Response headers in Flask

Introduction

Response headers send extra information from the server to the browser. They help control how the browser handles the response.

When you want to tell the browser the type of content it received, like JSON or HTML.

When you want to control caching so the browser knows when to save or refresh data.

When you want to add security rules, like telling the browser to only allow secure connections.

When you want to set cookies or control how cookies behave.

When you want to customize how the browser treats the response, like forcing a file download.

Syntax

Flask

from flask import Flask, Response

app = Flask(__name__)

@app.route('/')
def index():
    response = Response('Hello, world!')
    response.headers['Content-Type'] = 'text/plain'
    response.headers['X-Custom-Header'] = 'MyValue'
    return response

You create a Response object or use the one Flask gives you.

Headers are key-value pairs you add to the response.headers dictionary.

Examples

This sets the content type to JSON so the browser knows to expect JSON data.

Flask

response.headers['Content-Type'] = 'application/json'

This tells the browser not to save the response in its cache.

Flask

response.headers['Cache-Control'] = 'no-cache'

This adds a cookie to the response with extra security flags.

Flask

response.headers.add('Set-Cookie', 'user=123; HttpOnly')

This forces the browser to download the response as a file named file.txt.

Flask

response.headers['Content-Disposition'] = 'attachment; filename="file.txt"'

Sample Program

This Flask app returns a plain text message with custom headers. It tells the browser the content is plain text, adds a custom header, and disables caching.

Flask

from flask import Flask, Response

app = Flask(__name__)

@app.route('/')
def index():
    response = Response('Welcome to my site!')
    response.headers['Content-Type'] = 'text/plain'
    response.headers['X-Powered-By'] = 'Flask'
    response.headers['Cache-Control'] = 'no-store'
    return response

if __name__ == '__main__':
    app.run(debug=True)

Important Notes

Headers names are case-insensitive but usually written in Title-Case.

Some headers like Content-Type are important for browsers to understand the response.

Be careful not to overwrite important default headers unless you know why.

Summary

Response headers send extra info from server to browser.

They control content type, caching, security, and more.

In Flask, add headers by setting keys on response.headers dictionary.

Practice

1. What is the purpose of response headers in a Flask web application?

easy

A. To send extra information from the server to the browser

B. To store user data on the server

C. To write data into the database

D. To create HTML content dynamically

Solution

1. Step 1: Understand what response headers do

   Response headers carry extra information about the response, like content type or caching rules.

2. Step 2: Identify their role in Flask

   In Flask, response headers are sent from the server to the browser to control how the browser handles the response.

3. Final Answer:

   To send extra information from the server to the browser -> Option A

4. Quick Check:

   Response headers = extra info sent to browser [OK]

Hint: Response headers send info from server to browser [OK]

Common Mistakes:

• Confusing headers with server-side data storage
• Thinking headers create HTML content
• Mixing headers with database operations

2. Which of the following is the correct way to add a custom header X-Custom-Header with value Test to a Flask response?

easy

A. response.set_header('X-Custom-Header', 'Test')

B. response.add_header('X-Custom-Header', 'Test')

C. response.header('X-Custom-Header', 'Test')

D. response.headers['X-Custom-Header'] = 'Test'

Solution

1. Step 1: Recall Flask response header syntax

   In Flask, headers are set by assigning values to keys in the response.headers dictionary.

2. Step 2: Match the correct syntax

   Only response.headers['X-Custom-Header'] = 'Test' uses the correct dictionary assignment syntax.

3. Final Answer:

   response.headers['X-Custom-Header'] = 'Test' -> Option D

4. Quick Check:

   Use dictionary assignment for headers [OK]

Hint: Set headers by assigning to response.headers dict [OK]

Common Mistakes:

• Using non-existent methods like add_header or set_header
• Calling response.header() which does not exist
• Trying to set headers as function calls

3. Given this Flask code snippet, what will be the value of the Content-Type header in the response?

from flask import Flask, make_response
app = Flask(__name__)

@app.route('/')
def index():
    response = make_response('Hello')
    response.headers['Content-Type'] = 'application/json'
    return response

medium

A. text/html

B. application/json

C. text/plain

D. application/xml

Solution

1. Step 1: Check the header assignment in the code

   The code sets response.headers['Content-Type'] = 'application/json', which overrides the default.

2. Step 2: Understand the effect on response

   This means the response will have the Content-Type header set to application/json.

3. Final Answer:

   application/json -> Option B

4. Quick Check:

   Content-Type header = application/json [OK]

Hint: Headers set in code override defaults [OK]

Common Mistakes:

• Assuming default Content-Type without checking header override
• Confusing Content-Type with response body content
• Ignoring header assignment line

4. What is wrong with this Flask code that tries to add a header?

from flask import Flask
app = Flask(__name__)

@app.route('/')
def home():
    response = 'Hello'
    response.headers['X-Test'] = 'Value'
    return response

medium

A. You cannot add headers to a string response directly

B. The header key must be lowercase

C. The header value must be an integer

D. Flask does not support custom headers

Solution

1. Step 1: Identify the response type

   The response is a plain string, not a response object.

2. Step 2: Understand header assignment requirements

   Headers can only be set on response objects, not on strings.

3. Final Answer:

   You cannot add headers to a string response directly -> Option A

4. Quick Check:

   Headers require response object, not string [OK]

Hint: Headers need response object, not plain string [OK]

Common Mistakes:

• Trying to set headers on string instead of response
• Thinking header keys must be lowercase
• Believing Flask disallows custom headers

5. You want to add a header Cache-Control with value no-store to all responses in your Flask app. Which is the best way to do this?

hard

A. Set Cache-Control in the HTML meta tags

B. Add response.headers['Cache-Control'] = 'no-store' inside every route function

C. Use a @app.after_request function to add the header to all responses

D. Modify the Flask source code to add the header globally

Solution

1. Step 1: Understand the goal

   You want to add the header to all responses automatically, not manually in each route.

2. Step 2: Identify Flask feature for global response modification

   The @app.after_request decorator lets you modify every response before sending it.

3. Step 3: Evaluate options

   The @app.after_request decorator adds the header to every response automatically, which is efficient and clean.

4. Final Answer:

   Use a @app.after_request function to add the header to all responses -> Option C

5. Quick Check:

   Use after_request to set headers globally [OK]

Hint: Use @app.after_request to add headers globally [OK]

Common Mistakes:

• Adding header manually in every route (Option B)
• Trying to set headers in HTML meta tags (Option C)
• Editing Flask source code (Option D)