Decorators introduction · CAB106

A decorator is a function that takes a function and returns a new function, usually a wrapper that adds behaviour before or after the original. The syntax @deco above a definition is shorthand for f = deco(f), so it rebinds the function's name to the wrapper.

Idea	Detail
Timing	A decorator runs once, when the function is defined
Stacking	Applied bottom-up: `@a` above `@b` means `a(b(f))`
Arguments	`@repeat(3)` calls `repeat(3)` first, which returns the real decorator
Metadata	`functools.wraps(f)` keeps the original `__name__` and `__doc__`
Return value	A decorator must return a callable; returning `None` breaks the function

Wrappers usually accept *args and **kwargs so they work with any signature, and pass the result back. Functions are objects, so a wrapper may store state on itself, such as a call counter attribute.

Exam trap: without functools.wraps the decorated function reports the wrapper's name, not its own, and a decorator that forgets to return the wrapper replaces the function with None.

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Question 1 · Tier 1, Recall

A decorator is best described as

a comment that explains what a function is meant to do
a class that is not allowed to contain any methods
a function that takes a function and returns a new function wrapping it
a keyword that deletes a function after its first use

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C. A decorator adds behaviour around a function without editing its code.

Why not A: A comment is ignored by Python and does not change behaviour.

Why not B: Decorators are usually functions, and classes may have methods.

Why not D: Python has no such keyword.

Question 2 · Tier 1, Recall

Which symbol is used to apply a decorator above a function definition?

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B. Writing @name above a def applies the decorator name to that function.

Why not A: # starts a comment.

Why not C: & is the bitwise AND operator.

Why not D: % is the remainder operator.

Key points

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