Recap: Single Responsibility

The single responsibility principle gives each class one reason to change.

• A responsibility is an actor: a group of people who ask for changes. Count actors, not methods.
• The symptom is a change for one group breaking the work of another, as finance's rounding broke marketing's layout.
• The refactor moves each actor's code into its own class, one step at a time, checking the output after each step.
• Accidental duplication is code that looks shared but serves two actors; give each actor its own copy, as with payroll's and HR's regular hours.
• Libraries split the same way: Python's logging separates what to record, where it goes and how it looks.
• Over-splitting is the opposite mistake. Code that always changes together for the same people belongs together, however many methods it needs.