Folder structure patterns in FastAPI - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding FastAPI folder structure for scalability

Which folder structure below best supports adding multiple API routers and models in a FastAPI project for better scalability?

A

app/
  main.py
  api/
    v1/
      endpoints.py
      models.py
  core/
    config.py
  utils.py

B

app/
  main.py
  models.py
  routes.py
  utils.py

C

app/
  main.py
  api.py
  models.py
  helpers.py

D

app/
  main.py
  endpoints/
    users.py
    items.py
  models.py

💡 Hint

Think about how to organize code to easily add new API versions and separate concerns.

Option A organizes the project into clear folders for API versions, core configuration, and utilities. This structure supports scalability by isolating endpoints and models per version.

❓ component_behavior

intermediate

Behavior of FastAPI app with nested routers

Given this folder structure and code, what will be the URL path to access the 'get_users' endpoint? Structure: app/ main.py api/ v1/ users.py Code in users.py: from fastapi import APIRouter router = APIRouter(prefix='/users') @router.get('/') async def get_users(): return ['Alice', 'Bob'] Code in main.py: from fastapi import FastAPI from api.v1.users import router as users_router app = FastAPI() app.include_router(users_router, prefix='/v1')

FastAPI

from fastapi import APIRouter
router = APIRouter(prefix='/users')

@router.get('/')
async def get_users():
    return ['Alice', 'Bob']

from fastapi import FastAPI
from api.v1.users import router as users_router
app = FastAPI()
app.include_router(users_router, prefix='/v1')

A /users/

B /v1/users/

C /users/v1/

D /v1/

💡 Hint

Remember how prefixes combine when including routers.

The router in users.py has prefix '/users'. It is included in main.py with prefix '/v1'. These prefixes combine, so the full path is '/v1/users/'.

📝 Syntax

advanced

Identify the syntax error in FastAPI folder import

In a FastAPI project with this folder structure: app/ main.py api/ __init__.py v1/ __init__.py users.py Which import statement in main.py correctly imports the 'users' router?

A import api.v1.users.router

B from api/v1/users import router

C from api.v1.users import router

D from api.v1 import users.router

💡 Hint

Python imports use dots, not slashes.

Option C uses correct Python import syntax with dots and imports the router object directly.

Option C uses slashes which is invalid syntax.

Option C tries to import a module path as a single name, which is invalid.

Option C tries to import an attribute with dot notation inside import, which is invalid.

🔧 Debug

advanced

Debugging missing endpoint error in FastAPI project

You have this folder structure: app/ main.py api/ v1/ users.py In users.py: from fastapi import APIRouter router = APIRouter(prefix='/users') @router.get('/users') async def read_users(): return ['user1', 'user2'] In main.py: from fastapi import FastAPI from api.v1 import users app = FastAPI() app.include_router(users.router, prefix='/v1') When you run the app and visit '/v1/users', you get 404 Not Found. What is the most likely cause?

A The router in users.py has prefix '/users', so the endpoint is at '/v1/users/users' instead of '/v1/users'.

B The users.py file is missing an __init__.py, so the import fails silently.

C The endpoint function read_users is missing the @app.get decorator.

D The include_router call should not have a prefix argument.

💡 Hint

Check how prefixes combine in routers.

Option A is correct. The router has prefix '/users' and the endpoint path is '/users', combined with the app's include_router prefix '/v1', making the full path '/v1/users/users'. Visiting '/v1/users' therefore returns 404.

Option A is incorrect because @router.get is the proper decorator when using APIRouter.

Option A is incorrect; users.py is a module and doesn't require __init__.py.

Option A is incorrect; prefix on include_router is valid and useful for versioning.

❓ state_output

expert

Output of FastAPI app with multiple routers and prefixes

Consider this FastAPI app folder structure: app/ main.py api/ v1/ users.py items.py users.py: from fastapi import APIRouter router = APIRouter(prefix='/users') @router.get('/') async def get_users(): return ['user1'] items.py: from fastapi import APIRouter router = APIRouter(prefix='/items') @router.get('/') async def get_items(): return ['item1'] main.py: from fastapi import FastAPI from api.v1.users import router as users_router from api.v1.items import router as items_router app = FastAPI() app.include_router(users_router, prefix='/v1') app.include_router(items_router, prefix='/v1') What will be the JSON response when a client requests GET /v1/items/ ?

FastAPI

from fastapi import APIRouter

# users.py
router = APIRouter(prefix='/users')
@router.get('/')
async def get_users():
    return ['user1']

# items.py
router = APIRouter(prefix='/items')
@router.get('/')
async def get_items():
    return ['item1']

# main.py
from fastapi import FastAPI
from api.v1.users import router as users_router
from api.v1.items import router as items_router
app = FastAPI()
app.include_router(users_router, prefix='/v1')
app.include_router(items_router, prefix='/v1')

A ['user1', 'item1']

B ['user1']

C 404 Not Found error

D ['item1']

💡 Hint

Check how prefixes combine for each router and which endpoint is called.

The items router has prefix '/items' and is included with prefix '/v1', so the full path is '/v1/items/'. The endpoint returns ['item1'], so the response is ['item1'].