Problem statements for practice — attempt them before checking solutions. Focus on design, applying OOP principles, and writing clean Python code.
Design a BankAccount class with deposit, withdraw, and transfer functionality. Ensure proper validations and error handling. How would you make it thread-safe?
Implement a Singleton in Python. Explain why singletons might be problematic in testing and how dependency injection helps.
Create an Event system where functions can subscribe to an event and be notified when it is triggered. Demonstrate with a small example (e.g., a logger or notifier).
Design a context class that can use different strategies (algorithms) interchangeably at runtime. Provide at least two strategies (e.g., JSON serializer and XML serializer).
Implement a LRUCache class with get and put methods. Ensure least recently used items are evicted when capacity is reached.
Design a notification system that can send messages via email, SMS, and push notifications. Make the design extensible for adding new channels later.
Design a plugin system for a test runner where plugins can register setup/teardown hooks. Show how new plugins can be added without modifying core code.
Create an immutable class in Python. Show at least two different approaches (e.g., using @dataclass(frozen=True) and __slots__).
Design an example where composition is better than inheritance. Refactor an inheritance-based design into composition.
You are given a God-class that handles DB, validation, and reporting. Refactor the design by splitting responsibilities into smaller, testable classes.