Custom error response models in FastAPI - Mini Project: Build & Apply

Custom error response models in FastAPI

📖 Scenario: You are building a simple API that returns user information. You want to handle errors clearly by sending custom error messages in a structured way.

🎯 Goal: Create a FastAPI app that returns user data for a given user ID. If the user ID is not found, return a custom error response model with a clear message and error code.

📋 What You'll Learn

Create a dictionary called users with user IDs as keys and names as values.

Create a Pydantic model called ErrorResponse with fields error_code (int) and message (str).

Create a GET endpoint /users/{user_id} that returns the user name if found.

If the user ID is not found, return a JSON response using the ErrorResponse model with error_code 404 and a message User not found.

💡 Why This Matters

🌍 Real World

APIs often need to send clear error messages to clients. Custom error response models help keep error data consistent and easy to understand.

💼 Career

Backend developers use FastAPI and Pydantic to build APIs with clear error handling, improving user experience and debugging.

1

Create the initial 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 values "Alice", "Bob", "Charlie".

2

Define the custom error response model

Import BaseModel from pydantic and create a class called ErrorResponse that inherits from BaseModel. It should have two fields: error_code of type int and message of type str.

FastAPI

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

# Import BaseModel and create ErrorResponse model
# Your code here

Need a hint?

Use class ErrorResponse(BaseModel): and define the two fields with type annotations.

3

Create the FastAPI app and GET endpoint

Import FastAPI and HTTPException from fastapi. Create a FastAPI app called app. Define a GET endpoint /users/{user_id} that takes user_id as an int path parameter. If user_id is in users, return a dictionary with key name and the user name as value.

FastAPI

from pydantic import BaseModel

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

class ErrorResponse(BaseModel):
    error_code: int
    message: str

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

Need a hint?

Use @app.get("/users/{user_id}") decorator and an async function with user_id: int parameter.

4

Return custom error response when user not found

In the get_user function, if user_id is not in users, raise HTTPException with status_code=404 and detail set to an instance of ErrorResponse with error_code=404 and message="User not found".

FastAPI

from fastapi import FastAPI, HTTPException
from pydantic import BaseModel

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

class ErrorResponse(BaseModel):
    error_code: int
    message: str

app = FastAPI()

@app.get("/users/{user_id}")
async def get_user(user_id: int):
    if user_id in users:
        return {"name": users[user_id]}
    # Raise HTTPException with ErrorResponse detail if user not found
    # Your code here

Need a hint?

Use raise HTTPException(status_code=404, detail=ErrorResponse(error_code=404, message="User not found").dict()) to send the custom error response.