Mastering Flask: 18 Common Interview Questions and Answers

Flask is a popular web framework for Python that allows developers to build web applications quickly and efficiently. If you’re preparing for a Flask interview, it’s important to familiarize yourself with some of the common questions that may be asked. In this article, we will explore 18 common interview questions for Flask and provide detailed answers to help you ace your interview. Whether you’re a beginner or an experienced developer, these questions will test your knowledge of Flask and showcase your expertise in web development.

Understanding Flask: An Overview

Before diving into the interview questions, let’s have a brief overview of Flask. Flask is a micro web framework written in Python. It is lightweight, flexible, and easy to use, making it a popular choice among developers. Flask follows the Model-View-Controller (MVC) architectural pattern and provides a range of features and extensions to simplify web development. With its simplicity and extensibility, Flask is widely used for building small to medium-sized web applications and APIs.

18 Common Interview Questions for Flask

1. What is Flask and how does it differ from other web frameworks?

Flask is a micro web framework for Python that is designed to be simple and lightweight. Unlike other web frameworks, Flask does not impose any specific tools or libraries, allowing developers to make their own choices. Flask provides the essentials for web development, such as routing, request handling, and templating, without forcing any additional components. This flexibility makes Flask highly customizable and suitable for a wide range of projects.

2. What is the difference between Flask and Django?

Flask and Django are both popular web frameworks for Python, but they have different design philosophies and use cases. Flask is a micro framework that provides the bare essentials for web development, allowing developers to choose their own tools and libraries. On the other hand, Django is a full-featured framework that includes everything needed for web development out of the box. Flask is ideal for small to medium-sized projects, while Django is better suited for larger, more complex applications.

3. How do you create a Flask application?

To create a Flask application, you need to follow a few simple steps:

  • Create a virtual environment: Set up a virtual environment to isolate your Flask project from other Python projects.
  • Install Flask: Use pip to install Flask in your virtual environment.
  • Create a Flask app: Create a new Python file and import the Flask module. Define a Flask app instance.
  • Define routes and views: Use the app.route decorator to define routes and associate them with view functions.
  • Run the app: Use the method to start the Flask development server.

4. How does routing work in Flask?

In Flask, routing is the process of mapping URLs to view functions. You can define routes using the app.route decorator, which takes a URL pattern as an argument. When a user visits a URL that matches a route pattern, Flask calls the associated view function and returns the result as an HTTP response. Routes can also include variables and converters to capture dynamic parts of the URL. For example, you can define a route like @app.route('/user/<username>') to handle URLs like /user/johndoe.

5. What is a view function in Flask?

In Flask, a view function is a Python function that handles a specific URL route. When a user visits a URL that matches a route, Flask calls the associated view function, which generates a response to be sent back to the user. View functions can return a string, a template, or a response object. They can also access and modify request data, such as form inputs or URL parameters. View functions are the building blocks of Flask applications and are responsible for handling user requests and generating responses.

6. How do you handle form submissions in Flask?

Handling form submissions in Flask involves two steps: defining a form class and handling the form data in a view function. To define a form class, you can use the Flask-WTF extension, which provides a set of classes and helpers for working with forms in Flask. In the view function, you can use the request object to access the form data and perform validation. Flask-WTF also includes built-in validation and CSRF protection to enhance the security of your forms.

7. What is a template engine in Flask?

A template engine in Flask is a tool that allows you to separate the presentation logic from the application logic. It provides a way to generate dynamic HTML pages by combining static HTML templates with dynamic content. Flask uses the Jinja2 template engine by default, which is a powerful and flexible template engine for Python. Jinja2 templates can include variables, control structures, filters, and macros, making it easy to generate complex HTML pages.

8. How do you use sessions in Flask?

In Flask, sessions are used to store user-specific data across multiple requests. To use sessions, you need to enable the SESSION configuration in your Flask app. Once sessions are enabled, you can use the session object to store and retrieve data. For example, you can use session['username'] = 'johndoe' to store the username in the session, and username = session.get('username') to retrieve it in another request. Sessions are stored on the server side, and a session identifier is sent to the client as a cookie.

9. How can you handle errors and exceptions in Flask?

In Flask, you can handle errors and exceptions using error handlers and try-except blocks. Flask provides built-in error handlers for common HTTP errors, such as 404 Not Found and 500 Internal Server Error. You can define your own error handlers using the @app.errorhandler decorator. For example, you can define an error handler for a 404 error like @app.errorhandler(404). Alternatively, you can use try-except blocks to catch and handle exceptions within your view functions.

10. What are Flask blueprints?

Flask blueprints are a way to organize and structure Flask applications. Blueprints allow you to define reusable components, such as routes, views, and templates, that can be registered with an application or mounted on a specific URL prefix. Blueprints are particularly useful for building large applications or modularizing your code. With blueprints, you can create separate modules for different parts of your application and easily plug them together.

