Model inheritance in FastAPI - Step-by-Step Execution

Concept Flow - Model inheritance

Define Base Model

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Define Child Model inherits Base

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Create instance of Child Model

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Access inherited and own fields

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Use model in FastAPI endpoint

Model inheritance means creating a new model that reuses fields from a base model and adds new ones, making code simpler and organized.

Execution Sample

FastAPI

from pydantic import BaseModel

class UserBase(BaseModel):
    username: str

class UserCreate(UserBase):
    password: str

Defines a base user model with username, then a child model adding password.

Execution Table

Step	Action	Evaluation	Result
1	Define UserBase model	UserBase has field username:str	UserBase ready
2	Define UserCreate model inheriting UserBase	UserCreate has username:str + password:str	UserCreate ready
3	user = UserCreate(username='alice', password='secret')	Instance fields set	user = UserCreate(username='alice', password='secret')
4	Access user.username	Returns 'alice'	'alice'
5	Access user.password	Returns 'secret'	'secret'

💡 All fields inherited and added are accessible in UserCreate instance

Variable Tracker

Variable	Start	After Step 3	After Step 4	After Step 5
user	undefined	UserCreate(username='alice', password='secret')	UserCreate(username='alice', password='secret')	UserCreate(username='alice', password='secret')

Key Moments - 2 Insights

Why can UserCreate access username even though it is not defined inside it?

Because UserCreate inherits from UserBase, it automatically includes all fields from UserBase as shown in execution_table step 2 and 3.

What happens if you try to create UserCreate without password?

It will raise an error because password is required in UserCreate, as seen in step 3 where both fields must be provided.

Visual Quiz - 3 Questions

Look at the execution table, what fields does UserCreate have after step 2?

A username and password

B Only password

C Only username

D No fields

💡 Hint

Check step 2 in execution_table where UserCreate fields are described

At which step is the UserCreate instance created?

A Step 4

B Step 1

C Step 3

D Step 5

💡 Hint

Look for the step mentioning instance creation in execution_table

If you remove inheritance from UserBase in UserCreate, what changes in variable_tracker after step 3?

A user will have only username field

B user will be undefined

C user will have only password field

D user will have username and password fields

💡 Hint

Without inheritance, step 3 fails (extra 'username' field not allowed), so user undefined

Concept Snapshot

Model inheritance in FastAPI uses Pydantic models.
Define a base model with common fields.
Create child models inheriting base to add fields.
Child models have all base fields plus new ones.
Use child models in endpoints for clean code.

Full Transcript

Model inheritance in FastAPI means making a new model that reuses fields from an existing model. We start by defining a base model with common fields like username. Then we create a child model that inherits from the base and adds more fields like password. When we create an instance of the child model, it has both the inherited and new fields. This helps keep code simple and organized. For example, UserCreate inherits username from UserBase and adds password. Accessing these fields works as expected. If you forget to provide required fields, FastAPI will raise an error. This pattern is useful for sharing common data structures in your API.