Optional and nullable fields in FastAPI - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output when sending a JSON with missing optional field?

Consider this FastAPI Pydantic model:

from pydantic import BaseModel
from typing import Optional

class Item(BaseModel):
    name: str
    description: Optional[str] = None

If you send a POST request with JSON {"name": "Book"}, what will be the value of description in the parsed Item?

FastAPI

from pydantic import BaseModel
from typing import Optional

class Item(BaseModel):
    name: str
    description: Optional[str] = None

item = Item(name="Book")
print(item.description)

A Missing attribute error

B None

C Raises a validation error

D Empty string ''

💡 Hint

Optional fields with a default value are set to that default if missing.

The description field is optional and defaults to None. If the JSON omits it, Pydantic sets it to None without error.

📝 Syntax

intermediate

Which code correctly defines a nullable string field in Pydantic?

In FastAPI, you want a field comment that can be None or a string. Which of these definitions is correct?

A comment: Optional[str] = None

B comment: str = None

C comment: str | None

D comment: Optional[str]

💡 Hint

Use Optional with a default None to allow null values.

Option A correctly uses Optional[str] with a default None. Option A is invalid because str cannot be assigned None without Optional. Option A is valid Python 3.10+ but lacks default value, so field is required. Option A lacks default value, so field is required.

❓ state_output

advanced

What is the output of this FastAPI endpoint with nullable field?

Given this FastAPI endpoint:

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

app = FastAPI()

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

@app.post("/users")
async def create_user(user: User):
    return {"username": user.username, "bio": user.bio or "No bio provided"}

If you POST {"username": "alice", "bio": null}, what is the JSON response?

A {"username": "alice", "bio": ""}

B {"username": "alice", "bio": null}

C Validation error because bio is null

D {"username": "alice", "bio": "No bio provided"}

💡 Hint

Check how user.bio or "No bio provided" works when bio is None.

The bio field is None because JSON null maps to Python None. The expression user.bio or "No bio provided" evaluates to the string because None is falsy.

🔧 Debug

advanced

Why does this FastAPI model raise a validation error on null input?

Consider this Pydantic model:

from pydantic import BaseModel

class Product(BaseModel):
    name: str
    price: float
    description: str = None

When sending JSON {"name": "Pen", "price": 1.5, "description": null}, a validation error occurs. Why?

A Because name is missing a default value

B Because price cannot be float with decimal

C Because description is typed as str but default None does not allow null

D Because null is not valid JSON

💡 Hint

Check the type of description and how Pydantic treats default values.

The field description is typed as str but has a default None. This means it is not Optional and does not accept null. To accept null, it should be typed as Optional[str].

🧠 Conceptual

expert

How does FastAPI handle missing vs null fields in request bodies?

In FastAPI, what is the difference between a missing optional field and a field explicitly set to null in the JSON request body?

A Missing optional fields get default values; null fields map to None if type allows it

B Both missing and null fields cause validation errors

C Missing fields are ignored; null fields are converted to empty strings

D Missing fields are set to None; null fields cause validation errors

💡 Hint

Think about how Pydantic treats defaults and null values.

When a field is missing, Pydantic uses the default value if provided. When a field is present with null, it maps to Python None if the type allows it (e.g., Optional). Otherwise, it raises a validation error.