What Is the Liskov Substitution Principle? Subtypes, Contracts and the Promises a Base Class Makes

Wed, 21 Oct 2026 00:03:00 +0000

A function that sorts a list of shapes by area has never heard of the square. It was written against a base type, and it will be handed whatever subclasses exist now and whatever subclasses someone writes next year. Whether it keeps working depends on a promise nobody wrote down: that every one of those classes behaves the way the base type said it would. The Liskov substitution principle is that promise, made explicit. It is the third of the five SOLID principles, and this is part 3 of a series on SOLID. This article explains what the principle says, where it came from, the rules a subtype has to keep, the violations that ship inside widely used platforms, and how to repair one. Every output shown here was produced by running the code on Windows 11, with Python 3.14.6, mypy 2.4.0 and the .NET 10 SDK, not written by hand.

Subtyping on Artur Rios Lab

What Is the Liskov Substitution Principle? Subtypes, Contracts and the Promises a Base Class Makes