HTTPException usage in FastAPI - Mini Project: Build & Apply

Using HTTPException in FastAPI

📖 Scenario: You are building a simple FastAPI web service that returns user information. You want to handle cases where the requested user does not exist by sending a proper HTTP error response.

🎯 Goal: Create a FastAPI app with a route /users/{user_id} that returns user data if found. If the user ID is not in the data, raise an HTTPException with status code 404 and a clear error message.

📋 What You'll Learn

Create a dictionary called users with these exact entries: 1: {'name': 'Alice'}, 2: {'name': 'Bob'}, 3: {'name': 'Charlie'}

Create a FastAPI app instance called app

Create a GET route /users/{user_id} that accepts an integer user_id

Inside the route, check if user_id is in users

If user_id is not found, raise HTTPException(status_code=404, detail='User not found')

If found, return the user data dictionary for that user_id

💡 Why This Matters

🌍 Real World

Web APIs often need to handle missing data gracefully by sending proper HTTP error codes and messages.

💼 Career

Knowing how to use HTTPException in FastAPI is essential for backend developers building robust APIs.

1

Create the user data dictionary

Create a dictionary called users with these exact entries: 1: {'name': 'Alice'}, 2: {'name': 'Bob'}, 3: {'name': 'Charlie'}

FastAPI

# Create the users dictionary with user IDs and names
# Your code here

Hint

Use curly braces to create a dictionary. Keys are numbers 1, 2, 3. Values are dictionaries with key 'name' and the exact names.

2

Create the FastAPI app instance

Import FastAPI from fastapi and create an app instance called app by calling FastAPI()

FastAPI

users = {
    1: {'name': 'Alice'},
    2: {'name': 'Bob'},
    3: {'name': 'Charlie'}
}

# Import FastAPI and create app instance
# Your code here

Hint

Use from fastapi import FastAPI and then app = FastAPI() to create the app.

3

Create the GET route with user_id parameter

Import HTTPException from fastapi. Create a GET route /users/{user_id} using @app.get. Define a function get_user that takes an integer parameter user_id. Inside the function, check if user_id is in users. If not, raise HTTPException(status_code=404, detail='User not found'). If found, return users[user_id].

FastAPI

from fastapi import FastAPI
from fastapi import HTTPException

users = {
    1: {'name': 'Alice'},
    2: {'name': 'Bob'},
    3: {'name': 'Charlie'}
}

app = FastAPI()

# Create GET route /users/{user_id} with function get_user(user_id: int)
# Check if user_id in users, else raise HTTPException
# Return users[user_id] if found
# Your code here

Hint

Use @app.get('/users/{user_id}') to create the route. Use if user_id not in users: to check. Raise HTTPException with status 404 and detail message if missing.

4

Complete the FastAPI app with proper imports and route

Ensure the code imports FastAPI and HTTPException from fastapi. The app instance is called app. The GET route /users/{user_id} is defined with function get_user(user_id: int). The function raises HTTPException(status_code=404, detail='User not found') if the user is missing, else returns the user data from users dictionary.

FastAPI

from fastapi import FastAPI
from fastapi import HTTPException

users = {
    1: {'name': 'Alice'},
    2: {'name': 'Bob'},
    3: {'name': 'Charlie'}
}

app = FastAPI()

@app.get('/users/{user_id}')
def get_user(user_id: int):
    if user_id not in users:
        raise HTTPException(status_code=404, detail='User not found')
    return users[user_id]

# Final check: all imports, app, route, and exception usage are correct
# Your code here

Hint

Review your code to confirm all parts are present and correct.

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