Why error handling ensures reliability in FastAPI - See It in Action

Why error handling ensures reliability in FastAPI

📖 Scenario: You are building a simple FastAPI web service that returns user data by user ID. Sometimes, the user ID might not exist, or the input might be invalid. Proper error handling helps keep the service reliable and user-friendly.

🎯 Goal: Create a FastAPI app that handles errors gracefully by returning clear messages and proper HTTP status codes when a user is not found or input is invalid.

📋 What You'll Learn

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

Add a variable called max_user_id set to 3

Write a GET endpoint /users/{user_id} that returns the user name for a valid user_id

Add error handling to return 404 status with message 'User not found' if user_id is not in users

Add error handling to return 400 status with message 'Invalid user ID' if user_id is less than 1 or greater than max_user_id

💡 Why This Matters

🌍 Real World

Web services often need to handle invalid inputs and missing data gracefully to avoid crashes and provide clear feedback to users.

💼 Career

Understanding error handling in FastAPI is essential for backend developers to build reliable and user-friendly APIs.

1

Create the user data dictionary

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

FastAPI

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

Need a hint?

Use curly braces to create a dictionary with keys 1, 2, 3 and their corresponding names as values.

2

Add a max_user_id variable

Add a variable called max_user_id and set it to 3

FastAPI

users = {1: 'Alice', 2: 'Bob', 3: 'Charlie'}
# Add max_user_id variable here
# Your code here

Need a hint?

Just assign the number 3 to the variable max_user_id.

3

Create the GET endpoint to return user names

Import FastAPI and create an app instance called app. Then write a GET endpoint /users/{user_id} that takes user_id as an integer path parameter and returns the user name from users if user_id is valid.

FastAPI

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

# Import FastAPI and create app
# Write GET endpoint /users/{user_id}
# Your code here

Need a hint?

Use @app.get decorator with the path /users/{user_id}. Define an async function with user_id: int parameter. Return the user name from the users dictionary.

4

Add error handling for invalid and missing users

Import HTTPException from fastapi. In the get_user function, add error handling: if user_id is less than 1 or greater than max_user_id, raise HTTPException with status code 400 and detail 'Invalid user ID'. If user_id is not in users, raise HTTPException with status code 404 and detail 'User not found'.

FastAPI

from fastapi import FastAPI

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

app = FastAPI()

@app.get('/users/{user_id}')
async def get_user(user_id: int):
    # Add error handling for invalid user_id and missing user
    # Your code here

Need a hint?

Use raise HTTPException(status_code=..., detail=...) to send error responses. Check the user_id range first, then check if it exists in users.