Explain Python and React performance fundamentals

In a phone screen for a full-stack software engineering role, you may be asked a group of fundamentals questions such as: - Explain what a Python context manager is, how the `with` statement works, and when you would use it. - Discuss React rendering behavior, hooks, memoization, and common frontend performance issues. - During on-call, if a teammate reports that the application is rendering very slowly, describe how you would investigate, identify the root cause, and mitigate the issue. Answer these as practical engineering questions, focusing on correctness, trade-offs, and debugging methodology.