Model inheritance in FastAPI

Introduction

Model inheritance helps you reuse and organize data models by building new models from existing ones. It keeps your code simple and avoids repetition.

When you have several data models that share common fields.

When you want to extend a base model with extra information for specific cases.

When you want to create a clear structure for your data models in an API.

When you want to validate similar data with small differences.

When you want to keep your code clean and easy to maintain.

Syntax

FastAPI

from pydantic import BaseModel

class BaseUser(BaseModel):
    username: str
    email: str

class UserInDB(BaseUser):
    hashed_password: str

Use normal Python class inheritance to extend models.

Child models inherit all fields and validation from parent models.

Examples

Dog inherits the name field from Animal and adds breed.

FastAPI

from pydantic import BaseModel

class Animal(BaseModel):
    name: str

class Dog(Animal):
    breed: str

Employee has all Person fields plus employee_id.

FastAPI

from pydantic import BaseModel

class Person(BaseModel):
    first_name: str
    last_name: str

class Employee(Person):
    employee_id: int

Sample Program

This example shows how to use model inheritance in FastAPI. UserCreate adds a password field to the base user model. The endpoint accepts UserCreate and returns a User with an id.

FastAPI

from fastapi import FastAPI
from pydantic import BaseModel

app = FastAPI()

class UserBase(BaseModel):
    username: str
    email: str

class UserCreate(UserBase):
    password: str

class User(UserBase):
    id: int

@app.post('/users/')
async def create_user(user: UserCreate):
    # Imagine saving user and returning user with id
    return User(id=1, username=user.username, email=user.email)

Important Notes

Model inheritance works because Pydantic models are normal Python classes.

Inherited models keep validation rules from parent models.

You can override fields or add new ones in child models.

Summary

Model inheritance helps reuse fields and validation in FastAPI data models.

Use it to keep your code clean and avoid repeating fields.

Child models extend parent models with new fields or changes.