Exercise 2: Singleton as a Decorator

Goal

Turn Singleton behavior into a reusable decorator.

In Exercise 1, the singleton logic was written directly inside AppSettings. That works, but the same _instance and __new__ logic would be repeated if another class also needed Singleton behavior.

In this exercise, you will move the Singleton behavior into a decorator:

@singleton
class MetricsRegistry:
    ...

What you will practice

• Writing a decorator that receives a class
• Returning a wrapper function that controls instance creation
• Storing the cached instance in a closure
• Adding a small reset_instance() helper for exercises and tests
• Seeing the main tradeoff: the class name now points to a function

Starting point

Open exercise2.py.

Fill the TODOs in singleton(cls) so that:

1. The first call creates the decorated object.
2. Later calls return the cached object.
3. reset_instance() clears the cached instance.
4. The same decorator works for more than one class.

Expected behavior

metrics1 = MetricsRegistry(namespace="billing")
metrics2 = MetricsRegistry(namespace="email")

assert metrics1 is metrics2
assert metrics2.namespace == "billing"

The first call wins because the second call returns the cached object.

Run the tests

python -m pytest exercise2.py