Route decorator syntax in FastAPI - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this FastAPI route?

Consider this FastAPI route definition. What will the server respond with when a GET request is made to /hello?

FastAPI

from fastapi import FastAPI
app = FastAPI()

@app.get('/hello')
async def say_hello():
    return {'message': 'Hello, FastAPI!'}

A A 404 Not Found error

B A plain text response with 'Hello, FastAPI!'

C A JSON response with {"message": "Hello, FastAPI!"}

D A server error 500

💡 Hint

Look at the decorator and the return type of the function.

The @app.get('/hello') decorator creates a GET route at '/hello'. The function returns a dictionary, which FastAPI automatically converts to JSON. So the response is JSON with the key 'message' and value 'Hello, FastAPI!'.

📝 Syntax

intermediate

Which route decorator syntax is correct for a POST method?

You want to create a POST route at /submit. Which decorator syntax is correct?

A @app.post('/submit')

B @app.route('/submit', methods=['POST'])

C @app.get('/submit')

D @app.post_method('/submit')

💡 Hint

FastAPI uses HTTP method names as decorator functions.

FastAPI uses decorators like @app.post() for POST routes. The syntax @app.route() is from Flask, not FastAPI. There is no @app.post_method() in FastAPI.

🔧 Debug

advanced

What error does this FastAPI route raise?

Examine this code snippet. What error will occur when starting the FastAPI app?

FastAPI

from fastapi import FastAPI
app = FastAPI()

@app.get('/items/{item_id}')
async def read_item(item_id: int):
    return {'item_id': item_id}

@app.get('/items/{item_id}')
async def read_item_duplicate(item_id: int):
    return {'item_id_duplicate': item_id}

A SyntaxError: Duplicate function names

B RuntimeError: Duplicate route path '/items/{item_id}'

C No error, both routes work fine

D TypeError: Missing return statement

💡 Hint

Check if two routes have the same path and method.

FastAPI does not allow two routes with the same path and HTTP method. Defining two @app.get('/items/{item_id}') routes causes a runtime error about duplicate routes.

❓ state_output

advanced

What is the response when using path and query parameters?

Given this FastAPI route, what is the JSON response for a GET request to /users/42?active=true?

FastAPI

from fastapi import FastAPI
app = FastAPI()

@app.get('/users/{user_id}')
async def get_user(user_id: int, active: bool = False):
    return {'user_id': user_id, 'active': active}

A {"user_id": 42, "active": false}

B {"user_id": "42", "active": "true"}

C 404 Not Found

D {"user_id": 42, "active": true}

💡 Hint

Query parameters are converted to their declared types.

The path parameter user_id is 42 as an integer. The query parameter active is passed as 'true' and converted to boolean True. So the response JSON reflects these values.

🧠 Conceptual

expert

Which option correctly explains FastAPI route decorator behavior with multiple decorators?

What happens if you decorate a FastAPI function with multiple HTTP method decorators like @app.get() and @app.post() on the same function?

FastAPI

from fastapi import FastAPI
app = FastAPI()

@app.get('/multi')
@app.post('/multi')
async def multi_method():
    return {'message': 'Works for GET and POST'}

A The route responds to both GET and POST requests at '/multi'.

B Only the last decorator (@app.post) is effective; GET requests return 404.

C FastAPI raises a runtime error about multiple decorators on one function.

D The route responds only to GET requests; POST requests return 405 Method Not Allowed.

💡 Hint

Think about how decorators stack in Python and FastAPI's routing.

FastAPI allows stacking multiple HTTP method decorators on the same function. This creates multiple routes for the same path but different methods. So the function handles both GET and POST requests at '/multi'.