Expected Answer (Python 3.14)

A decorator is a callable that takes a function or class and returns a replacement. In @log_calls syntax, Python evaluates it like process = log_calls(process). A function decorator usually returns a wrapper that performs work before or after calling the original function.

from functools import wraps

def log_calls(func):
    @wraps(func)
    def wrapper(*args, **kwargs):
        print(f"calling {func.__name__}")
        return func(*args, **kwargs)
    return wrapper

@log_calls
def greet(name):
    "Return a greeting."
    return f"Hello, {name}!"

assert greet("Ada") == "Hello, Ada!"
assert greet.__name__ == "greet"

functools.wraps copies metadata and exposes __wrapped__, which helps introspection, documentation tools, debuggers, and frameworks that inspect function signatures.

Why It Matters

Decorators are widely used for cross-cutting behavior such as logging, authorization, retries, validation, caching, and route registration. Understanding their mechanics helps you debug framework code and avoid wrappers that break tooling.

Common Mistakes

• Omitting @wraps(func): The wrapper hides the original function’s name, docstring, and other metadata.
• Forgetting to return the wrapped function’s result: The decorated function unexpectedly returns None.
• Using a fixed wrapper signature: A wrapper that does not accept *args, **kwargs fails for valid calls to the wrapped function.

Follow-up Questions

• How does a parameterized decorator work? (Answer: An outer function receives configuration and returns the actual decorator.)
• In what order are stacked decorators applied? (Answer: Bottom-up; @a above @b becomes a(b(function)).)