Big File Upload Resumable Chunked Uploads (TUS) Strategy, Repository, State Machine
21 Order Matching Engine Order Book, Price-Time Priority, Sequencer Command, Event Sourcing
22 Circuit Breaker & Resilience Retry, Backoff, Bulkhead, Timeout State, Decorator
23 Food Delivery Order State Machine, Dispatch, Idempotent Checkout State, Strategy, Observer
24 Logging Framework Logger Hierarchy, Async Handlers, Correlation IDs Chain of Responsibility, Strategy

🎯 New: start with the LLD Interview Playbook: time-boxed round script, Senior vs Staff signals, a concurrency toolkit for Python/Java/Go, and a 3-week practice plan.

Every project ships a test_<module>.py (stdlib unittest):

cd python-low-level-design/<project-name>
python -m unittest test_<module>

🎯 SOLID Principles Applied

Each project is designed following SOLID principles:

  • S - Single Responsibility: Each class has one clear purpose
  • O - Open/Closed: Open for extension, closed for modification
  • L - Liskov Substitution: Subtypes are substitutable for base types
  • I - Interface Segregation: Small, focused interfaces
  • D - Dependency Inversion: Depend on abstractions, not concretions

🧩 Design Patterns Used

Pattern Usage
Strategy Pricing, fee calculation, eviction strategies, matching algorithms
Factory Vehicle, piece, player, document creation
Observer Notifications, display updates, event handling
State Vending machine, ATM, game states
Facade Unified service interfaces
Decorator Adding features without modifying core classes
Command Job execution, undo/redo operations
Chain of Responsibility Validation pipelines, fraud checks
Singleton Cache manager, broker instance

🚀 How to Run

Each project is self-contained. Navigate to the project directory and run:

cd python-low-level-design/<project-name>
python <project_file>.py

Or run all projects from root:

for dir in python-low-level-design/*/; do
    echo "=== Running $dir ==="
    cd "$dir" && python *.py && cd - > /dev/null
done

📖 Interview Preparation

Each project includes an INTERVIEW_QUESTIONS.md file with: - Core design questions with follow-ups - Expected solution points - Design pattern discussions - Trade-off analyses - Scalability considerations

  1. Start with basics: Parking Lot, Vending Machine, Tic-Tac-Toe
  2. Move to games: Chess, Snakes & Ladders
  3. Learn data structures: LRU Cache, Rate Limiter
  4. Build systems: Library Management, Inventory, Car Rental
  5. Master real-world: Splitwise, Movie Booking, Uber
  6. Advanced: Payment Processing, Job Scheduling, Search Platform

📚 Additional Resources