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.

Practice

1. What is the main purpose of model inheritance in FastAPI?

easy

A. To reuse fields and validation rules from a parent model in child models

B. To create unrelated models with no shared fields

C. To automatically generate database tables

D. To handle HTTP requests directly

Solution

1. Step 1: Understand model inheritance concept

   Model inheritance allows child models to reuse fields and validation from a parent model, avoiding repetition.

2. Step 2: Identify the purpose in FastAPI context

   FastAPI uses Pydantic models where inheritance helps keep code clean by extending existing models.

3. Final Answer:

   To reuse fields and validation rules from a parent model in child models -> Option A

4. Quick Check:

   Model inheritance = reuse fields and validation [OK]

Hint: Inheritance means child models get parent fields automatically [OK]

Common Mistakes:

• Thinking inheritance creates database tables automatically
• Confusing model inheritance with HTTP request handling
• Believing inheritance creates unrelated models

2. Which of the following is the correct way to inherit a Pydantic model in FastAPI?

easy

A. class ChildModel inherits ParentModel: pass

B. class ChildModel extends ParentModel: pass

C. class ChildModel(ParentModel): pass

D. class ChildModel : ParentModel

Solution

1. Step 1: Recall Python class inheritance syntax

   In Python, inheritance is done by placing the parent class name in parentheses after the child class name.

2. Step 2: Match with FastAPI model inheritance

   FastAPI uses Pydantic models which follow Python syntax, so class ChildModel(ParentModel): pass is correct.

3. Final Answer:

   class ChildModel(ParentModel): pass -> Option C

4. Quick Check:

   Python inheritance syntax = parentheses [OK]

Hint: Use parentheses for inheritance in Python classes [OK]

Common Mistakes:

• Using 'extends' like in JavaScript or Java
• Using 'inherits' keyword which doesn't exist in Python
• Using colon without parentheses for inheritance

3. Given the following code, what fields will ChildModel have?

from pydantic import BaseModel

class ParentModel(BaseModel):
    id: int
    name: str

class ChildModel(ParentModel):
    age: int

medium

A. age only

B. id, name, age

C. id, name

D. name, age only

Solution

1. Step 1: Identify fields in ParentModel

   ParentModel has fields id and name.

2. Step 2: Identify fields added in ChildModel

   ChildModel inherits ParentModel and adds age.

3. Final Answer:

   id, name, age -> Option B

4. Quick Check:

   ChildModel fields = parent fields + new fields [OK]

Hint: Child model fields = parent fields + added fields [OK]

Common Mistakes:

• Ignoring inherited fields from parent
• Assuming child model only has new fields
• Confusing field names or missing fields

4. Which of the following correctly describes this FastAPI model inheritance code?

from pydantic import BaseModel

class ParentModel(BaseModel):
    id: int

class ChildModel(ParentModel):
    id: str

medium

A. Field names must be unique across all models

B. ParentModel must inherit from ChildModel

C. BaseModel cannot be inherited

D. ChildModel can override a field with a different type

Solution

1. Step 1: Check field types in parent and child

   ParentModel has id as int, ChildModel overrides id as str.

2. Step 2: Understand Pydantic field overriding rules

   Pydantic allows overriding the type of an inherited field; the child's definition takes precedence.

3. Final Answer:

   ChildModel can override a field with a different type -> Option D

4. Quick Check:

   Field type override allowed [OK]

Hint: Inherited fields can be overridden with different types [OK]

Common Mistakes:

• Thinking child cannot change field types
• Confusing inheritance direction
• Believing BaseModel cannot be inherited

5. You want to create a UserUpdate model that inherits from UserBase but makes all fields optional for partial updates. Which approach correctly uses model inheritance in FastAPI?

hard

A. class UserUpdate(UserBase):
id: Optional[int] = None
name: Optional[str] = None

B. class UserUpdate(UserBase):
pass

C. class UserUpdate(BaseModel):
id: int
name: str

D. class UserUpdate(UserBase):
id: int
name: str

Solution

1. Step 1: Understand the goal of partial updates

   Partial updates require fields to be optional so they can be omitted.

2. Step 2: Use inheritance and make fields optional

   Inheriting from UserBase and redefining fields as Optional with default None allows partial updates.

3. Final Answer:

   class UserUpdate(UserBase): id: Optional[int] = None name: Optional[str] = None -> Option A

4. Quick Check:

   Make fields optional in child model for partial updates [OK]

Hint: Override fields as Optional with default None for partial updates [OK]

Common Mistakes:

• Leaving fields required in update model
• Not inheriting from base model
• Copying fields without making them optional