Exercise 3: Adding SlackNotification and SlackNotificationCreator — Possible Solution
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THE NEW CLASSES, FOLLOWING THIS CHAPTER'S OWN PATTERN EXACTLY
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    class SlackNotification(Notification):
        def send(self, message):
            return f'Slack sent: {message}'

    class SlackNotificationCreator(NotificationCreator):
        def create_notification(self):
            return SlackNotification()

These follow the exact same shape as this chapter's own
EmailNotification/EmailNotificationCreator pair: SlackNotification
implements the shared send() interface, and SlackNotificationCreator
implements the one required factory method, create_notification(),
returning an instance of the new class.

VERIFYING IT WORKS THROUGH THE UNMODIFIED notify() METHOD
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Calling SlackNotificationCreator().notify('Your order shipped')
returns 'Slack sent: Your order shipped' - the correct result,
produced entirely by the base class's own existing notify() method
(unchanged from this chapter's own definition), which internally calls
create_notification() and then send() without ever needing to know
that a Slack-specific class exists.

WHY THIS CONFIRMS THE PATTERN'S OWN CORE CLAIM AGAIN
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This is the third notification type added to this exact hierarchy
across this chapter (following Email, SMS, and Push) - each one
required only a new pair of small, self-contained classes, and none of
them required a single line of NotificationCreator.notify() itself to
be edited. This directly confirms, for a fourth time, this chapter's
own verified zero-change extensibility claim, and demonstrates that the
pattern's benefit doesn't degrade or require special-casing as more
notification types accumulate - each new type is exactly as
self-contained as the first one added.

WHY THIS WORKS AS AN ANSWER
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The new classes are built by directly mirroring this chapter's own
established EmailNotification/EmailNotificationCreator structure
rather than inventing a different shape, the result is verified by
actually running notify() rather than assuming it would work, and the
answer explicitly connects this fourth successful extension back to
the pattern's own core, already-verified promise.