Setup

# install dependencies
pip install -r requirements.txt

# run
python app.py

After this, just open http://127.0.0.1:5000 in your browser.

Youtube Presentation

https://www.youtube.com/watch?v=sd_cQRbV6rE

Design Patterns Used

Pattern	File	Used for
Singleton	patterns/singleton.py	SessionManager, one shared instance across the app
Observer	patterns/observer.py	WatchlistManager, notifies renters when a watched car drops in price
Mediator	patterns/mediator.py	NotificationMediator, handles booking, payment, and message events
Builder	patterns/builder.py	CarListingBuilder, constructs car listings with required + optional fields
Proxy	patterns/proxy.py	PaymentProxy, validates before delegating to RealPaymentService
Chain of Responsibility	patterns/chain.py	PasswordRecoveryChain, three security question handlers