SOLID Principles in Python — ELI5

Imagine you have a toy toolbox. Each tool does one job: the hammer hits nails, the screwdriver turns screws. You would never glue them together into one mega-tool because if anything broke, the whole thing would be useless.

SOLID is a set of five rules that help programmers keep their code like that toolbox — each piece does one clear job, and you can swap or fix pieces without breaking everything else.

Here is what the letters stand for, in everyday language:

• S — Each piece of code should do one thing. A toaster toasts. It should not also brew coffee.
• O — You should be able to add new features without rewriting the old ones. Think of phone apps: you install new ones, you do not rebuild the phone.
• L — If something promises to act a certain way, anything replacing it should keep that promise. A substitute teacher still teaches the class.
• I — Do not force someone to depend on stuff they do not need. A TV remote should not require you to learn the washing-machine buttons too.
• D — Important code should not depend on tiny details. A lamp plugs into any outlet — it does not care about the wiring behind the wall.

Python makes these rules easier to follow than many other languages because it uses things like duck typing and simple inheritance. But the ideas work everywhere.

The one thing to remember: SOLID keeps each part of your code independent so you can change one part without accidentally breaking three others.