Solution 1: Basic Decorator

Implementation

class LoggingDecorator(Notifier):
    def __init__(self, wrapped: Notifier):
        self._wrapped = wrapped

    def send(self, recipient: str, message: str) -> None:
        print(f"SENDING notification to {recipient}")
        self._wrapped.send(recipient, message)
        print(f"SENT notification to {recipient}")


class UppercaseDecorator(Notifier):
    def __init__(self, wrapped: Notifier):
        self._wrapped = wrapped

    def send(self, recipient: str, message: str) -> None:
        self._wrapped.send(recipient, message.upper())

Composition:

notifier = LoggingDecorator(UppercaseDecorator(EmailNotifier()))
notifier.send("bob@example.com", "server is down")

Output:

SENDING notification to bob@example.com
EMAIL to bob@example.com: SERVER IS DOWN
SENT notification to bob@example.com

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What makes this work

Every decorator:

1. Implements Notifier — so the caller never knows it is wrapped.
2. Receives the wrapped object in its constructor — so the chain can be any depth.
3. Calls self._wrapped.send(...) — so the real work still happens.
4. Only does one thing — LoggingDecorator only logs, UppercaseDecorator only transforms.

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The key structural insight

Notice that UppercaseDecorator wraps the inner object and LoggingDecorator wraps that:

LoggingDecorator
    └── UppercaseDecorator
            └── EmailNotifier

The call flows inward:

LoggingDecorator.send
    prints "SENDING..."
    calls UppercaseDecorator.send
        uppercases message
        calls EmailNotifier.send
            prints "EMAIL to..."
    prints "SENT..."

This is why the logged message is already uppercase in the output — uppercasing happens before the email is sent, and logging wraps the entire operation.

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Possible pitfall: forgetting to call self._wrapped.send

class BrokenDecorator(Notifier):
    def __init__(self, wrapped: Notifier):
        self._wrapped = wrapped

    def send(self, recipient: str, message: str) -> None:
        print(f"SENDING to {recipient}")
        # forgot to call self._wrapped.send !

This decorator silently swallows the notification. The log appears but nothing is actually sent. Always call through to the wrapped object unless you have a deliberate reason not to (like a stub or a null object).

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Possible improvement: a shared decorator base

If you have many decorators, you can put the forwarding in a base class so each decorator only overrides what it changes:

class NotifierDecorator(Notifier):
    def __init__(self, wrapped: Notifier):
        self._wrapped = wrapped

    def send(self, recipient: str, message: str) -> None:
        self._wrapped.send(recipient, message)


class LoggingDecorator(NotifierDecorator):
    def send(self, recipient: str, message: str) -> None:
        print(f"SENDING notification to {recipient}")
        super().send(recipient, message)
        print(f"SENT notification to {recipient}")

This is not required for this exercise, but becomes useful in exercise 2 when you have several decorators.

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Exercise 1 · Exercise 2