Model inheritance in FastAPI - Mini Project: Build & Apply

Model Inheritance with FastAPI and Pydantic

📖 Scenario: You are building a simple FastAPI app to manage users. You want to reuse common user data fields by using model inheritance with Pydantic models.

🎯 Goal: Create a base user model and extend it with an inherited model for user creation that adds a password field.

📋 What You'll Learn

Create a base Pydantic model called UserBase with username and email fields

Create a new model called UserCreate that inherits from UserBase and adds a password field

Use FastAPI to create a POST endpoint /users/ that accepts UserCreate data

Return the user data without the password in the response

💡 Why This Matters

🌍 Real World

Model inheritance helps reuse common data structures in web APIs, making code cleaner and easier to maintain.

💼 Career

Understanding Pydantic model inheritance is essential for building scalable FastAPI applications and handling data validation efficiently.

1

Create the base user model

Create a Pydantic model called UserBase with two fields: username of type str and email of type str.

FastAPI

from fastapi import FastAPI
from pydantic import BaseModel

app = FastAPI()

# Create UserBase model here

Hint

Use class UserBase(BaseModel): and define username and email as string fields.

2

Add the user creation model with inheritance

Create a new Pydantic model called UserCreate that inherits from UserBase and adds a new field password of type str.

FastAPI

from fastapi import FastAPI
from pydantic import BaseModel

app = FastAPI()

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

# Create UserCreate model inheriting UserBase here

Hint

Define UserCreate with (UserBase) after the class name and add password: str.

3

Create the POST endpoint to accept user creation data

Add a POST endpoint /users/ to the FastAPI app that accepts a UserCreate model as input in the request body.

FastAPI

from fastapi import FastAPI
from pydantic import BaseModel

app = FastAPI()

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

class UserCreate(UserBase):
    password: str

# Add POST endpoint /users/ here

Hint

Use @app.post('/users/') decorator and define an async function create_user with parameter user: UserCreate.

4

Return user data without the password

Modify the create_user function to return only the username and email fields from the user object, excluding the password.

FastAPI

from fastapi import FastAPI
from pydantic import BaseModel

app = FastAPI()

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

class UserCreate(UserBase):
    password: str

@app.post('/users/')
async def create_user(user: UserCreate):
    # Return only username and email here
    # Your code here

Hint

Return a dictionary with keys username and email from the user parameter.

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