APIRouter and application structure

Difficulty: Intermediate

Question

How do you organize a large FastAPI application using APIRouter?

Answer

`APIRouter` is a mini-application that groups related routes. It works exactly like the `FastAPI` app instance but is included into the main app with `app.include_router()`.

Benefits: - Split routes into multiple files/modules - Apply shared prefix, tags, dependencies, and responses to all routes in the router - Keep each domain area (users, items, orders) in separate files

Typical structure: ``` app/ main.py # FastAPI instance, includes routers routers/ users.py # APIRouter for /users items.py # APIRouter for /items models/ # Pydantic models database.py # DB setup ```

Code examples

APIRouter usage

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

router = APIRouter(
    prefix="/users",
    tags=["users"],
    responses={404: {"description": "Not found"}}
)

@router.get("/")
def list_users():
    return [{"id": 1, "name": "Alice"}]

@router.get("/{user_id}")
def get_user(user_id: int):
    return {"id": user_id, "name": "Alice"}

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

app = FastAPI()
app.include_router(users.router)
app.include_router(items.router, prefix="/api/v1")  # override prefix

Key points

• Router-level `dependencies` apply to all routes in the router
• `app.include_router()` can add additional prefix, tags, or dependencies on top of the router's own
• Routers can include other routers for deep nesting

Concepts covered

APIRouter, modular structure, prefix, tags