What Is the Single Responsibility Principle? Reasons to Change, Actors and Cohesion

Wed, 21 Oct 2026 00:01:00 +0000

An online shop has an Order class. Finance uses it to book revenue, the storefront uses it to print receipts, and the platform team uses it to store orders in the database. One day the storefront asks for receipts to show prices with VAT, a developer makes the change in the obvious place, every test the storefront cares about passes, and at the end of the month finance’s revenue is twenty per cent too high. Nobody touched finance’s code. They touched code that finance shared. The single responsibility principle is the rule that would have kept those two apart, and it is the most quoted and least understood of the five SOLID principles. This is the first part of a series on SOLID: it explains what the principle says, where it came from, how the acronym itself was born, what it looks like in running code, and how far to take it. Every output shown here was produced by running the code on Windows 11, with Python 3.14.6, not written by hand.

Cohesion on Artur Rios Lab

What Is the Single Responsibility Principle? Reasons to Change, Actors and Cohesion