Flask – Interview Questions

  1. What is Flask?
    Flask is a micro web framework in Python that is used to build web applications. It is simple, lightweight, and follows the WSGI (Web Server Gateway Interface) standard.
    Flask is a lightweight framework, which makes it easy to get started with. It is also very flexible, which allows you to customize your application to your specific needs.
  2. What are the disadvantages of using Flask?
    Flask is a microframework, which means that it does not come with a lot of features out of the box. You may need to install additional packages to add features like database support or authentication.
  3. What is the difference between Flask and Django?
    Flask and Django are both web frameworks for Python. However, they have different philosophies. Flask is a microframework, while Django is a full-featured framework. This means that Flask is more lightweight and flexible, while Django is more powerful and has more features.
  4. How do you install Flask?
    You can install Flask using pip:
  5. Explain how to create a basic Flask application.

    from flask import Flask

app = Flask(__name__)

@app.route('/')
def hello():
    return "Hello, Flask!"

     

  6. What are some of the popular Flask extensions?
    Some of the popular Flask extensions include:
    * Flask-SQLAlchemy: This extension provides database support for Flask.
    * Flask-Login: This extension provides authentication support for Flask.
    * Flask-WTF: This extension provides form validation support for Flask.
  7. What is route in Flask?
    A route in Flask is a URL pattern associated with a view function. When a user requests a specific URL, Flask maps the URL to the corresponding view function defined in the application.
    To define routes in Flask, you can use the @app.route() decorator. The @app.route() decorator takes a URL as the argument and a function as the value. The function that you pass to the @app.route() decorator will be called when the URL is requested.
  8. How do you run a Flask application?

    @app.route('/user/<username>')
def show_user(username):
    return f"Hello, {username}!"
  9. How do you perform URL redirect in Flask?
    You can use the redirect() function to redirect to another URL.

    from flask import redirect

@app.route('/old-url')
def old_url():
    return redirect('/new-url')

     

  10. How do you handle errors in Flask?
    To handle errors in Flask, you can use the @app.errorhandler() decorator. The @app.errorhandler() decorator takes an error code as the argument and a function as the value. The function that you pass to the @app.errorhandler() decorator will be called when the error code is encountered.In Flask, you can handle errors using error handlers and exception handling. Flask provides ways to customize error pages for different HTTP status codes and also allows you to handle exceptions that might occur during the execution of your application. Let’s look at both methods:Error Handlers: Flask allows you to create custom error handlers for specific HTTP status codes. You can use the @app.errorhandler decorator to define error handlers for various status codes.

    from flask import Flask, render_template

app = Flask(__name__)

@app.route('/')
def index():
    return "Hello, Flask!"

# Custom error handler for 404 Not Found
@app.errorhandler(404)
def page_not_found(error):
    return render_template('404.html'), 404

# Custom error handler for 500 Internal Server Error
@app.errorhandler(500)
def internal_server_error(error):
    return "Internal Server Error", 500

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

    Exception handling in Flask allows you to catch and handle exceptions that occur during the execution of your application. You can use the try, except, and finally blocks to handle exceptions and respond accordingly.

    from flask import Flask, render_template

app = Flask(__name__)

@app.route('/')
def index():
    return "Hello, Flask!"

@app.route('/divide/<int:num>')
def divide_by_zero(num):
    try:
        result = 10 / num
        return f"The result is: {result}"
    except ZeroDivisionError:
        return "Error: Cannot divide by zero."
    except Exception as e:
        return f"An error occurred: {str(e)}"

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

     

  11. What is Flask-WTF and how is it used? Flask-WTF is an extension that provides integration with WTForms, allowing you to create and handle forms in Flask applications. 
    from flask_wtf import FlaskForm
from wtforms import StringField, SubmitField

class MyForm(FlaskForm):
    name = StringField('Name')
    submit = SubmitField('Submit')

     

  12. What are Flask Blueprints? Flask Blueprints are used to organize a Flask application into smaller, reusable components. They allow you to register views, templates, and static files for a specific feature or module.
  13. Explain the use of Flask-SQLAlchemy. Flask-SQLAlchemy is an extension that integrates SQLAlchemy with Flask, providing an easy way to work with databases in Flask applications.
  14. How can you connect a Flask application to a database using Flask-SQLAlchemy?

    from flask_sqlalchemy import SQLAlchemy

app = Flask(__name__)
app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:///mydatabase.db'
db = SQLAlchemy(app)

     

     

  15. How do you define a model in Flask-SQLAlchemy?

    class User(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    username = db.Column(db.String(80), unique=True, nullable=False)
    email = db.Column(db.String(120), unique=True, nullable=False)

    def __repr__(self):
        return f'<User {self.username}>'

     

  16. Explain Flask-Migrate and its purpose. Flask-Migrate is an extension that allows you to manage database migrations in Flask-SQLAlchemy applications. It helps to keep the database schema in sync with the application models.
  17. What is Flask-RESTful used for? Flask-RESTful is an extension that simplifies the process of building RESTful APIs in Flask. It provides features for defining resources, request parsing, and handling HTTP methods.
  18. Explain how to create a simple RESTful API using Flask-RESTful.

