G object for request-scoped data in Flask - Mini Project: Build & Apply

Using Flask's g Object for Request-Scoped Data

📖 Scenario: You are building a simple Flask web app that needs to store and access data specific to each web request. This data should be available throughout the request but reset for the next request.

🎯 Goal: Learn how to use Flask's g object to store and access request-scoped data safely and easily.

📋 What You'll Learn

Create a Flask app with a route

Use the g object to store data during a request

Access the stored data later in the same request

Demonstrate that data is reset for each new request

💡 Why This Matters

🌍 Real World

Web apps often need to keep track of user info or other data during a single web request without mixing it with other requests. Flask's <code>g</code> object is perfect for this.

💼 Career

Understanding request-scoped data is essential for backend web development with Flask, helping you write clean, safe, and maintainable code.

1

Set up a basic Flask app with a route

Import Flask from flask and create a Flask app called app. Then define a route / with a function index that returns the string 'Hello, Flask!'.

Flask

# Your code here

Hint

Start by importing Flask and creating an app instance. Then add a route decorator and a simple function that returns a greeting.

2

Import g and store data in it

Import g from flask. Inside the index function, set g.user to the string 'Alice' to simulate storing user data for this request.

Flask

from flask import Flask, g

app = Flask(__name__)

@app.route('/')
def index():
    # Store user name in g
    # Your code here
    return 'Hello, Flask!'

Hint

Remember to import g from flask. Then assign the user name to g.user inside the route function.

3

Access the stored g.user data in the response

Modify the index function to return a greeting that includes the user name stored in g.user. Use an f-string to return f'Hello, {g.user}!'.

Flask

from flask import Flask, g

app = Flask(__name__)

@app.route('/')
def index():
    g.user = 'Alice'
    # Return greeting with g.user
    # Your code here

Hint

Use an f-string to include g.user in the returned string.

4

Add a before_request function to set g.user

Define a function load_user decorated with @app.before_request that sets g.user to 'Alice'. Remove the g.user assignment from the index function.

Flask

from flask import Flask, g

app = Flask(__name__)

@app.before_request
def load_user():
    # Set g.user here
    # Your code here

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

Hint

Use @app.before_request to run code before each request. Set g.user there so it is available in all routes.

Practice

1. What is the main purpose of the g object in Flask?

easy

A. To store data temporarily during a single request

B. To store data permanently across all requests

C. To manage database connections globally

D. To handle user sessions across multiple requests

Solution

1. Step 1: Understand the scope of g

   The g object is designed to hold data only during the life of one request.

2. Step 2: Compare with other storage options

   Unlike session or global variables, g does not persist data beyond the current request.

3. Final Answer:

   To store data temporarily during a single request -> Option A

4. Quick Check:

   g = request-scoped storage [OK]

Hint: Remember: g lasts only for one request [OK]

Common Mistakes:

• Thinking g stores data across requests
• Confusing g with session or global variables
• Assuming g manages database connections automatically

2. Which of the following is the correct way to store a value in the g object inside a Flask view function?

easy

A. g.user = 'Alice'

B. g.add('user', 'Alice')

C. g.set('user', 'Alice')

D. g['user'] = Alice

Solution

1. Step 1: Recall how to assign attributes in g

   g acts like a simple object where you assign attributes using dot notation.

2. Step 2: Check each option's syntax

   g.user = 'Alice' uses g.user = 'Alice', which is the correct way. g['user'] = Alice uses dictionary style which is invalid. Options B and C use methods that g does not have.

3. Final Answer:

   g.user = 'Alice' -> Option A

4. Quick Check:

   Use dot notation to set g attributes [OK]

Hint: Use dot notation: g.attribute = value [OK]

Common Mistakes:

• Using dictionary syntax like g['key']
• Trying to call non-existent methods on g
• Confusing g with Flask's session object

3. Consider this Flask code snippet:

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

@app.before_request
def before():
    g.value = 10

@app.route('/')
def index():
    g.value += 5
    return str(g.value)

What will be the output when you visit the '/' route?

medium

A. 10

B. 5

C. 15

D. Error: AttributeError

Solution

1. Step 1: Understand the before_request function

   Before each request, g.value is set to 10.

2. Step 2: Analyze the index function

   It adds 5 to g.value, making it 15, then returns it as a string.

3. Final Answer:

   15 -> Option C

4. Quick Check:

   g.value starts 10, adds 5 = 15 [OK]

Hint: Remember g resets each request; additions apply after setup [OK]

Common Mistakes:

• Assuming g.value keeps old values between requests
• Forgetting before_request runs before route
• Expecting an error due to missing attribute

4. What is wrong with this Flask code using g?

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

@app.route('/')
def index():
    if not hasattr(g, 'counter'):
        g.counter = 0
    g.counter += 1
    return str(g.counter)

medium

A. It will raise an error because g cannot have new attributes

B. The code will cause a syntax error due to missing import

C. The counter will increase across requests, causing unexpected results

D. The counter resets on every request, so it always returns 1

Solution

1. Step 1: Understand g scope

   g is reset for each request, so counter does not persist between requests.

2. Step 2: Analyze the counter behavior

   Each request starts fresh, so counter is set to 0 then incremented to 1 every time.

3. Final Answer:

   The counter resets on every request, so it always returns 1 -> Option D

4. Quick Check:

   g data is request-scoped, no persistence [OK]

Hint: Remember g resets each request; no data persists [OK]

Common Mistakes:

• Expecting g.counter to keep increasing across requests
• Thinking g forbids new attributes
• Confusing g with session or global variables

5. You want to share a database connection during a single Flask request using g. Which code snippet correctly opens the connection once per request and reuses it in multiple functions?

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

def get_db():
    if not hasattr(g, 'db'):
        g.db = open_database_connection()
    return g.db

@app.route('/')
def index():
    db = get_db()
    # use db
    return 'OK'

What is the main benefit of this pattern?

hard

A. It opens a new database connection on every function call

B. It shares one connection per request, avoiding multiple opens

C. It stores the connection globally for all requests

D. It closes the connection automatically after each function

Solution

1. Step 1: Analyze the get_db function

   It checks if g.db exists; if not, it opens the connection and saves it to g.

2. Step 2: Understand request scope of g

   The connection is stored only for the current request, so multiple calls reuse the same connection.

3. Step 3: Compare options

   It shares one connection per request, avoiding multiple opens correctly describes this behavior. It opens a new database connection on every function call is wrong because it avoids multiple opens. It stores the connection globally for all requests is wrong because g is not global. It closes the connection automatically after each function is incorrect; closing is not automatic here.

4. Final Answer:

   It shares one connection per request, avoiding multiple opens -> Option B

5. Quick Check:

   Use g to cache per-request resources [OK]

Hint: Cache resources in g to reuse during request [OK]

Common Mistakes:

• Thinking g stores data globally across requests
• Opening new connections on every function call
• Assuming g auto-closes resources