Including routers in main app in FastAPI - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output when accessing /items/42?

Given this FastAPI app with a router included, what will be the JSON response when you visit /items/42?

FastAPI

from fastapi import FastAPI, APIRouter

app = FastAPI()

items_router = APIRouter()

@items_router.get('/items/{item_id}')
async def read_item(item_id: int):
    return {"item_id": item_id, "name": f"Item {item_id}"}

app.include_router(items_router)

A 404 Not Found error

B {"detail": "Not Found"}

C {"item_id": 42, "name": "Item 42"}

D {"item_id": "42", "name": "Item 42"}

💡 Hint

Check how the router path and parameter types match the request URL.

The router defines a GET endpoint at /items/{item_id} where item_id is an integer. Accessing /items/42 matches this route and returns the JSON with item_id as 42 and the name formatted accordingly.

📝 Syntax

intermediate

Which option correctly includes a router with prefix '/users'?

Choose the correct way to include a router named users_router in the main FastAPI app with the prefix /users.

FastAPI

from fastapi import FastAPI, APIRouter

app = FastAPI()
users_router = APIRouter()

# Assume users_router has some routes defined

# Which line correctly includes the router with prefix '/users'?

A app.include(users_router, prefix='/users')

B app.include_router(users_router, prefix='/users')

C app.add_router(users_router, prefix='/users')

D app.include_router(users_router, path='/users')

💡 Hint

Check the FastAPI method name and parameter for adding routers with prefixes.

The correct method to add a router is include_router and the parameter to add a prefix is prefix. Other options use incorrect method names or parameters.

🔧 Debug

advanced

Why does this router path not respond as expected?

Consider this router code included in the main app. Why does accessing /api/data return 404?

FastAPI

from fastapi import FastAPI, APIRouter

app = FastAPI()

api_router = APIRouter(prefix='/api')

@api_router.get('/data')
async def get_data():
    return {"data": "value"}

app.include_router(api_router, prefix='/api')

A The route path becomes '/api/api/data' due to double prefix, so '/api/data' is not found.

B The router prefix is ignored, so '/data' is the actual path.

C The function get_data is missing async keyword causing runtime error.

D The app.include_router call is missing parentheses.

💡 Hint

Check how prefixes combine when including routers.

The router is defined with prefix '/api' and then included with prefix '/api' again. This results in the full path being '/api/api/data'. Accessing '/api/data' returns 404 because it does not match any route.

❓ state_output

advanced

What is the response after calling /counter twice?

Given this FastAPI app with a router that keeps a counter, what JSON response do you get on the second call to /counter?

FastAPI

from fastapi import FastAPI, APIRouter

app = FastAPI()
counter_router = APIRouter()

count = 0

@counter_router.get('/counter')
async def get_counter():
    global count
    count += 1
    return {"count": count}

app.include_router(counter_router)

A {"count": 0}

B {"count": 1}

C 500 Internal Server Error due to global variable

D {"count": 2}

💡 Hint

Think about how the global variable changes with each request.

The global variable count starts at 0 and increments by 1 on each call. The first call returns 1, the second call returns 2.

🧠 Conceptual

expert

What error occurs if you include the same router twice without a prefix?

What happens if you include the same APIRouter instance twice in a FastAPI app without using different prefixes?

FastAPI

from fastapi import FastAPI, APIRouter

app = FastAPI()

router = APIRouter()

@router.get('/hello')
async def say_hello():
    return {"msg": "Hello"}

app.include_router(router)
app.include_router(router)

A RuntimeError: Duplicate route path '/hello' detected

B The second include_router call is ignored silently

C The app runs but only one route is registered

D SyntaxError due to duplicate function definitions

💡 Hint

Think about route conflicts when including routers multiple times.

Including the same router twice without prefixes causes FastAPI to detect duplicate routes with the same path and method, raising a RuntimeError.