Application factory pattern deep dive in Flask - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this Flask app factory code?

Consider this Flask application factory code snippet. What will be printed when the app runs and the root URL is accessed?

Flask

from flask import Flask

def create_app():
    app = Flask(__name__)

    @app.route('/')
    def home():
        return 'Hello from factory!'

    print('App created')
    return app

app = create_app()

if __name__ == '__main__':
    app.run()

A App created is printed once before the server starts, and visiting '/' shows 'Hello from factory!'

B App created is printed every time '/' is accessed, and the page shows 'Hello from factory!'

C Nothing is printed, and visiting '/' returns a 404 error

D App created is printed once, but visiting '/' returns an empty page

💡 Hint

Remember when the print statement inside the factory function runs.

The print statement runs once when create_app() is called, which happens before the server starts. The route returns the string when accessed.

❓ state_output

intermediate

What is the value of app.config['DEBUG'] after app creation?

Given this factory function, what will be the value of app.config['DEBUG'] after calling create_app()?

Flask

from flask import Flask

def create_app():
    app = Flask(__name__)
    app.config['DEBUG'] = True
    return app

app = create_app()

A Raises a KeyError

B False

C None

D True

💡 Hint

Check how config values are set inside the factory.

The config key 'DEBUG' is explicitly set to True inside the factory before returning the app.

🔧 Debug

advanced

Why does this factory pattern code raise an error?

Examine this code. Why does it raise an error when running?

Flask

from flask import Flask

def create_app():
    app = Flask(__name__)

    @app.route('/')
    def home():
        return message

    message = 'Hello from factory'
    return app

app = create_app()

if __name__ == '__main__':
    app.run()

A NameError because 'message' is used before assignment in the route function

B TypeError because 'message' is not a string

C SyntaxError due to incorrect indentation

D No error, it runs and returns 'Hello from factory'

💡 Hint

Look at when 'message' is defined versus when it is used inside the route.

The route function tries to return message before it is assigned, causing a NameError.

🧠 Conceptual

advanced

Which statement about Flask application factory pattern is true?

Select the correct statement about the Flask application factory pattern.

A It prevents registering blueprints after app creation

B It requires using global variables for app state

C It allows creating multiple app instances with different configurations

D It disables Flask's debug mode by default

💡 Hint

Think about why factories are useful in software design.

The factory pattern lets you create multiple app instances with different settings, useful for testing or different environments.

📝 Syntax

expert

Which option correctly registers a blueprint inside an app factory?

Given a blueprint bp, which code snippet correctly registers it inside the Flask application factory?

Flask

from flask import Flask, Blueprint

bp = Blueprint('bp', __name__)

@bp.route('/hello')
def hello():
    return 'Hello Blueprint!'

# Choose the correct factory code to register bp

A

def create_app():
    app = Flask(__name__)
    app.register(bp)
    return app

B

def create_app():
    app = Flask(__name__)
    app.register_blueprint(bp)
    return app

C

def create_app():
    app = Flask(__name__)
    bp.register_blueprint(app)
    return app

D

def create_app():
    app = Flask(__name__)
    app.blueprint_register(bp)
    return app

💡 Hint

Check the Flask method name for blueprint registration.

The correct method to register a blueprint on the app is app.register_blueprint(bp).