Exercise 2: Multiple Decorators and Ordering

The scenario

The notification system from Exercise 1 has grown. There are now three notifier types:

class EmailNotifier(Notifier):
    def send(self, recipient: str, message: str) -> None:
        print(f"EMAIL to {recipient}: {message}")

class SmsNotifier(Notifier):
    def send(self, recipient: str, message: str) -> None:
        print(f"SMS to {recipient}: {message[:160]}")  # SMS has a 160-char limit

class PushNotifier(Notifier):
    def send(self, recipient: str, message: str) -> None:
        print(f"PUSH to {recipient}: {message[:100]}")  # push has a 100-char limit

And four cross-cutting behaviours have been requested:

• Logging — log before and after each send (from Exercise 1).
• Retry — if the inner notifier raises an exception, retry up to N times before giving up.
• Rate limiting — track how many notifications have been sent; raise RateLimitExceeded if a per-instance limit is exceeded.
• Prefix — prepend a fixed string to every message (e.g. "[URGENT] " for critical alerts).

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Your task

Part A — implement the four decorators

Implement LoggingDecorator, RetryDecorator, RateLimitDecorator, and PrefixDecorator.

RetryDecorator constructor:

RetryDecorator(wrapped, max_retries=3)

It should print "Retry {n}/{max_retries}..." on each retry attempt.

RateLimitDecorator constructor:

RateLimitDecorator(wrapped, limit=5)

It should raise RateLimitExceeded when the limit is reached.

PrefixDecorator constructor:

PrefixDecorator(wrapped, prefix="[URGENT] ")

Part B — ordering matters

Consider these two compositions and predict the output for each before running them:

# Composition A
notifier_a = RateLimitDecorator(
    LoggingDecorator(EmailNotifier()),
    limit=5,
)

# Composition B
notifier_b = LoggingDecorator(
    RateLimitDecorator(EmailNotifier(), limit=5)
)

Call notifier_a.send("alice@example.com", "hello") six times on composition A, then the same on composition B.

Answer these questions:

1. In which composition does logging still happen when the rate limit is exceeded?
2. Why?
3. Which composition would you use if you wanted to audit every attempted send, including ones that were blocked?
4. Which would you use if you only wanted to log sends that actually went through?

Part C — a real-world composition

Build a notifier for a production alert system with these requirements:

• Uses EmailNotifier.
• Prepends "[ALERT] " to every message.
• Retries up to 2 times on failure.
• Logs all send attempts.
• Enforces a rate limit of 10 alerts per instance.

Write the composition and explain why you chose the order you did.

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Skeleton

See exercise2.py.

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What to focus on

• Each decorator still does exactly one thing.
• Decorator order is not arbitrary — it determines what each layer sees and when.
• The outermost decorator acts first on the way in and last on the way out.

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Exercise 1 · Solution 2 · Exercise 3