Response model exclude and include in FastAPI

Introduction

Sometimes you want to send only some parts of your data to users. Exclude and include help you pick what to show or hide in the response.

You want to hide sensitive info like passwords from API responses.

You want to show only a few fields from a big data object.

You want to customize the response for different users or roles.

You want to reduce data size by excluding unnecessary fields.

You want to include extra fields only in some responses.

Syntax

FastAPI

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

class User(BaseModel):
    id: int
    name: str
    email: Optional[str] = None
    password: Optional[str] = None

app = FastAPI()

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

response_model_exclude hides fields from the response.

response_model_include shows only listed fields.

Examples

This hides the password field from the response.

FastAPI

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

This shows only the id and name fields in the response.

FastAPI

@app.get("/user", response_model=User, response_model_include={"id", "name"})
async def get_user():
    return User(id=1, name="Alice", email="[e-mail]", password="secret")

This excludes fields that were not set in the returned object.

FastAPI

@app.get("/user", response_model=User, response_model_exclude_unset=True)
async def get_user():
    return User(id=1, name="Alice")

Sample Program

This FastAPI app defines a User model with four fields. The endpoint /user returns a User but excludes the password field from the response. So users will see id, name, and email only.

FastAPI

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

class User(BaseModel):
    id: int
    name: str
    email: Optional[str] = None
    password: Optional[str] = None

app = FastAPI()

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

Important Notes

You can use sets or dictionaries for exclude/include to specify fields.

Exclude and include can be combined with nested models using dot notation or dictionaries.

response_model_exclude_unset hides fields not set in the returned object, useful for partial updates.

Summary

Exclude and include control what fields appear in API responses.

Use response_model_exclude to hide fields like passwords.

Use response_model_include to show only specific 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