After_request hooks in Flask - Step-by-Step Execution

Concept Flow - After_request hooks

Request received

↓

Route function runs

↓

Response object created

↓

After_request hook runs

↓

Modified or original response returned

↓

Response sent to client

After_request hooks run after the route function creates a response, allowing you to modify it before sending it back.

Execution Sample

Flask

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

@app.after_request
def add_header(response):
    response.headers['X-Hello'] = 'World'
    return response

This code adds a custom header to every response after the route function finishes.

Execution Table

Step	Action	Input/State	Output/State	Notes
1	Request received	Client sends GET /	Request object created	Flask starts processing request
2	Route function runs	Request object	Response object created	Route returns a response
3	After_request hook runs	Response object without X-Hello header	Response object with X-Hello: World header	Hook adds custom header
4	Response sent	Modified Response object	Client receives response with header	Response sent back to client

💡 After_request hook modifies response, then response is sent to client

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
response.headers	{}	{...default headers...}	{...default headers..., 'X-Hello': 'World'}	{...default headers..., 'X-Hello': 'World'}

Key Moments - 3 Insights

Why does the after_request hook receive a response object?

Because after_request hooks run after the route function returns a response, so they get that response to modify or inspect before sending it out (see execution_table step 3).

Can the after_request hook change the response content?

Yes, the hook can modify headers, status code, or body by changing the response object before returning it (execution_table step 3 shows header added).

What happens if the after_request hook does not return the response?

Flask requires the hook to return a response object; otherwise, it will cause an error or no response will be sent (see code sample return statement).

Visual Quiz - 3 Questions

Look at the execution_table, what does the after_request hook add to the response at step 3?

A A new cookie

B A new header 'X-Hello' with value 'World'

C Changes the response status code

D Modifies the request object

💡 Hint

Check the 'Output/State' column at step 3 in the execution_table

At which step does the route function create the response object?

A Step 2

B Step 1

C Step 3

D Step 4

💡 Hint

Look at the 'Action' column in execution_table for when the response is created

If the after_request hook did not return the response object, what would happen?

A Flask would send the original response anyway

B The request would be processed twice

C Flask would raise an error or fail to send a response

D Nothing changes, the hook is optional

💡 Hint

Refer to key_moments about the importance of returning the response from the hook

Concept Snapshot

After_request hooks run after a route returns a response.
They receive the response object and can modify it.
Always return the response from the hook.
Used to add headers, cookies, or logging.
Runs before sending response to client.

Full Transcript

In Flask, after_request hooks run after the route function finishes and returns a response. The hook receives this response object and can modify it, for example by adding headers. The hook must return the response object, or Flask will error. The modified response is then sent to the client. This lets you change responses globally after routes run.