HTTPException usage in FastAPI - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when HTTPException is raised in FastAPI?

Consider this FastAPI endpoint code:

from fastapi import FastAPI, HTTPException
app = FastAPI()

@app.get('/items/{item_id}')
async def read_item(item_id: int):
    if item_id < 1:
        raise HTTPException(status_code=400, detail='Invalid item ID')
    return {'item_id': item_id}

What will the client receive if they request /items/0?

FastAPI

from fastapi import FastAPI, HTTPException
app = FastAPI()

@app.get('/items/{item_id}')
async def read_item(item_id: int):
    if item_id < 1:
        raise HTTPException(status_code=400, detail='Invalid item ID')
    return {'item_id': item_id}

A A JSON response with status 400 and body {"detail": "Invalid item ID"}

B A server crash with a traceback error

C A JSON response with status 200 and body {"item_id": 0}

D A redirect to the root path '/'

💡 Hint

Think about what HTTPException does in FastAPI when raised inside a route.

Raising HTTPException in FastAPI immediately returns an HTTP response with the given status code and detail as JSON. It does not crash the server or redirect.

📝 Syntax

intermediate

Identify the correct way to raise HTTPException with a custom header

Which of the following code snippets correctly raises an HTTPException with status 403 and a custom header X-Error: Forbidden?

A raise HTTPException(status_code=403, detail='Forbidden', headers={'X-Error': 'Forbidden'})

B raise HTTPException(403, 'Forbidden', headers={'X-Error': 'Forbidden'})

C raise HTTPException(status=403, message='Forbidden', headers={'X-Error': 'Forbidden'})

D raise HTTPException(code=403, detail='Forbidden', header={'X-Error': 'Forbidden'})

💡 Hint

Check the exact parameter names in HTTPException constructor.

The HTTPException constructor expects named parameters: status_code, detail, and headers. Option A uses correct parameter names and syntax.

🔧 Debug

advanced

Why does this HTTPException raise a TypeError?

Given this code snippet:

from fastapi import HTTPException

raise HTTPException(status_code=404, detail=123)

What error will occur and why?

FastAPI

from fastapi import HTTPException

raise HTTPException(status_code=404, detail=123)

A SyntaxError due to missing parentheses

B No error, it raises HTTPException with detail 123

C ValueError because status_code 404 is invalid

D TypeError because detail must be a string or dict, not an integer

💡 Hint

Check the expected type for the detail parameter in HTTPException.

The detail parameter should be a string, dict, or list. Passing an integer causes a TypeError when FastAPI tries to serialize the response.

❓ state_output

advanced

What is the response status code after catching HTTPException?

Consider this FastAPI code:

from fastapi import FastAPI, HTTPException
app = FastAPI()

@app.get('/test')
async def test():
    try:
        raise HTTPException(status_code=401, detail='Unauthorized')
    except HTTPException as e:
        return {'caught': True, 'status': e.status_code}

What will the HTTP status code of the response be when calling /test?

FastAPI

from fastapi import FastAPI, HTTPException
app = FastAPI()

@app.get('/test')
async def test():
    try:
        raise HTTPException(status_code=401, detail='Unauthorized')
    except HTTPException as e:
        return {'caught': True, 'status': e.status_code}

A 500 Internal Server Error due to unhandled exception

B 401 Unauthorized with JSON body {'caught': True, 'status': 401}

C 200 OK with JSON body {'caught': True, 'status': 401}

D No response because the server crashes

💡 Hint

Think about what happens when you catch the exception and return a normal dict.

Catching HTTPException prevents it from propagating. Returning a dict sends a 200 OK response with that JSON body.

🧠 Conceptual

expert

Which statement about HTTPException in FastAPI is true?

Select the one correct statement about how HTTPException works in FastAPI.

A HTTPException can only be raised inside dependency functions, not route handlers.

B Raising HTTPException immediately returns an HTTP response with the given status and detail without executing further code in the route.

C HTTPException automatically logs the error to a file named 'http_errors.log'.

D Raising HTTPException changes the global server status code for all future requests.

💡 Hint

Think about the purpose of HTTPException in request handling.

Raising HTTPException stops the current route execution and sends the specified HTTP error response immediately. It does not log automatically or affect global server state.

Practice

1. What is the main purpose of using HTTPException in FastAPI?

easy

A. To create database models automatically

B. To define API routes

C. To handle user authentication

D. To send HTTP error responses with a status code and message

Solution

1. Step 1: Understand HTTPException role

   HTTPException is designed to send HTTP error responses with specific status codes and messages.

2. Step 2: Compare with other options

   Other options like database models, authentication, or route definition are unrelated to HTTPException's purpose.

