Why Your Python Functions Are Secretly Changing Data You Never Passed to Them

Python's mutable default argument bug silently accumulates state across function calls — because default values are evaluated once at definition time, not per call. A list default like `cart=[]` is stored inside the function object and mutated on every call. The fix: default to `None` and initialize inside the function body. Intentional persistence (like memoization caches) is the one valid exception.