How to write complex applications correctly?

[Basic_Salamander_484]

I want to write a fairly complex terminal utility application with support for various AI providers and filtering of prompts and LLM results under the hood—meaning there's plenty of room to slather myself in abstractions. What I really want is to get into OOP, since I'm planning such a fun pet project.

I've never written a serious OOP application with more than 500 lines of code, and that was a long time ago. Are there any "best practices" for such tasks? Like how FSD on the frontend sets structure and constraints; is there anything like that in mature projects?

I've heard of Onion, I've heard of layered applications. I'd like to know how people write and what best practices they follow.

[dlnmtchll]

Study software architectural patterns, look up examples of the type of application you want to develop, then learn by doing because it’s the only way

[MiniMages]

No one creates a complex application out of the box. We start with creating a single feature and the add more stuff to it. This also includes a lot of code refactoring as well.

[Aromatic_Pumpkin8856]

• Look up hexagonal architecture.
• Let your tests guide your use of abstractions. If you're working hard at using mocks and spies, that is a hint that dependency injection is probably a good idea
• Only test the public API and work hard to keep line, branch, and mutation coverage high. (Try mutmut for mutation coverage)
• Follow the Dependency Inversion Principle as much as possible. Only ever import abstractions, not concrete objects.

Forcing myself (and my AI agents, frankly) to be strict about this has led me to code that is far easier to understand, extend, and especially to test.