I recently spent three days chasing down a production bug that traced back to a function I'd written three years ago. The function was only 15 lines long, but it used every Python trick in the book: list comprehensions nested inside generator expressions, bitwise operators for boolean logic, and a custom decorator that modified function behavior based on runtime introspection. When a coworker asked me to explain how it worked during a code review, I had to admit I wasn't 100% sure myself. That should have been my first warning sign.

The bug itself was caused by an edge case where the bitwise operator behaved differently with None values than expected. Instead of simply checking if user_input and config_flag , I had written if user_input & config_flag , which threw a cryptic TypeError when either variable was None. What made this worse was that the function's complexity meant only three people in the company understood how it worked, and one of them had quit six months ago.…