Field validation rules in FastAPI - Mini Project: Build & Apply

Field validation rules

📖 Scenario: You are building a simple API to register users. You want to make sure the data sent by users follows certain rules to keep your app safe and clean.

🎯 Goal: Create a FastAPI app with a user registration endpoint that validates the user's name, age, and email using field validation rules.

📋 What You'll Learn

Create a Pydantic model called User with fields name, age, and email

Add validation rules: name must be a string with minimum length 3, age must be an integer between 18 and 100, email must be a valid email address

Create a POST endpoint /register that accepts a User object

Return the user data if validation passes

💡 Why This Matters

🌍 Real World

APIs often need to check that incoming data is correct and safe before using it. Field validation helps catch mistakes early and improves user experience.

💼 Career

Backend developers use FastAPI and Pydantic to build reliable APIs with automatic data validation, which is a key skill in web development jobs.

1

Create the User model with fields

Create a Pydantic model called User with fields name as str, age as int, and email as str.

FastAPI

from fastapi import FastAPI
from pydantic import BaseModel

app = FastAPI()

# Create User model with name, age, email fields
# Your code here

Need a hint?

Use class User(BaseModel): and define the fields with their types inside.

2

Add validation rules to User model fields

Add validation rules to the User model: name must have min_length=3, age must be between 18 and 100, and email must be a valid email using EmailStr from pydantic.

FastAPI

from fastapi import FastAPI
from pydantic import BaseModel, EmailStr, Field

app = FastAPI()

class User(BaseModel):
    # Add validation rules here
    name: str = Field(..., min_length=3)
    age: int = Field(..., ge=18, le=100)
    email: EmailStr

# Your code here

Need a hint?

Use Field to add min_length for name and ge, le for age. Use EmailStr type for email.

3

Create the POST /register endpoint

Create a POST endpoint /register that accepts a User object as input parameter called user.

FastAPI

from fastapi import FastAPI
from pydantic import BaseModel, EmailStr, Field

app = FastAPI()

class User(BaseModel):
    name: str = Field(..., min_length=3)
    age: int = Field(..., ge=18, le=100)
    email: EmailStr

@app.post("/register")
async def register(user: User):
    # Your code here

Need a hint?

Use @app.post("/register") decorator and define an async function register with parameter user: User. Return the user object.

4

Complete the FastAPI app with validation

Ensure the FastAPI app is complete with the User model and the /register endpoint that returns the validated user data.

FastAPI

from fastapi import FastAPI
from pydantic import BaseModel, EmailStr, Field

app = FastAPI()

class User(BaseModel):
    name: str = Field(..., min_length=3)
    age: int = Field(..., ge=18, le=100)
    email: EmailStr

@app.post("/register")
async def register(user: User):
    return user

# The app is complete

Need a hint?

Check that the model and endpoint are defined and the endpoint returns the user data.