    from flask import Flask
from flask_restful import Resource, Api

app = Flask(__name__)
api = Api(app)

class HelloWorld(Resource):
    def get(self):
        return {'message': 'Hello, World!'}

api.add_resource(HelloWorld, '/')

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

     

     

  19. What is the use of Flask-JWT? Flask-JWT (JSON Web Tokens) is an extension that provides a secure way to authenticate users and generate tokens for access control in Flask applications.
  20. Explain how to secure a Flask API using Flask-JWT.

    from flask_jwt import JWT, jwt_required, current_identity
from werkzeug.security import safe_str_cmp

# Sample user class
class User:
    def __init__(self, id, username, password):
        self.id = id
        self.username = username
        self.password = password

# Dummy user data
users = [
    User(1, 'user1', 'password1'),
    User(2, 'user2', 'password2')
]

# JWT authentication functions
def authenticate(username, password):
    user = next((user for user in users if user.username == username), None)
    if user and safe_str_cmp(user.password.encode('utf-8'), password.encode('utf-8')):
        return user

def identity(payload):
    user_id = payload['identity']
    return next((user for user in users if user.id == user_id), None)

app.config['SECRET_KEY'] = 'super-secret-key'
jwt = JWT(app, authenticate, identity)

@app.route('/protected')
@jwt_required()
def protected():
    return f'Hello, {current_identity.username}!'

     

     

  21. What is Flask-Caching and how is it used? Flask-Caching is an extension that adds caching support to Flask applications, improving the performance of repetitive tasks.
  22. Explain how to implement caching in Flask using Flask-Caching.

    from flask_caching import Cache

app = Flask(__name__)
app.config['CACHE_TYPE'] = 'simple'
cache = Cache(app)

@app.route('/slow')
@cache.cached(timeout=60)
def slow_function():
    # Expensive calculation or operation
    return "Slow response"

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

     

     

  23. Explain the use of Flask-SocketIO. Flask-SocketIO is an extension that provides WebSocket support to Flask applications, allowing real-time bidirectional communication between the server and clients.
  24. Explain how to use Flask-SocketIO to implement real-time communication.

    from flask import Flask, render_template
from flask_socketio import SocketIO

app = Flask(__name__)
app.config['SECRET_KEY'] = 'secret_key'
socketio = SocketIO(app)

@app.route('/')
def index():
    return render_template('index.html')

@socketio.on('message')
def handle_message(message):
    print('Received message: ', message)
    socketio.send('Message received: ' + message)

if __name__ == '__main__':
    socketio.run(app, debug=True)

     

     

  25. How do you deploy a Flask application?
    To deploy a Flask application, you can use a variety of methods. One common method is to use a web server like Gunicorn or uWSGI. You can also deploy a Flask application to a cloud platform like Heroku or AWS.
  26. What is the MVC pattern? The MVC (Model-View-Controller) pattern is a software design pattern used to organize the structure of web applications. It separates the application into three interconnected components:Model: Represents the data and business logic of the application.
    View: Handles the presentation and user interface.
    Controller: Manages the user input, processes requests, and updates the model and view accordingly.
    In Flask, the View and Controller components are often combined into views and routed using decorators.
  27.  
  28. What are the different types of HTTP requests? The main types of HTTP requests are:
    • GET: Used to request data from a specified resource.
    • POST: Used to submit data to be processed to a specified resource.
    • PUT: Used to update a resource or create one if it doesn’t exist.
    • DELETE: Used to delete a specified resource.
    • PATCH: Used to partially update a resource.
    • HEAD: Similar to GET, but retrieves only the headers of a resource.

    In Flask, you can handle these HTTP requests using different route decorators (@app.route) to specify the URL patterns for each type.

  29. What is the difference between a GET and a POST request?
          The main differences between a GET and a POST request are:

    • GET: Requests data from a specified resource, and the data is appended to the URL as query parameters. GET requests are less secure as the data is visible in the URL, making it unsuitable for sensitive information.
    • POST: Submits data to be processed to a specified resource, and the data is sent in the request body. POST requests are more secure and can handle larger data payloads compared to GET requests.

    In Flask, you can handle both GET and POST requests using the route decorators (@app.route) and request methods in view functions.

  30. How do you use the Flask debugger?
    The Flask debugger is a built-in feature that helps in debugging Flask applications. To enable the debugger, you can set the debug parameter to True when creating the Flask application:

    from flask import Flask

app = Flask(__name__)
app.debug = True

     

  31. How do you use the Flask test client? The Flask test client is used to simulate HTTP requests to the application during testing. It allows you to test views and routes without making actual requests to the server. You can use the test client as follows: 
    from flask import Flask

app = Flask(__name__)

@app.route('/')
def index():
    return "Hello, Flask!"

with app.test_client() as client:
    response = client.get('/')
    print(response.data)  # Output: b'Hello, Flask!'

     

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