G object for request-scoped data in Flask - Step-by-Step Execution

Concept Flow - G object for request-scoped data

Start Request

↓

Create g object

↓

Store data in g

↓

Use g data in view

↓

Request ends

↓

g data cleared

Each web request creates a fresh g object to store data accessible only during that request.

Execution Sample

Flask

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

@app.before_request
def before():
    g.user = request.headers.get('User')

@app.route('/')
def index():
    return f"Hello, {g.user}!"

This code saves the user from request headers into g before handling the request, then uses it in the response.

Execution Table

Step	Action	g.user value	Output
1	Start request, create g	unset	No output yet
2	before_request runs, sets g.user = 'Alice'	'Alice'	No output yet
3	index() reads g.user	'Alice'	Returns 'Hello, Alice!'
4	Request ends, g cleared	unset	No output

💡 Request ends, g object data is cleared and not shared between requests

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
g.user	unset	'Alice'	'Alice'	unset (cleared after request)

Key Moments - 2 Insights

Why does g.user reset to None after the request ends?

Because g is request-scoped, it only lives during one request. After the request finishes (see step 4 in execution_table), g data is cleared automatically.

Can g.user be shared between different requests?

No, each request gets a new g object. Sharing data between requests requires other methods like sessions or databases.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of g.user at Step 3?

A 'Alice'

B None

C 'Bob'

D Undefined

💡 Hint

Check the 'g.user value' column at Step 3 in the execution_table.

At which step does the g object get cleared?

A Step 2

B Step 4

C Step 3

D Never

💡 Hint

Look at the 'Action' and 'g.user value' columns in the execution_table for Step 4.

If we did not set g.user in before_request, what would index() return?

A 'Hello, None!'

B 'Hello, !'

C Error because g.user is missing

D 'Hello, g.user!'

💡 Hint

g.user is not set, so accessing it raises AttributeError.

Concept Snapshot

Flask's g object stores data only during one request.
Use g to save info like current user or db connection.
Set g values in before_request or view functions.
Data in g is cleared after request ends.
Do not use g for data sharing between requests.

Full Transcript

In Flask, the g object is a special place to store data that lasts only during one web request. When a request starts, Flask creates a fresh g object. You can put information there, like the current user or database connection, so different parts of your code can access it easily. For example, in a before_request function, you can save the user from the request headers into g.user. Then, in your route function, you read g.user to personalize the response. After the request finishes, Flask clears g automatically, so the data does not leak into other requests. This makes g perfect for temporary, request-scoped data but not for sharing data between requests.