20. Design Patterns

1. Singleton

A class that has only ONE instance (e.g., Database Connection).

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

2. Factory

A function that creates objects without exposing the logic to the client.

3. Observer

A subscription mechanism (EventListener).