Setup

pip install fastapi uvicorn[standard]
# optional extras
pip install fastapi[all]          # includes uvicorn, pydantic, python-multipart, etc.
pip install sqlalchemy alembic    # DB
pip install python-jose[cryptography] passlib[bcrypt]  # auth

from fastapi import FastAPI

app = FastAPI()

@app.get("/")
def root():
    return {"message": "Hello, World"}

# development (auto-reload)
uvicorn main:app --reload

# specify host/port
uvicorn main:app --host 0.0.0.0 --port 8080 --reload

# production (multiple workers)
uvicorn main:app --workers 4

# or with gunicorn (process manager)
gunicorn main:app -k uvicorn.workers.UvicornWorker -w 4

app = FastAPI(
    title="My API",
    description="API description shown in docs",
    version="1.0.0",
    docs_url="/docs",          # Swagger UI  (None to disable)
    redoc_url="/redoc",        # ReDoc       (None to disable)
    openapi_url="/openapi.json",
    terms_of_service="https://example.com/terms",
    contact={"name": "Dev", "email": "dev@example.com"},
    license_info={"name": "MIT"},
    root_path="/api/v1",       # useful behind a proxy/gateway
    openapi_tags=[             # tag metadata for docs grouping
        {"name": "users", "description": "User operations"},
    ],
)

project/
├── app/
│   ├── __init__.py
│   ├── main.py          # FastAPI() instance + include_router
│   ├── dependencies.py  # shared Depends()
│   ├── models.py        # Pydantic schemas
│   ├── database.py      # SQLAlchemy engine / session
│   └── routers/
│       ├── users.py
│       └── items.py
├── tests/
└── requirements.txt

# app/routers/users.py
from fastapi import APIRouter

router = APIRouter(prefix="/users", tags=["users"])

@router.get("/")
def list_users():
    return []

# app/main.py
from fastapi import FastAPI
from app.routers import users

app = FastAPI()
app.include_router(users.router)
app.include_router(
    items.router,
    prefix="/items",
    tags=["items"],
    dependencies=[Depends(verify_token)],   # apply to all routes
    responses={404: {"description": "Not found"}},
)

# Modern: lifespan context manager (FastAPI >= 0.93)
from contextlib import asynccontextmanager

@asynccontextmanager
async def lifespan(app: FastAPI):
    # startup logic
    db.connect()
    yield
    # shutdown logic
    db.disconnect()

app = FastAPI(lifespan=lifespan)

# Legacy (still works but deprecated):
@app.on_event("startup")
async def startup():
    await db.connect()

@app.on_event("shutdown")
async def shutdown():
    await db.disconnect()

# pip install pydantic-settings
from pydantic_settings import BaseSettings

class Settings(BaseSettings):
    database_url: str
    secret_key: str
    debug: bool = False

    model_config = {"env_file": ".env", "env_file_encoding": "utf-8"}

settings = Settings()   # reads from environment / .env automatically

# in a route or dependency:
from functools import lru_cache

@lru_cache
def get_settings() -> Settings:
    return Settings()