Application factory pattern preview in Flask - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define the Flask application factory function.

Flask

from flask import Flask

def create_app():
    app = Flask(__name__)
    return [1]

Drag options to blanks, or click blank then click option'

AFlask

Bapp

Ccreate_app

DNone

Attempts:

3 left

💡 Hint

Common Mistakes

Returning the Flask class instead of the app instance.

Forgetting to return anything from the function.

The function must return the Flask app instance named app.

2fill in blank

medium

Complete the code to register a blueprint inside the application factory.

Flask

from flask import Flask, Blueprint

bp = Blueprint('main', __name__)

def create_app():
    app = Flask(__name__)
    app.[1](bp)
    return app

Drag options to blanks, or click blank then click option'

Aregister_blueprint

Badd_url_rule

Crun

Dinit_app

Attempts:

3 left

💡 Hint

Common Mistakes

Using add_url_rule instead of register_blueprint.

Calling run inside the factory.

Use register_blueprint to add a blueprint to the Flask app.

3fill in blank

hard

Fix the error in the application factory by completing the code to load configuration from a file.

Flask

def create_app():
    app = Flask(__name__)
    app.config.[1]('config.py')
    return app

Drag options to blanks, or click blank then click option'

Afrom_pyfile

Bload_config

Cload_from_file

Dread_config

Attempts:

3 left

💡 Hint

Common Mistakes

Using a non-existent method like load_config.

Trying to assign config directly without using a method.

The correct method to load config from a file is from_pyfile.

4fill in blank

hard

Fill both blanks to create a route inside a blueprint and return a greeting.

Flask

from flask import Blueprint

bp = Blueprint('main', __name__)

@bp.route([1])
def hello():
    return [2]

Drag options to blanks, or click blank then click option'

A"/"

B"Hello, Flask!"

C"/home"

D"Welcome!"

Attempts:

3 left

💡 Hint

Common Mistakes

Using a wrong route path like "/home" when root is expected.

Returning a variable instead of a string.

The route decorator needs the path "/" and the function returns the greeting string "Hello, Flask!".

5fill in blank

hard

Fill all three blanks to create an application factory that registers a blueprint and loads config.

Flask

from flask import Flask, Blueprint

bp = Blueprint('main', __name__)

@bp.route("/")
def index():
    return "Welcome!"

def create_app():
    app = Flask(__name__)
    app.config.[1]('config.py')
    app.[2](bp)
    return [3]

Drag options to blanks, or click blank then click option'

Afrom_pyfile

Bregister_blueprint

Capp

Dload_config

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong method names for config loading or blueprint registration.

Forgetting to return the app instance.

The factory loads config with from_pyfile, registers the blueprint with register_blueprint, and returns the app instance.

Practice

1. What is the main purpose of the application factory pattern in Flask?

easy

A. To avoid using blueprints and extensions

B. To write all routes directly in the main script without functions

C. To automatically generate HTML templates

D. To create the Flask app inside a function for better configuration and testing

Solution

1. Step 1: Understand the pattern's purpose

   The application factory pattern creates the Flask app inside a function instead of a global variable.

2. Step 2: Benefits of this approach

   This allows flexible configuration, easier testing, and clean registration of extensions and blueprints.

3. Final Answer:

   To create the Flask app inside a function for better configuration and testing -> Option D

4. Quick Check:

   Application factory = create app in function [OK]

Hint: Remember: factory means 'make inside a function' [OK]

Common Mistakes:

• Thinking app is created globally
• Confusing with blueprint usage
• Assuming it auto-generates templates

2. Which of the following is the correct way to define an application factory function in Flask?

easy

A. def create_app():\n app = Flask\n return app

B. app = Flask(__name__)\ndef create_app():\n return app

C. def create_app():\n app = Flask(__name__)\n return app

D. def create_app():\n return Flask

Solution

1. Step 1: Check function definition and app creation

   The factory function must create a Flask instance inside it using Flask(__name__)

2. Step 2: Verify return of the app instance

   The function should return the created app instance, not the class or a global variable.

3. Final Answer:

   def create_app():\n app = Flask(__name__)\n return app -> Option C

4. Quick Check:

   Factory returns new Flask instance [OK]

Hint: Create and return app inside function with Flask(__name__) [OK]

Common Mistakes:

• Returning Flask class instead of instance
• Using global app outside function
• Not calling Flask with parentheses

3. Given this code snippet, what will print(app.config['DEBUG']) output?

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

app = create_app()
print(app.config['DEBUG'])

medium

A. True

B. False

C. None

D. KeyError

Solution

1. Step 1: Analyze config setting inside factory

   The factory sets app.config['DEBUG'] = True before returning the app.

2. Step 2: Check printed value after app creation

   After calling create_app(), app.config['DEBUG'] is True, so printing it outputs True.

3. Final Answer:

   True -> Option A

4. Quick Check:

   Config set to True inside factory [OK]

Hint: Config set inside factory is available after app creation [OK]

Common Mistakes:

• Assuming default DEBUG is False
• Expecting KeyError for missing key
• Confusing config with environment variables

4. Identify the error in this application factory code:

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

app = create_app()

Assume main is a blueprint imported correctly.

medium

A. No error, code is correct

B. Missing app context push before registering blueprint

C. Blueprint must be registered before app creation

D. Blueprint registration should happen inside a separate function

Solution

1. Step 1: Check blueprint registration timing

   Blueprints are registered on the app instance inside the factory function, which is correct.

2. Step 2: Verify app context and function structure

   No app context push is needed for registration; the code is valid and follows the pattern.

3. Final Answer:

   No error, code is correct -> Option A

4. Quick Check:

   Blueprint registered inside factory is valid [OK]

Hint: Register blueprints inside factory before returning app [OK]

Common Mistakes:

• Thinking app context is needed for registration
• Trying to register blueprint before app exists
• Separating registration unnecessarily

5. You want to add a Flask extension (e.g., Flask-Migrate) using the application factory pattern. Which approach is correct?

hard

A. Create the extension instance inside the factory function only

B. Initialize the extension globally, then call init_app(app) inside the factory

C. Initialize the extension after app creation outside the factory

D. Pass the app instance as a parameter to the extension constructor inside the factory

Solution

1. Step 1: Understand extension initialization pattern

   Extensions are usually created globally without app, then initialized with app inside the factory using init_app(app).

2. Step 2: Why not create inside factory?

   Creating inside factory loses global access; initializing outside factory after app creation breaks pattern.

3. Final Answer:

   Initialize the extension globally, then call init_app(app) inside the factory -> Option B

4. Quick Check:

   Global extension + init_app(app) inside factory [OK]

Hint: Create extension globally, init inside factory [OK]

Common Mistakes:

• Creating extension instance inside factory only
• Initializing extension outside factory after app creation
• Passing app to extension constructor inside factory