Response model exclude and include in FastAPI - Mini Project: Build & Apply

FastAPI Response Model Exclude and Include

📖 Scenario: You are building a simple API for a bookstore. You want to control which book details are sent back to users in different situations.

🎯 Goal: Create a FastAPI app that returns book data. Use response_model with exclude and include options to control which fields appear in the response.

📋 What You'll Learn

Create a Pydantic model Book with fields title, author, year, and price

Create a FastAPI app with two routes: one that excludes price from the response, and one that includes only title and author

Use response_model with response_model_exclude and response_model_include parameters

Test that the API returns the correct fields for each route

💡 Why This Matters

🌍 Real World

APIs often need to send different views of the same data to different users or clients. Controlling response fields helps keep data minimal and secure.

💼 Career

Knowing how to customize API responses is important for backend developers working with FastAPI or similar frameworks to build efficient and secure web services.

1

Create the Book model

Create a Pydantic model called Book with the fields title (str), author (str), year (int), and price (float).

FastAPI

from fastapi import FastAPI
from pydantic import BaseModel

app = FastAPI()

# Create the Book model with title, author, year, and price
# Your code here

Hint

Use class Book(BaseModel): and define each field with its type.

2

Create a sample book data

Create a variable called sample_book and assign it a Book instance with title='The FastAPI Guide', author='Alex', year=2023, and price=29.99.

FastAPI

from fastapi import FastAPI
from pydantic import BaseModel

app = FastAPI()

class Book(BaseModel):
    title: str
    author: str
    year: int
    price: float

# Create sample_book with the given values
# Your code here

Hint

Create sample_book by calling Book(...) with the exact values.

3

Create route excluding the price field

Create a GET route at /book_no_price that returns sample_book. Use response_model=Book and set response_model_exclude={'price'} to exclude the price field from the response.

FastAPI

from fastapi import FastAPI
from pydantic import BaseModel

app = FastAPI()

class Book(BaseModel):
    title: str
    author: str
    year: int
    price: float

sample_book = Book(title='The FastAPI Guide', author='Alex', year=2023, price=29.99)

# Create GET route /book_no_price with response_model=Book and exclude price
# Your code here

Hint

Use @app.get decorator with response_model=Book and response_model_exclude={'price'}.

4

Create route including only title and author

Create a GET route at /book_title_author that returns sample_book. Use response_model=Book and set response_model_include={'title', 'author'} to include only the title and author fields in the response.

FastAPI

from fastapi import FastAPI
from pydantic import BaseModel

app = FastAPI()

class Book(BaseModel):
    title: str
    author: str
    year: int
    price: float

sample_book = Book(title='The FastAPI Guide', author='Alex', year=2023, price=29.99)

@app.get('/book_no_price', response_model=Book, response_model_exclude={'price'})
def get_book_no_price():
    return sample_book

# Create GET route /book_title_author with response_model=Book and include only title and author
# Your code here

Hint

Use @app.get with response_model_include={'title', 'author'} to include only those fields.

Practice

1. What does the response_model_exclude parameter do in FastAPI?

easy

A. It includes only specified fields in the API response.

B. It hides specified fields from the API response.

C. It changes the data type of response fields.

D. It validates the response data against the model.

Solution

1. Step 1: Understand the purpose of response_model_exclude

   This parameter is used to remove or hide certain fields from the response data.

2. Step 2: Compare with other parameters

   Unlike response_model_include which shows only specified fields, response_model_exclude hides specified fields.

3. Final Answer:

   It hides specified fields from the API response. -> Option B

4. Quick Check:

   Exclude means hide fields [OK]

Hint: Exclude hides fields; include shows fields [OK]

Common Mistakes:

• Confusing exclude with include
• Thinking it changes data types
• Assuming it validates data

2. Which of the following is the correct way to exclude the field password from a FastAPI response model?

easy

A. response_model_include=['password']

B. response_model_include={'password'}

C. response_model_exclude={'password'}

D. response_model_exclude=['password']

Solution

1. Step 1: Identify correct data type for exclude

   The response_model_exclude parameter expects a set or dict of field names to exclude.

