Three SOLID Principles, One Design Decision: OCP, ISP, and LSP in a Real FastAPI Service

Thu, 21 May 2026 00:00:00 +0000

Most SOLID explainers cover all five principles in one pass, give you a definition and a toy example for each, and move on. What they skip is showing up in a codebase that already works, finding where a principle is being violated, and making the case that fixing it actually changed anything.

This post focuses on three: OCP, ISP, and LSP. They came up together in a single design decision on a FastAPI invoice-reminder service — and the interesting thing is that one construct in Python’s type system addresses all three at once.

Clean-Architecture on ppl-blog

Three SOLID Principles, One Design Decision: OCP, ISP, and LSP in a Real FastAPI Service