Exercise 4: Monostate

Goal

Implement Monostate: many objects, one shared state.

Singleton says:

one object

Monostate says:

many objects, one shared state

In this exercise, Preferences() should create different objects, but all Preferences objects should share the same attributes.

What you will practice

• Redirecting each object’s __dict__ to shared storage
• Keeping object identity separate from object state
• Guarding initialization so later objects do not reset shared state
• Giving each subclass its own shared state dictionary
• Comparing Monostate with Singleton

Starting point

Open exercise4.py.

Fill the TODOs so that:

1. Preferences() creates different objects.
2. Different Preferences objects share the same __dict__.
3. Changing one object’s attribute is visible through another object.
4. Later Preferences(...) calls do not reset the first initialization.
5. Different subclasses do not accidentally share state.

Expected behavior

first = Preferences(theme="light")
second = Preferences(theme="dark")

assert first is not second
assert first.__dict__ is second.__dict__
assert second.theme == "light"

first.theme = "dark"
assert second.theme == "dark"

Run the tests

python -m pytest exercise4.py