The Single Responsibility Principle states that a class or module should have one responsibility and one reason to change.
The Single Responsibility Principle assigns each class one responsibility and one reason to change. In practice, separate business logic from orchestration and communications. section 11.8section 7.1
Apply the principle by:
Splitting a class when mocking a concrete dependency requires preserving part of its functionality. section 11.8
Moving domain logic into one class and communication with out-of-process dependencies into another. section 11.8
Keeping complex, important code narrow while allowing orchestration code to coordinate many collaborators. section 7.1
Refactoring an overcomplicated class
Extract business logic from a class that also coordinates difficult dependencies. Leave a humble object to orchestrate, and test the extracted logic without those dependencies. section 7.1
How Unit Testing Principles, Practices, and Patterns covers it
How deep it goes
Explains responsibility separation through dependency examples, Humble Objects, and code depth versus width, without prescribing exhaustive design rules.
Examples
Uses concrete-class mocking, difficult dependencies, extracted logic, functional cores, domain layers, controllers, and presenters as examples.
What it assumes
Assumes familiarity with classes, collaborators, unit testing, and architectural separation; teaches responsibility through refactoring examples.
What you can do after
Refactor mixed classes into focused domain logic and simpler orchestration components that are easier to test.
How it connects
Connects to unit testing, black-box testing, hexagonal architecture, functional architecture, Humble Object, MVC, MVP, and Domain-Driven Design.
Software Design for Python ProgrammersThe Single Responsibility Principle states that a class should have one primary responsibility and remain cohesive around it.
.NET MAUI in ActionThe single-responsibility principle is a software design principle stating that a component should have one primary reason to change.
ASP.NET Core in Action, 3eThe single-responsibility principle is a design principle stating that a class should have responsibility for one piece of functionality and change only when that functionality changes.
principlekind of · A principle is a general rule, guideline, or value that guides decisions, behavior, or judgments.
cohesive classimplemented by · A class whose methods and instance variables collectively serve one closely related primary responsibility.
SOLID principleskind of · The SOLID principles are five object-oriented software-design principles concerning single responsibility, openness to extension, substitutability, interface segregation, and dependency inversion.
separation of concernsimplements · Separation of concerns is a design principle that organizes a system so distinct responsibilities are handled by separate, independently manageable parts.
functionconfigures · A function is a callable program unit that performs an operation, may accept inputs, and can produce a result or side effect.
pipeline componentimplemented by · A pipeline component is a discrete, executable stage in a data or machine-learning workflow that performs an operation and may consume or produce data artifacts.
This explainer was generated by Manning's knowledge graph from the text of Unit Testing Principles, Practices, and Patterns. The section links point to the parts of the book it draws on.
