Setting and reading cookies in Flask - Step-by-Step Execution

Concept Flow - Setting and reading cookies

Client sends request

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Server reads cookies from request

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Server processes request

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Server sets cookies in response

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Response sent back to client with Set-Cookie header

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Client stores cookies

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Next request includes stored cookies

This flow shows how a client sends a request with cookies, the server reads them, processes the request, sets new cookies in the response, and the client stores them for future requests.

Execution Sample

Flask

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

@app.route('/')
def index():
    user = request.cookies.get('user')
    resp = make_response(f'Hello {user}')
    resp.set_cookie('user', 'Alice')
    return resp

This Flask code reads a cookie named 'user' from the request and sets a cookie 'user' with value 'Alice' in the response.

Execution Table

Step	Action	Request Cookies	Read Cookie 'user'	Set Cookie 'user'	Response Content
1	Client sends first request	{}	None	Set 'user'='Alice'	Hello None
2	Client stores cookie 'user'='Alice'	{}	N/A	N/A	N/A
3	Client sends second request with cookie	{'user': 'Alice'}	Alice	Set 'user'='Alice'	Hello Alice

💡 Execution stops after response is sent and client stores cookie for next requests.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3
request.cookies	{}	{}	{}	{'user': 'Alice'}
user	None	None	None	Alice
response cookie 'user'	None	Alice	Alice	Alice

Key Moments - 3 Insights

Why is 'user' None in the first response content?

Because the client did not send any 'user' cookie in the first request (see execution_table step 1), so request.cookies.get('user') returns None.

How does the cookie 'user' get sent back to the server in the second request?

After the first response, the client stores the cookie 'user=Alice' (execution_table step 2). The browser automatically includes this cookie in the next request headers (step 3).

Why does the server set the cookie 'user'='Alice' on every response?

The code calls resp.set_cookie('user', 'Alice') every time the route runs, so the server instructs the client to keep the cookie updated each response.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'user' read from cookies at step 1?

A '' (empty string)

B 'Alice'

C None

D Error

💡 Hint

Check the 'Read Cookie user' column at step 1 in execution_table.

At which step does the client start sending the 'user' cookie back to the server?

A Step 3

B Step 2

C Step 1

D Never

💡 Hint

Look at the 'Request Cookies' column in execution_table to see when 'user' appears.

If the server did not call set_cookie, what would happen to the 'user' cookie in the client?

A It would be deleted immediately

B It would remain stored and sent back

C It would never be stored

D It would cause an error

💡 Hint

Cookies stay stored on the client until expired or deleted; server setting cookie updates or removes it.

Concept Snapshot

Flask cookies flow:
- Read cookies from request: request.cookies.get('name')
- Set cookies in response: resp.set_cookie('name', 'value')
- Client stores cookies and sends them in future requests
- Server can update cookies each response
- Cookies enable state across requests

Full Transcript

In Flask, cookies are small pieces of data stored on the client browser. When a client sends a request, the server can read cookies from request.cookies. The server can send cookies back by setting them in the response using resp.set_cookie. The client stores these cookies and includes them in future requests automatically. This allows the server to remember information about the client between requests. In the example, the server reads a cookie named 'user' and sets it to 'Alice' in every response. The first request has no cookies, so 'user' is None. After the first response, the client stores the cookie and sends it back in the next request, so the server reads 'Alice' as the user cookie.