FastAPI Fundamentals

FastAPI is the dominant modern Python web framework — built on Starlette + Pydantic, with automatic validation and interactive docs generated from your type hints.

Minimal app:

python from fastapi import FastAPI app = FastAPI() @app.get("/") def read_root(): return {"message": "Hello World"} # Run with: uvicorn main:app --reload

Path and query parameters — type hints drive validation:

python from typing import Optional @app.get("/items/{item_id}") def get_item(item_id: int, q: Optional[str] = None): return {"item_id": item_id, "q": q} - item_id is a **path parameter** — it appears in {} in the route. int means FastAPI validates and converts it, returning a 422 error if the URL segment isn't a valid int. - q is a **query parameter** (?q=...) — any function parameter not in the path or declared as a body model becomes a query parameter. Optional[str] = None` makes it optional.

Request bodies with Pydantic:

python from pydantic import BaseModel class Item(BaseModel): name: str price: float in_stock: bool = True @app.post("/items") def create_item(item: Item): return {"name": item.name, "total": item.price} FastAPI parses the JSON body, validates field types, and gives you a typed Item` object. Invalid bodies (missing fields, wrong types) return 422 automatically — no manual validation code needed.

Dependency Injection with Depends():

python from fastapi import Depends, HTTPException def get_db(): db = SessionLocal() try: yield db finally: db.close() def get_current_user(token: str = Depends(oauth2_scheme)): user = decode_token(token) if not user: raise HTTPException(status_code=401, detail="Invalid token") return user @app.get("/profile") def read_profile(user=Depends(get_current_user), db=Depends(get_db)): return user Depends()` tells FastAPI to call a function (or generator, for setup/teardown) and inject its return value as an argument — used for DB sessions, auth checks, and shared logic across routes.

Async endpoints:

python @app.get("/users/{user_id}") async def get_user(user_id: int): user = await db.fetch_one("SELECT * FROM users WHERE id = :id", {"id": user_id}) return user Use async def when the body awaits I/O (DB, HTTP calls). A plain def` endpoint still works fine — FastAPI runs it in a thread pool.

Route ordering matters:

python @app.get("/users/me") # must come first def read_current_user(): return {"user": "me"} @app.get("/users/{user_id}") # would otherwise match "/users/me" first def read_user(user_id: int): return {"user_id": user_id}

Automatic docs: every FastAPI app gets interactive docs at /docs (Swagger UI) and /redoc, generated entirely from your type hints and Pydantic models.