3. Final Answer:

   To send HTTP error responses with a status code and message -> Option D

4. Quick Check:

   HTTPException = send error response [OK]

Hint: HTTPException sends errors, not data or routes [OK]

Common Mistakes:

• Confusing HTTPException with route creation
• Thinking it manages database or authentication
• Using it to send successful responses

2. Which of the following is the correct way to raise an HTTP 404 error with a message in FastAPI?

easy

A. return HTTPException(404, "Item not found")

B. raise HTTPException(status_code=404, detail="Item not found")

C. HTTPException(404, detail="Item not found")

D. raise HTTPException(404, message="Item not found")

Solution

1. Step 1: Check correct syntax for raising HTTPException

   The correct syntax uses raise HTTPException(status_code=404, detail="Item not found").

2. Step 2: Identify errors in other options

   return HTTPException(404, "Item not found") wrongly uses return instead of raise. HTTPException(404, detail="Item not found") misses raise keyword. raise HTTPException(404, message="Item not found") uses incorrect keyword 'message' instead of 'detail'.

3. Final Answer:

   raise HTTPException(status_code=404, detail="Item not found") -> Option B

4. Quick Check:

   Use raise + status_code + detail [OK]

Hint: Always use raise with status_code and detail keys [OK]

Common Mistakes:

• Using return instead of raise
• Missing status_code or detail keywords
• Using wrong keyword like message

3. What will happen when this FastAPI endpoint is called and the item is not found?

from fastapi import FastAPI, HTTPException
app = FastAPI()

items = {"apple": "A juicy fruit"}

@app.get("/items/{item_name}")
async def read_item(item_name: str):
    if item_name not in items:
        raise HTTPException(status_code=404, detail="Item not found")
    return {"item": items[item_name]}

medium

A. The API returns the string 'Item not found'

B. The API returns an empty JSON response

C. The API returns a 404 error with message 'Item not found'

D. The API crashes with a server error

Solution

1. Step 1: Analyze the condition for missing item

   If the requested item_name is not in the items dictionary, the code raises HTTPException with status 404 and detail message.

2. Step 2: Understand FastAPI behavior on HTTPException

   FastAPI catches HTTPException and sends an HTTP response with the given status code and detail message as JSON.

3. Final Answer:

   The API returns a 404 error with message 'Item not found' -> Option C

4. Quick Check:

   Missing item triggers HTTPException 404 [OK]

Hint: Missing item triggers HTTPException with 404 [OK]

Common Mistakes:

• Expecting empty or string response instead of error
• Thinking the server crashes
• Ignoring the raise keyword effect

4. Identify the error in this FastAPI code snippet:

from fastapi import FastAPI, HTTPException
app = FastAPI()

@app.get("/users/{user_id}")
def get_user(user_id: int):
    if user_id < 0:
        HTTPException(status_code=400, detail="Invalid user ID")
    return {"user_id": user_id}

medium

A. Missing raise keyword before HTTPException

B. Incorrect status_code value for error

C. user_id should be a string, not int

D. The endpoint path is invalid

Solution

1. Step 1: Check how HTTPException is used

   The code calls HTTPException but does not use raise, so the exception is not actually raised.

2. Step 2: Verify other parts

   Status code 400 is valid for bad request. user_id as int is correct. Endpoint path syntax is valid.

3. Final Answer:

   Missing raise keyword before HTTPException -> Option A

4. Quick Check:

   Always use raise before HTTPException [OK]

Hint: Use raise before HTTPException to trigger error [OK]

Common Mistakes:

• Calling HTTPException without raise
• Using wrong status codes for errors
• Confusing parameter types

5. You want to create a FastAPI endpoint that returns a 403 Forbidden error with a custom message if a user is not an admin. Which code snippet correctly implements this?

hard

A. if not is_admin:
raise HTTPException(status_code=403, detail="Access denied: Admins only")

B. if not is_admin:
return HTTPException(status_code=403, detail="Access denied: Admins only")

C. if not is_admin:
raise HTTPException(403, message="Access denied: Admins only")

D. if not is_admin:
HTTPException(status_code=403, detail="Access denied: Admins only")

Solution

1. Step 1: Check correct way to raise HTTPException with 403

   The correct way is to use raise with status_code=403 and detail message.

2. Step 2: Identify errors in other options

   if not is_admin: return HTTPException(status_code=403, detail="Access denied: Admins only") uses return instead of raise, so no error is raised. if not is_admin: raise HTTPException(403, message="Access denied: Admins only") uses wrong keyword 'message'. if not is_admin: HTTPException(status_code=403, detail="Access denied: Admins only") calls HTTPException without raise, so no exception is triggered.

3. Final Answer:

   if not is_admin: raise HTTPException(status_code=403, detail="Access denied: Admins only") -> Option A

4. Quick Check:

   Use raise + status_code + detail for errors [OK]

Hint: Raise HTTPException with status_code and detail for errors [OK]

Common Mistakes:

• Using return instead of raise
• Wrong keyword 'message' instead of 'detail'
• Calling HTTPException without raise