Optional and nullable fields in FastAPI - Mini Project: Build & Apply

Optional and Nullable Fields in FastAPI

📖 Scenario: You are building a simple API to register users. Some user details are optional or can be empty.

🎯 Goal: Create a FastAPI app with a user model that has optional and nullable fields, and an endpoint to receive user data.

📋 What You'll Learn

Create a Pydantic model called User with required, optional, and nullable fields

Use Optional and None correctly for optional and nullable fields

Create a POST endpoint /users/ that accepts a User model

Return the received user data as JSON

💡 Why This Matters

🌍 Real World

APIs often receive data where some fields are optional or can be empty. Handling optional and nullable fields correctly is important for flexible and robust APIs.

💼 Career

Understanding how to use Pydantic models with optional and nullable fields is essential for backend developers working with FastAPI or similar frameworks.

1

Create the User model with required fields

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

FastAPI

from fastapi import FastAPI
from pydantic import BaseModel

app = FastAPI()

# Create User model with required fields username and email
# Your code here

Need a hint?

Use class User(BaseModel): and define username: str and email: str inside.

2

Add an optional field with default value

Add an optional field called full_name of type str to the User model. It should default to None using Optional[str].

FastAPI

from fastapi import FastAPI
from pydantic import BaseModel
from typing import Optional

app = FastAPI()

class User(BaseModel):
    username: str
    email: str
    # Add optional full_name field here
    # Your code here

Need a hint?

Use full_name: Optional[str] = None to make the field optional and nullable.

3

Add a nullable field explicitly allowing None

Add a field called bio of type str to the User model. It should allow None values explicitly by using Optional[str] and default to None.

FastAPI

from fastapi import FastAPI
from pydantic import BaseModel
from typing import Optional

app = FastAPI()

class User(BaseModel):
    username: str
    email: str
    full_name: Optional[str] = None
    # Add nullable bio field here
    # Your code here

Need a hint?

Use bio: Optional[str] = None to allow the field to be null.

4

Create POST endpoint to receive User data

Create a POST endpoint /users/ that accepts a User model as JSON body and returns the same User data as response.

FastAPI

from fastapi import FastAPI
from pydantic import BaseModel
from typing import Optional

app = FastAPI()

class User(BaseModel):
    username: str
    email: str
    full_name: Optional[str] = None
    bio: Optional[str] = None

# Create POST endpoint /users/ here
# Your code here

Need a hint?

Use @app.post("/users/") decorator and define an async function create_user that takes user: User and returns it.