Response headers in Flask - Step-by-Step Execution

Concept Flow - Response headers

Client sends HTTP request

↓

Flask app processes request

↓

Create response object

↓

Add headers to response

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Send response with headers back to client

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Client receives response with headers

This flow shows how a Flask app receives a request, creates a response, adds headers, and sends it back to the client.

Execution Sample

Flask

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

@app.route('/')
def index():
    resp = make_response('Hello World')
    resp.headers['X-Custom-Header'] = 'MyValue'
    return resp

This Flask route returns 'Hello World' with a custom response header 'X-Custom-Header'.

Execution Table

Step	Action	Response Content	Headers Before	Headers After
1	Request received at '/' route		{}	{}
2	Create response with body 'Hello World'	Hello World	{}	{}
3	Add header 'X-Custom-Header: MyValue'	Hello World	{}	{'X-Custom-Header': 'MyValue'}
4	Return response to client	Hello World	{'X-Custom-Header': 'MyValue'}	{'X-Custom-Header': 'MyValue'}

💡 Response sent with body and custom header; request handling complete.

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
resp.headers	{}	{}	{'X-Custom-Header': 'MyValue'}	{'X-Custom-Header': 'MyValue'}
resp.data	null	b'Hello World'	b'Hello World'	b'Hello World'

Key Moments - 2 Insights

Why do we add headers after creating the response object?

Because the response object must exist first to modify its headers, as shown in step 3 of the execution_table.

Can we add multiple headers the same way?

Yes, you can add multiple headers by assigning different keys in resp.headers before returning, similar to step 3.

Visual Quiz - 3 Questions

Look at the execution_table at step 3, what header is added to the response?

A X-Custom-Header with value 'MyValue'

B Content-Type with value 'text/html'

C Server with value 'Flask'

D Authorization with value 'Bearer token'

💡 Hint

Check the 'Headers After' column at step 3 in the execution_table.

At which step does the response body get set to 'Hello World'?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'Response Content' column in the execution_table.

If you add another header 'Cache-Control' before returning, how would the 'Headers After' look at step 4?

A {'X-Custom-Header': 'MyValue'}

B {}

C {'X-Custom-Header': 'MyValue', 'Cache-Control': 'no-cache'}

D {'Cache-Control': 'no-cache'}

💡 Hint

Refer to variable_tracker and how headers accumulate after step 3.

Concept Snapshot

Flask Response Headers Quick Guide:
- Create response object with make_response()
- Add headers via resp.headers['Header-Name'] = 'value'
- Return response object from route
- Headers sent with response to client
- Use headers to control caching, security, or custom info

Full Transcript

In Flask, when a client sends a request, the app processes it and creates a response object. We can add headers to this response by modifying resp.headers before returning it. For example, adding a custom header 'X-Custom-Header' with a value. The response body and headers are then sent back to the client. This process allows us to control extra information sent with the response, like caching or security details.