Decorators: wrappers you can see

You will be able to

• ✓ Wrap a call with start/end logs without changing the inner spec
• ✓ Keep the inner function testable without the wrapper
• ✓ See @timer as syntax for wrap(fn)

Before the slow report, say start and end

That wrapper is the idea behind decorators. Here it is wrap calling percent — visible, testable, no mystery. On a laptop, @timer above def report(): is syntax that builds the same wrapping. If you cannot write wrap(fn), you are not ready to decorate.

The inner function stays pure: percent returns a number. The wrapper logs. Tests call percent directly. Ops look at INFO lines. Mixing them is how a unit test needs a log file.

Wrappers you actually want

Timing a slow attendance CSV. Checking that the caller is allowed to run a job. Retrying a weather GET a bounded number of times. Each is a wrapper. None of them should change the meaning of the inner result — only observe or decide whether to call.

functools.wraps keeps the inner name in traces. Class decorators wrap a whole type — same idea, bigger surface. Copy the sample; do not paste @ into this Run box.

Words that matter

Wrapper

A function that calls another, adding logging, timing or checks.

Decorator

Laptop syntax (@name) that applies a wrapper.

Inner function

The job itself — still testable alone.

Common mistakes

On a full Python install — functools.wraps + a timer

Keep the inner name in traces. Time utilisation without editing percent’s body.

No pip — stdlib.

Decorator form of the same wrap() you will run in the sandbox.

from functools import wraps
from time import perf_counter

def timed(fn):
    @wraps(fn)
    def wrapper(*args, **kwargs):
        t0 = perf_counter()
        out = fn(*args, **kwargs)
        print("INFO ms", round((perf_counter() - t0) * 1000, 1))
        return out
    return wrapper

@timed
def percent(used, seats):
    return used * 100 / seats

Example program — Check, then call

A guard wrapper. Inner percent never sees seats 0.

Your turn — Wrap with INFO

Call a function that returns 90. Print INFO and 90.

Self-assessment

Check your understanding before you mark the lesson complete.