Validation error responses in FastAPI - Mini Project: Build & Apply

Handling Validation Error Responses in FastAPI

📖 Scenario: You are building a simple API that accepts user data. You want to make sure the data is correct and send clear error messages if it is not.

🎯 Goal: Create a FastAPI app that validates incoming user data and returns clear validation error responses when the data is invalid.

📋 What You'll Learn

Create a Pydantic model called User with fields name (string) and age (integer).

Add a FastAPI app instance called app.

Create a POST endpoint /users/ that accepts a User model.

Customize the validation error response to return a JSON with detail key showing the errors.

💡 Why This Matters

🌍 Real World

APIs often need to check that incoming data is correct and send clear error messages when it is not. This project shows how to do that in FastAPI.

💼 Career

Backend developers use FastAPI and Pydantic to build APIs that validate data and handle errors gracefully, improving user experience and reliability.

1

Create the User data model

Create a Pydantic model called User with two fields: name as a string and age as an integer.

FastAPI

# Your code here to create User model

Need a hint?

Use class User(BaseModel): and define name and age with correct types.

2

Create the FastAPI app instance

Create a FastAPI app instance called app by importing FastAPI and assigning app = FastAPI().

FastAPI

from fastapi import FastAPI

# Your code here to create app instance

Need a hint?

Import FastAPI and create app = FastAPI().

3

Create POST endpoint to accept User data

Create a POST endpoint /users/ using @app.post("/users/") that accepts a parameter user of type User.

FastAPI

# Your code here to create POST endpoint /users/ accepting user: User

Need a hint?

Use @app.post("/users/") decorator and define async function create_user(user: User).

4

Customize validation error response

Add an exception handler for RequestValidationError that returns a JSON response with detail key containing the validation errors. Use @app.exception_handler(RequestValidationError) and return JSONResponse with status code 422.

FastAPI

# Your code here to add validation error handler

Need a hint?

Use @app.exception_handler(RequestValidationError) and return JSONResponse with detail key from exc.errors().