What are decorators in Python?

Question presented to candidate: "You see @something above function definitions all over Python code. What is a decorator, what does the @ line actually do, and can you write one for me, say one that logs or changes what a function returns?"

What a strong answer should cover:

• A decorator is a callable that takes a function (or class) and returns a replacement; @deco above def f means f = deco(f), applied once when the def statement runs.
• It works because functions are first-class objects and the inner wrapper is a closure that remembers the original function.
• The standard shape: an inner wrapper(*args, **kwargs) that calls the original, returns its result, and is decorated with functools.wraps.
• Stacking order: the decorator nearest the def is applied first, the top one wraps everything (so it runs first at call time).
• Real uses: logging, timing, caching (functools.lru_cache), access checks, retries, registration (web routes, plugins), property / classmethod / staticmethod, and class decorators such as dataclasses.dataclass.
• A version fact: function decorators came with PEP 318 (Python 2.4), class decorators with PEP 3129, and PEP 614 (Python 3.9) allows any expression after @.

Code / implementation expected: Yes — write a small decorator (logging, timing or a result transform) live, with *args, **kwargs and functools.wraps.