Application factory pattern in Flask - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What does the Flask app factory function return?

Consider this Flask app factory function:

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

What does create_app() return?

Flask

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

A A function that returns 'Hello!'

B A Flask application instance ready to run

C None

D A string 'Hello!'

💡 Hint

Think about what the variable 'app' is inside the function.

The function creates a Flask app instance, sets a route, and returns the app instance itself. It does not return the route function or its output.

❓ state_output

intermediate

What is the output when running the app created by the factory?

Given this Flask app factory:

def create_app():
    app = Flask(__name__)
    @app.route('/greet')
    def greet():
        return 'Hi there!'
    return app

app = create_app()
with app.test_client() as client:
    response = client.get('/greet')
    print(response.data.decode())

What will be printed?

Flask

def create_app():
    app = Flask(__name__)
    @app.route('/greet')
    def greet():
        return 'Hi there!'
    return app

app = create_app()
with app.test_client() as client:
    response = client.get('/greet')
    print(response.data.decode())

A Hello!

B 404 Not Found

C Hi there!

D TypeError

💡 Hint

Check the route defined inside the factory.

The route '/greet' returns 'Hi there!'. The test client requests this route and prints the response data decoded to string.

❓ lifecycle

advanced

When is the Flask app instance created in the factory pattern?

In the application factory pattern, when does the Flask app instance get created?

def create_app():
    app = Flask(__name__)
    # setup code
    return app

Flask

def create_app():
    app = Flask(__name__)
    # setup code
    return app

A Each time create_app() is called

B Only once when the module is imported

C When the server starts automatically

D When the first request is received

💡 Hint

Think about what happens inside the function.

The app instance is created inside the function, so every call to create_app() makes a new app instance.

📝 Syntax

advanced

Which option correctly registers a blueprint inside the factory?

You want to register a blueprint inside the Flask app factory. Which code snippet is correct?

from flask import Flask, Blueprint
bp = Blueprint('bp', __name__)
@bp.route('/hello')
def hello():
    return 'Hello Blueprint!'

def create_app():
    app = Flask(__name__)
    # register blueprint here
    return app

Flask

from flask import Flask, Blueprint
bp = Blueprint('bp', __name__)
@bp.route('/hello')
def hello():
    return 'Hello Blueprint!'

def create_app():
    app = Flask(__name__)
    # register blueprint here
    return app

A register_blueprint(app, bp)

B bp.register_blueprint(app)

C app.blueprint_register(bp)

D app.register_blueprint(bp)

💡 Hint

Check the Flask app method to add blueprints.

The Flask app instance has a method register_blueprint to add blueprints. The blueprint does not register the app.

🔧 Debug

expert

Why does this app factory cause a runtime error?

Examine this code:

def create_app():
    app = Flask(__name__)
    @app.route('/count')
    def count():
        return str(counter)
    counter = 0
    return app

What error occurs when accessing '/count' and why?

Flask

def create_app():
    app = Flask(__name__)
    @app.route('/count')
    def count():
        return str(counter)
    counter = 0
    return app

A NameError because 'counter' is used before assignment

B AttributeError because 'counter' is not an attribute

C No error, returns '0'

D TypeError because 'counter' is not a string

💡 Hint

The route handler function body executes at request time, after the factory function has fully executed including the assignment.

A NameError occurs because the 'count' function tries to access 'counter' before it is assigned. The function 'count' is defined before 'counter' is assigned, so 'counter' is not in the closure at the time of function definition.