Custom request validation in FastAPI - Mini Project: Build & Apply

Custom request validation in FastAPI

📖 Scenario: You are building a simple web API to accept user registration data. You want to make sure the data is valid before processing it.

🎯 Goal: Create a FastAPI app that accepts a POST request with user data. Add custom validation to check that the username is at least 3 characters and the age is at least 18.

📋 What You'll Learn

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

Add a custom validator to User that checks username length is at least 3 characters.

Add a custom validator to User that checks age is at least 18.

Create a POST endpoint /register that accepts a User model in the request body.

💡 Why This Matters

🌍 Real World

APIs often need to check that incoming data is correct before saving or processing it. Custom validation helps enforce business rules.

💼 Career

Backend developers use FastAPI and Pydantic validation to build reliable and secure APIs that handle user input safely.

1

Create the User model

Create a Pydantic model called User with two fields: username of type str and age of type int.

FastAPI

from fastapi import FastAPI
from pydantic import BaseModel

app = FastAPI()

# Create User model here

Need a hint?

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

2

Add custom username validation

Add a custom validator method called validate_username inside the User model using the @validator('username') decorator. It should raise a ValueError if the username length is less than 3 characters.

FastAPI

from fastapi import FastAPI
from pydantic import BaseModel
from pydantic import validator

app = FastAPI()

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

    # Add username validator here

Need a hint?

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

3

Add custom age validation

Add a custom validator method called validate_age inside the User model using the @validator('age') decorator. It should raise a ValueError if the age is less than 18.

FastAPI

from fastapi import FastAPI
from pydantic import BaseModel
from pydantic import validator

app = FastAPI()

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

    @validator('username')
    def validate_username(cls, v):
        if len(v) < 3:
            raise ValueError('username must be at least 3 characters')
        return v

    # Add age validator here

Need a hint?

Use @validator('age') and define a method that checks the age value.

4

Create the POST /register endpoint

Create a POST endpoint /register in the FastAPI app that accepts a User model in the request body and returns the same user data.

FastAPI

from fastapi import FastAPI
from pydantic import BaseModel
from pydantic import validator

app = FastAPI()

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

    @validator('username')
    def validate_username(cls, v):
        if len(v) < 3:
            raise ValueError('username must be at least 3 characters')
        return v

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

# Create POST /register endpoint here

Need a hint?

Use @app.post('/register') and define a function that takes user: User and returns it.