Python Interview Questions

Lists are mutable and slightly heavier; tuples are immutable, hashable when their contents are hashable, and can therefore be used as dictionary keys. Interviewers usually follow up by asking where immutability helps — shared constants, cache keys and function defaults.

CPython uses reference counting plus a cyclic garbage collector for reference cycles. Objects are freed as soon as their refcount hits zero, which is why holding a reference in a global or a closure is the usual cause of a leak.

A decorator is a callable that takes a function and returns a wrapped function. They are used for cross-cutting behaviour — timing, retries, caching, authorisation — without touching the wrapped function's body.

The Global Interpreter Lock allows only one thread to execute Python bytecode at a time. IO-bound work still benefits from threads or asyncio; CPU-bound work needs multiprocessing, native extensions or a different runtime.

A shallow copy duplicates the outer container and shares references to the nested objects; a deep copy recursively duplicates everything. Mutating a nested list after a shallow copy is the classic bug this question probes.