Exercise 3: Singleton as a Metaclass

Goal

Implement Singleton behavior with a metaclass.

In Exercise 2, the decorator worked, but it replaced the class name with a wrapper function. In this exercise, the class should remain a real class.

That is the advantage of the metaclass approach.

What you will practice

• Understanding that normal classes use type as their metaclass
• Overriding __call__ on a custom metaclass
• Caching one instance per class
• Keeping isinstance(obj, Class) working
• Giving each singleton class its own cached instance

The normal behavior

A regular class behaves roughly like this:

type.__call__(RegularSettings, "development")

That creates a new object every time.

Your job is to change that call path by defining:

class SingletonMeta(type):
    def __call__(cls, *args, **kwargs):
        ...

Then classes can opt in:

class Settings(metaclass=SingletonMeta):
    ...

Starting point

Open exercise3.py.

Fill the TODOs so that:

1. Regular classes still create new objects every call.
2. Classes using SingletonMeta return one cached object.
3. __init__ runs only on the first creation.
4. Each singleton class has its own cached instance.
5. The class remains a real class, so isinstance(obj, Settings) works.

Run the tests

python -m pytest exercise3.py