What Is the Open-Closed Principle? Extension, Abstraction, Plug-ins and the Expression Problem

Wed, 21 Oct 2026 00:02:00 +0000

A shop adds a new way to ship parcels. In one codebase that means opening the function that prices shipping, adding a branch to the middle of it, and re-testing everything that calls it. In another it means writing one new class in one new file, and nothing that already works is touched. The difference between the two is what the open-closed principle is about: arranging code so that new behaviour arrives as new code, rather than as edits to code that is released, tested and depended on. It is the second of the five SOLID principles, and this is part 2 of a series on them, after the single responsibility principle. This article explains what the principle means, where it came from, what it looks like in running code, why no module can be closed against every change, how the expression problem sets its limits, and when the abstraction it calls for is worth paying for. Every output shown here was produced by running the code on Windows 11, with Python 3.14.6, not written by hand.

Open-Closed Principle on Artur Rios Lab

What Is the Open-Closed Principle? Extension, Abstraction, Plug-ins and the Expression Problem