Response model exclude and include in FastAPI - Performance & Optimization

Performance: Response model exclude and include

MEDIUM IMPACT

This affects the size of the JSON response sent to the client, impacting network load and client rendering speed.

Sending API responses with only necessary fields

FastAPI

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'})
async def get_user():
    user = User(id=1, username='alice', email='[e-mail]', password='secret')
    return user

Excludes password field from response, reducing payload size and improving security.

📈 Performance GainSmaller JSON response reduces network transfer and speeds up LCP.

Sending API responses with only necessary fields

FastAPI

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)
async def get_user():
    user = User(id=1, username='alice', email='[e-mail]', password='secret')
    return user

Returns all fields including sensitive data like password, increasing response size and security risk.

📉 Performance CostAdds unnecessary data to response, increasing payload size and slowing LCP.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Full response with all fields	N/A	N/A	Higher due to larger JSON parsing	[X] Bad
Response with exclude/include to limit fields	N/A	N/A	Lower due to smaller JSON parsing	[OK] Good

Rendering Pipeline

The response model exclude/include affects the JSON serialization stage before sending data to the client. Smaller JSON means less data to parse and render on the client side.

→JSON Serialization

→Network Transfer

→Client Rendering

⚠️ BottleneckNetwork Transfer due to payload size

Core Web Vital Affected

LCP

This affects the size of the JSON response sent to the client, impacting network load and client rendering speed.

Optimization Tips

1Always exclude sensitive or unnecessary fields to reduce response size.

2Use include to send only the fields the client needs.

3Smaller JSON responses improve network transfer and page load speed.

Performance Quiz - 3 Questions

What is the main performance benefit of using response_model_exclude in FastAPI?

A Adds more fields to the response

B Reduces response JSON size, improving load speed

C Increases server CPU usage

D Improves database query speed

DevTools: Network

How to check: Open DevTools, go to Network tab, make the API request, and inspect the response payload size and content.

What to look for: Look for smaller response size and absence of unnecessary fields in JSON to confirm good performance.

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