Application factory pattern in Flask - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the Flask class correctly.

Flask

from flask import [1]

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

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

AFlask

Bflask

CApp

Dapplication

Attempts:

3 left

💡 Hint

Common Mistakes

Using lowercase 'flask' instead of 'Flask'

Importing 'App' or 'application' which do not exist

The Flask class must be imported with a capital 'F' as Flask to create the app instance.

2fill in blank

medium

Complete the code to define the application factory function name correctly.

Flask

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

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

Acreate_app

Binit_app

Cmake_app

Dstart_app

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'init_app' which is usually a method, not the factory function

Using 'start_app' or 'make_app' which are not standard names

The conventional name for the application factory function in Flask is create_app.

3fill in blank

hard

Fix the error in the code to register a blueprint inside the factory function.

Flask

def create_app():
    app = Flask(__name__)
    from .views import main
    app.[1](main)
    return app

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

Aadd_blueprint

Bregister_blueprint

Cinclude_blueprint

Dattach_blueprint

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'add_blueprint' which does not exist

Using 'include_blueprint' or 'attach_blueprint' which are invalid

The correct method to add a blueprint to a Flask app is register_blueprint.

4fill in blank

hard

Fill both blanks to configure the app with a secret key and enable debug mode.

Flask

def create_app():
    app = Flask(__name__)
    app.config['[1]'] = '[2]'
    app.config['DEBUG'] = True
    return app

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

ASECRET_KEY

BDATABASE_URI

Cmysecretkey

Dproductionkey

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'DATABASE_URI' instead of 'SECRET_KEY' for the config key

Using 'productionkey' which is less common for examples

The secret key is set with the config key SECRET_KEY. The value can be any secret string like 'mysecretkey'.

5fill in blank

hard

Fill all three blanks to create a dictionary comprehension that maps blueprint names to their url prefixes.

Flask

blueprints = {
    '[1]': [2].url_prefix for [3] in app.blueprints.values()
}

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

Abp.name

Bbp

Dblueprint

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'blueprint' as the loop variable which is less common

Using incorrect keys or values that do not exist on blueprint objects

This comprehension creates a dictionary where each blueprint's name is the key and its url_prefix is the value. The variable iterating is commonly named 'bp'.

Practice

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

easy

A. To create the Flask app inside a function for better flexibility and configuration

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

C. To avoid using blueprints in the application

D. To hardcode configuration values in the app instance

Solution

1. Step 1: Understand the pattern's purpose

   The application factory pattern creates the Flask app inside a function instead of globally. This allows creating multiple app instances with different settings.

2. Step 2: Compare options to the pattern's goal

   The other options contradict the flexibility and organization benefits of the pattern. Only "To create the Flask app inside a function for better flexibility and configuration" correctly describes the main purpose.

3. Final Answer:

   To create the Flask app inside a function for better flexibility and configuration -> Option A

4. Quick Check:

   Application factory = create app in function [OK]

Hint: Remember: factory means create inside a function [OK]

Common Mistakes:

• Thinking app is created globally, not inside a function
• Confusing blueprints with the factory pattern
• Assuming configuration is hardcoded, not flexible

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

easy

A. def create_app():\n return Flask

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 app = Flask\n return app

Solution

1. Step 1: Check function definition and app creation

   def create_app():\n app = Flask(__name__)\n return app correctly defines a function named create_app that creates a Flask instance and returns it.

2. Step 2: Identify syntax errors or incorrect returns

   app = Flask(__name__)\ndef create_app():\n return app creates app outside the function, which is not factory pattern. def create_app():\n return Flask returns the Flask class, not an instance. def create_app():\n app = Flask\n return app tries to assign Flask class without parentheses, causing error.

3. Final Answer:

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

4. Quick Check:

   Factory function creates and returns app instance [OK]

Hint: Factory returns a new Flask instance inside a function [OK]

Common Mistakes:

• Returning the Flask class instead of an instance
• Creating app outside the factory function
• Missing parentheses when creating Flask instance

3. Given this code snippet using the application factory pattern, 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. False

B. True

C. None

D. KeyError

Solution

1. Step 1: Trace the factory function execution

   The function create_app creates a Flask app and sets app.config['DEBUG'] = True before returning the app.

2. Step 2: Check the printed config value

   After calling create_app(), the returned app has DEBUG set to True. So printing it outputs True.

3. Final Answer:

   True -> Option B

4. Quick Check:

   Config DEBUG set True, print shows True [OK]

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

Common Mistakes:

• Assuming default DEBUG is False without setting
• Expecting KeyError if config key is missing (it's set here)
• Confusing app instance with factory function

4. Identify the error in this application factory code:

def create_app():
    app = Flask(__name__)
    @app.route('/')
    def home():
        return 'Hello'
    return app

app = create_app()
app.run()

medium

A. Missing parentheses in Flask constructor

B. Calling app.run() outside the factory is incorrect

C. Route decorator used incorrectly inside factory

D. No error; code runs correctly

Solution

1. Step 1: Review the factory function structure

   The function creates a Flask app, defines a route inside it, and returns the app instance.

2. Step 2: Check usage of app.run()

   Calling app.run() outside the factory is normal to start the server. The code is valid and will run correctly.

3. Final Answer:

   No error; code runs correctly -> Option D

4. Quick Check:

   Defining routes inside factory and running app outside is valid [OK]

Hint: Routes can be inside factory; run app after creation [OK]

Common Mistakes:

• Thinking app.run() must be inside factory
• Assuming route decorators can't be inside factory
• Confusing Flask constructor syntax

5. You want to create a Flask app using the application factory pattern that registers a blueprint and initializes an extension. Which code snippet correctly applies this pattern?

hard

A. def create_app():
app = Flask(__name__)
from .views import main_bp
app.register_blueprint(main_bp)
from flask_sqlalchemy import SQLAlchemy
db = SQLAlchemy()
db.init_app(app)
return app

B. app = Flask(__name__)
from .views import main_bp
app.register_blueprint(main_bp)
from flask_sqlalchemy import SQLAlchemy
db = SQLAlchemy(app)

C. def create_app():
app = Flask(__name__)
from .views import main_bp
app.register_blueprint(main_bp)
from flask_sqlalchemy import SQLAlchemy
db = SQLAlchemy(app)
return app

D. def create_app():
app = Flask(__name__)
from .views import main_bp
main_bp.register_blueprint(app)
from flask_sqlalchemy import SQLAlchemy
db = SQLAlchemy(app)
return app

Solution

1. Step 1: Understand blueprint registration

   Blueprints are registered on the app using app.register_blueprint(). Two options do this correctly, while one incorrectly calls main_bp.register_blueprint(app).

2. Step 2: Check extension initialization

   For extensions like SQLAlchemy, the recommended pattern is to instantiate the extension without the app (db = SQLAlchemy()) and then call db.init_app(app). The option that does db = SQLAlchemy(); db.init_app(app) follows this correctly. The other factory option with correct blueprint registration uses db = SQLAlchemy(app), which binds it directly to the app and is less flexible.

3. Final Answer:

   def create_app(): app = Flask(__name__) from .views import main_bp app.register_blueprint(main_bp) from flask_sqlalchemy import SQLAlchemy db = SQLAlchemy() db.init_app(app) return app -> Option A

4. Quick Check:

   Blueprint registered on app; extension initialized with init_app [OK]

Hint: Register blueprints on app; init extensions with init_app(app) [OK]

Common Mistakes:

• Registering blueprint on blueprint instead of app
• Creating extension with app inside factory instead of init_app
• Not importing blueprint inside factory function