Teaching My Backend to Listen and Reply — FastAPI CRUD, Phase 2

Validate what comes in, shape what goes out, and give every outcome its proper status code. So Phase 1 gave my app a memory — a real database that remembers users and expenses even after a restart. Small problem: the only way to actually talk to it was a Python script I ran by hand. The app could remember things, but it was mute. Nobody on the internet could add an expense, list their spending, or delete a typo. Phase 2 fixes that. This is where the app grows a mouth — real HTTP endpoints you hit through the browser. The buzzword is CRUD : Create, Read, Update, Delete. The four things basically every app does to data. I went in thinking "it's just four functions, how hard can it be" and came out having learned about request/response contracts, dependency injection, and roughly six different HTTP status codes — mostly by triggering them wrong first. Let me dump what I learned [and the parts that tripped me up, because there were, uh, several]. The Structure is as follows. Lets call it PHASE 2 — The Endpoints: Set up the request/response contracts (two Pydantic schemas: one for input, one for output) Build a session dependency so every request gets a safe, auto-closing DB connection POST — create an expense GET — list them all + fetch one [with a proper 404] PATCH — update just the fields that changed DELETE — remove one cleanly First, the two new ideas [and no, nothing to install this time] Phase 1 had two new libraries. Phase 2 has two new ideas — and the nice part is there's nothing to pip install , because Pydantic already ships inside FastAPI. [Which means requirements.txt didn't change this phase. Still worth knowing the freeze habit is only for phases where you actually install something.] Pydantic — the bouncer at the door. Your SQLAlchemy models describe how data is stored ; Pydantic describes the shape data must have to cross the border of your API . It checks types, rejects garbage, and turns raw JSON into a clean Python object before it gets anywhere near