2. Step 2: Check syntax correctness

   Using a set like {'password'} is valid; lists are not accepted and cause ValueError.

3. Final Answer:

   response_model_exclude={'password'} -> Option C

4. Quick Check:

   Exclude uses set or dict of fields [OK]

Hint: Use set {'password'} to exclude fields [OK]

Common Mistakes:

• Using include instead of exclude
• Using a list ['password'] instead of set {'password'}
• Passing a string instead of set

3. Given this FastAPI endpoint:

from fastapi import FastAPI
from pydantic import BaseModel

app = FastAPI()

class User(BaseModel):
    id: int
    username: str
    email: str
    password: str

@app.get('/user', response_model=User, response_model_exclude={'password'})
def get_user():
    return User(id=1, username='alice', email='[e-mail]', password='secret')

What will the API response JSON include?

medium

A. {"id": 1, "username": "alice", "email": "[e-mail]"}

B. {"id": 1, "username": "alice", "email": "[e-mail]", "password": "secret"}

C. {"username": "alice", "email": "[e-mail]"}

D. {"id": 1, "password": "secret"}

Solution

1. Step 1: Understand response_model_exclude effect

   The password field is excluded from the response, so it will not appear in the JSON output.

2. Step 2: Identify included fields

   All other fields id, username, and email remain and appear in the response.

3. Final Answer:

   {"id": 1, "username": "alice", "email": "[e-mail]"} -> Option A

4. Quick Check:

   Excluded field missing in output [OK]

Hint: Excluded fields do not appear in response JSON [OK]

Common Mistakes:

• Expecting excluded fields to appear
• Confusing exclude with include
• Ignoring response_model parameter

4. You wrote this FastAPI endpoint:

@app.get('/item', response_model=Item, response_model_include=['name', 'price'])
def get_item():
    return Item(id=5, name='Book', price=10.0, description='A good read')

But the endpoint raises a ValueError instead of returning the filtered response. What is the likely error?

medium

A. Using a list instead of a set for response_model_include

B. The response_model_include parameter should be a set, not a list

C. The response_model_include parameter is ignored if not a set or dict

D. The response_model_include parameter must be a dictionary

Solution

1. Step 1: Check expected type for response_model_include

   FastAPI expects response_model_include to be a set or dict of fields, not a list.

2. Step 2: Understand why list fails

   Passing a list causes a ValueError because Pydantic requires set or dict for include/exclude.

3. Final Answer:

   Using a list instead of a set for response_model_include -> Option A

4. Quick Check:

   Include needs set or dict [OK]

Hint: Use set {} for include, not list [] [OK]

Common Mistakes:

• Using list instead of set for include
• Expecting include to filter with wrong type
• Not checking parameter types

5. You want to return a user response with only id and username fields included, but exclude password and email. Which is the best way to do this in FastAPI?

hard

A. Use response_model_exclude={'password', 'email'} to exclude unwanted fields.

B. Return a custom dictionary with only id and username instead of the model.

C. Use both response_model_include={'id', 'username'} and response_model_exclude={'password', 'email'} together.

D. Use response_model_include={'id', 'username'} to include only needed fields.

Solution

1. Step 1: Understand include vs exclude

   Including only id and username is simpler and clearer than excluding fields.

2. Step 2: Avoid mixing include and exclude

   Using both can cause confusion or unexpected results; better to use include to specify exactly what to show.

3. Final Answer:

   Use response_model_include={'id', 'username'} to include only needed fields. -> Option D

4. Quick Check:

   Include specifies exact fields to show [OK]

Hint: Use include to specify exact fields to show [OK]

Common Mistakes:

• Mixing include and exclude causing conflicts
• Excluding fields but missing some unwanted fields
• Returning raw dict instead of model