Applying SOLID Together

Real classes rarely break one SOLID principle at a time. This topic puts the five together: how they depend on one another, how to refactor a class that breaks all of them without breaking what it does, and how far to go.

It starts with the relationships between the principles, then refactors a checkout class that breaks all five principles, one principle at a time, with the same tests passing after every step. A notification service follows, reached by the same reasoning from a different starting point, and a comparison shows what changes when Python's duck typing replaces declared interfaces.

The last cards show a refactor that went wrong because nothing pinned today's behaviour, and give you five short classes to diagnose.

By the end you can name the principle a design breaks from its symptom, explain how the principles support each other, order a safe refactor, and say what duck typing changes in Python.