How do you organise a FastAPI application with multiple routers (APIRouter)?

APIRouter is FastAPI's equivalent of Flask Blueprints — it lets you group related routes in separate files, then include them in the main app. This keeps large codebases manageable.

# routers/items.py
from fastapi import APIRouter, Depends

router = APIRouter(
    prefix="/items",          # all routes here are prefixed
    tags=["items"],           # Swagger UI grouping label
    responses={404: {"description": "Not found"}},  # shared response docs
)

@router.get("/")
def list_items():
    return [{"name": "item1"}]

@router.get("/{item_id}")
def get_item(item_id: int):
    return {"item_id": item_id}

@router.post("/", status_code=201)
def create_item(name: str):
    return {"name": name}

# routers/users.py
from fastapi import APIRouter

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

@router.get("/")
def list_users():
    return [{"username": "alice"}]

# main.py
from fastapi import FastAPI
from routers import items, users

app = FastAPI()

app.include_router(items.router)
app.include_router(users.router)
app.include_router(
    users.router,
    prefix="/v2",         # override prefix for a second version
    dependencies=[Depends(verify_admin)],  # apply dep to all routes
)

@app.get("/")  # root route stays in main.py
def root():
    return {"message": "API root"}

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