Custom validators in FastAPI - Mini Project: Build & Apply

Custom Validators in FastAPI

📖 Scenario: You are building a simple FastAPI app to register users. You want to make sure the user's age is valid and the username follows specific rules.

🎯 Goal: Create a FastAPI app with a user model that uses custom validators to check the age and username fields.

📋 What You'll Learn

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

Add a custom validator to ensure age is at least 18.

Add a custom validator to ensure username contains only letters and numbers.

Create a FastAPI app with a POST endpoint /register that accepts a User model.

💡 Why This Matters

🌍 Real World

Custom validators help ensure user input meets specific rules before saving or processing, improving data quality.

💼 Career

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

1

Create the User model with fields

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

FastAPI

from pydantic import BaseModel

# Your code here

Need a hint?

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

2

Add a custom validator for age

Add a custom validator method called check_age inside the User model that ensures age is at least 18. Use the @validator('age') decorator from Pydantic.

FastAPI

from pydantic import BaseModel, validator

class User(BaseModel):
    username: str
    age: int

    # Your code here

Need a hint?

Use @validator('age') and define a method that raises ValueError if age is less than 18.

3

Add a custom validator for username

Add a custom validator method called check_username inside the User model that ensures username contains only letters and numbers. Use @validator('username') and Python's isalnum() method.

FastAPI

from pydantic import BaseModel, validator

class User(BaseModel):
    username: str
    age: int

    @validator('age')
    def check_age(cls, value):
        if value < 18:
            raise ValueError('Age must be at least 18')
        return value

    # Your code here

Need a hint?

Use @validator('username') and check if value.isalnum() is False. Raise ValueError if so.

4

Create FastAPI app with /register endpoint

Create a FastAPI app instance called app. Add a POST endpoint /register that accepts a User model as JSON and returns the same user data.

FastAPI

from pydantic import BaseModel, validator
from fastapi import FastAPI

class User(BaseModel):
    username: str
    age: int

    @validator('age')
    def check_age(cls, value):
        if value < 18:
            raise ValueError('Age must be at least 18')
        return value

    @validator('username')
    def check_username(cls, value):
        if not value.isalnum():
            raise ValueError('Username must be alphanumeric')
        return value

app = FastAPI()

# Your code here

Need a hint?

Create app = FastAPI(). Use @app.post('/register') and define an async function register that takes user: User and returns it.