Control Changes with Property Setters

Everything so far has pushed in one direction: fewer ways for the outside world to write to an object. Lesson 2 refused bad construction, Lesson 3 replaced assignment with events, and Lesson 5 made the score readable but not writable.

Sometimes a value genuinely should be changeable. A timer’s duration might be adjusted after a review. Someone simply changed their mind.

A property that accepts a value

A property can define a second half that runs when someone assigns to it:

The reading half is @property. The writing half is @minutes.setter, named after the property it belongs to. Both methods are called minutes: one runs during reading and the other during assignment. Notice that __init__ uses self.minutes = minutes, too, so construction cannot bypass the rule.

Now an invalid assignment is refused:

timer.minutes = 0

The class inspects the value before storing it. If the check raises, the assignment never happens and the old valid value remains.

Use a setter deliberately

Wrapping every attribute in a property adds code without adding safety. Reach for a setter when both conditions hold:

• callers genuinely need to assign this one attribute;

• the attribute has a rule worth enforcing.

A score computed from recorded answers fails the first condition, so it stays read-only. In the exercise, a question’s adjustable point value meets both.

The exercise adds one justified setter and checks the load-bearing failure case: a refused assignment leaves the previous value untouched. The next lesson handles a different problem: one real change that must update several attributes together.