Custom validation with validator decorator in FastAPI - Mini Project: Build & Apply

Custom validation with validator decorator in FastAPI

📖 Scenario: You are building a simple FastAPI app to register users. You want to make sure the user's username is at least 3 characters long and contains only letters and numbers.

🎯 Goal: Create a Pydantic model with a custom validator using the @validator decorator to check the username field. The validator should raise an error if the username is too short or contains invalid characters.

📋 What You'll Learn

Create a Pydantic model called User with a username field of type str.

Add a class method decorated with @validator('username') to validate the username.

The validator must check that username is at least 3 characters long.

The validator must check that username contains only letters and numbers.

Raise a ValueError with a clear message if validation fails.

💡 Why This Matters

🌍 Real World

Custom validation is essential in web APIs to ensure user input meets rules before saving or processing. This prevents errors and security issues.

💼 Career

Backend developers often write custom validators in FastAPI or similar frameworks to enforce business rules and data integrity.

1

Create the User model with username field

Create a Pydantic model called User with a single field username of type str.

FastAPI

from pydantic import BaseModel

# Create User model with username field
# Your code here

Need a hint?

Use class User(BaseModel): and define username: str inside.

2

Import the validator decorator

Add an import statement to import validator from pydantic.

FastAPI

from pydantic import BaseModel
# Import validator decorator
# Your code here

class User(BaseModel):
    username: str

Need a hint?

Use from pydantic import validator to import the decorator.

3

Add a custom validator for username

Inside the User model, add a class method decorated with @validator('username') named check_username. It should accept cls and value parameters. Check if value is at least 3 characters long and contains only letters and numbers using value.isalnum(). If not, raise ValueError with the message 'Username must be at least 3 characters and alphanumeric'. Return value if valid.

FastAPI

from pydantic import BaseModel, validator

class User(BaseModel):
    username: str

    # Add validator method for username
    # Your code here

Need a hint?

Use @validator('username') above a method that checks length and isalnum().

4

Complete the FastAPI app with POST endpoint

Add the necessary imports from fastapi and create a FastAPI app instance called app. Add a POST endpoint /users/ that accepts a User model as input and returns the same User object as JSON.

FastAPI

from pydantic import BaseModel, validator
from fastapi import FastAPI

class User(BaseModel):
    username: str

    @validator('username')
    def check_username(cls, value):
        if len(value) < 3 or not value.isalnum():
            raise ValueError('Username must be at least 3 characters and alphanumeric')
        return value

# Create FastAPI app and POST endpoint
# Your code here

Need a hint?

Import FastAPI, create app = FastAPI(), then add a POST route /users/ that returns the User object.