HTTPException usage in FastAPI - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is HTTPException used for in FastAPI?

HTTPException is used to send HTTP error responses from your API. It stops normal processing and returns an error status code and message to the client.

beginner

How do you raise a 404 error using HTTPException in FastAPI?

You use raise HTTPException(status_code=404, detail='Item not found') inside your path operation function.

beginner

What parameters does HTTPException accept?

HTTPException mainly accepts status_code (an integer like 404 or 400) and detail (a message explaining the error).

beginner

What happens when you raise HTTPException in a FastAPI route?

The request stops processing, and FastAPI sends an HTTP response with the given status code and detail message to the client.

intermediate

Can you customize the response headers when raising HTTPException?

Yes, you can pass a headers dictionary to HTTPException to add custom HTTP headers in the error response.

Which of these is the correct way to raise a 400 Bad Request error in FastAPI?

A return Response(status_code=400)

B return HTTPException(400, 'Bad request')

C raise Exception('400 Bad Request')

D raise HTTPException(status_code=400, detail='Bad request')

Only raise HTTPException(status_code=400, detail='Bad request') correctly raises the HTTP error in FastAPI.

What does the detail parameter in HTTPException do?

A Provides a message explaining the error

B Sets the HTTP status code

C Defines the response headers

D Specifies the request method

detail is used to give a clear message about the error to the client.

If you want to add a custom header to an error response, which HTTPException parameter do you use?

A headers

B status_code

C detail

D content

The headers parameter lets you add custom HTTP headers to the error response.

What happens after you raise HTTPException in a FastAPI route?

A The route continues running

B FastAPI sends the error response immediately

C The server crashes

D Nothing happens

Raising HTTPException stops the route and sends the error response to the client.

Which status code is commonly used with HTTPException to indicate 'Not Found'?

A 200

B 500

C 404

D 302

404 means the requested resource was not found.

Explain how to use HTTPException in a FastAPI route to handle errors.

Think about how you stop normal processing and tell the client about an error.

Describe how you can customize the error response when raising HTTPException.

Consider what extra info you can send besides status and message.

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