11. How do you implement authentication and authorization in Flask?

Authentication and authorization can be implemented in Flask using various strategies and extensions. Flask-Login is a popular extension that provides user authentication and session management. It allows you to easily handle user logins, logouts, and access control. Flask-Security is another extension that provides a more comprehensive set of features for user management, including user registration, password recovery, and role-based access control. You can also implement custom authentication and authorization logic using Flask’s built-in tools and libraries.

12. What is Flask-WTF and how do you use it?

Flask-WTF is an extension that integrates the WTForms library with Flask. WTForms is a flexible and powerful form validation and rendering library for Python. Flask-WTF provides a set of classes and helpers that make it easy to work with forms in Flask. With Flask-WTF, you can define forms as classes, add validation rules, and render them in your templates. Flask-WTF also includes built-in support for CSRF protection and file uploads.

13. How can you handle file uploads in Flask?

Handling file uploads in Flask involves a few steps. First, you need to set the enctype attribute of your HTML form to multipart/form-data. Then, in your view function, you can use the request.files object to access the uploaded files. Each file is represented as a FileStorage object, which provides methods and properties for working with the file. You can save the file to a specific location, retrieve its contents, or perform any other necessary operations.

14. How do you test Flask applications?

Testing Flask applications involves writing test cases and using a testing framework, such as pytest or unittest. Flask provides a test client that allows you to simulate requests and interact with your application. You can use the test client to send HTTP requests, check the response status and content, and assert the expected behavior of your application. Testing Flask applications is important to ensure the correctness and reliability of your code, as well as to catch and fix any bugs or regressions.

15. How can you deploy a Flask application?

There are several ways to deploy a Flask application, depending on your requirements and preferences. One common approach is to use a WSGI server, such as Gunicorn or uWSGI, to serve your Flask application. These servers are designed to handle multiple concurrent requests and provide better performance and scalability compared to the Flaskdevelopment server. Another option is to deploy your Flask application on a Platform as a Service (PaaS) provider, such as Heroku or AWS Elastic Beanstalk. These platforms provide a streamlined deployment process and handle infrastructure management for you. You can also deploy Flask applications on containers, such as Docker, or on virtual machines using tools like Vagrant. Regardless of the deployment method you choose, it’s important to ensure that your application is secure, scalable, and reliable.

16. What are some best practices for Flask development?

When developing Flask applications, it’s important to follow best practices to ensure code quality, maintainability, and performance. Here are some best practices to keep in mind:

  • Organize your code: Use blueprints or modules to organize your code into logical components. Separate your application logic from your presentation logic.
  • Use version control: Use a version control system, such as Git, to track changes to your code and collaborate with other developers.
  • Write tests: Write unit tests and integration tests to ensure the correctness and reliability of your code. Use a testing framework, such as pytest or unittest.
  • Follow the Flask conventions: Follow the naming conventions and coding style recommended by the Flask community. This will make your code more readable and maintainable.
  • Handle errors gracefully: Implement error handling and exception handling to provide meaningful error messages to users and handle unexpected situations.
  • Secure your application: Implement security measures, such as input validation, CSRF protection, and proper authentication and authorization mechanisms.
  • Optimize performance: Optimize your code and database queries to minimize response times and improve scalability. Use caching and pagination where appropriate.
  • Document your code: Write clear and concise documentation for your code to help other developers understand its purpose and usage.

17. How can you extend Flask functionality?

Flask provides a range of extensions and plugins that allow you to extend its functionality and add new features to your applications. Some popular extensions include:

  • Flask-WTF: Integrates the WTForms library for working with forms.
  • Flask-Login: Provides user authentication and session management.
  • Flask-Security: Offers a comprehensive set of features for user management and access control.
  • Flask-Mail: Enables sending email from your Flask application.
  • Flask-Cache: Provides caching capabilities to improve performance.
  • Flask-SQLAlchemy: Integrates the SQLAlchemy library for working with databases.
  • Flask-RESTful: Simplifies building RESTful APIs in Flask.

These extensions can save you time and effort by providing pre-built functionality that you can easily integrate into your Flask applications. You can find a wide range of Flask extensions in the Flask extension registry (

18. How can you contribute to the Flask community?

The Flask community is vibrant and welcoming, and there are several ways you can contribute to it:

  • Provide feedback: If you encounter any issues or have suggestions for improvement, don’t hesitate to share your feedback with the Flask community. This can be done through GitHub issues, mailing lists, or community forums.
  • Contribute code: If you have the skills and time, you can contribute code to the Flask project or its related extensions. This can involve fixing bugs, implementing new features, or improving documentation.
  • Write tutorials or articles: Share your knowledge and experience with the Flask community by writing tutorials, articles, or blog posts. This can help newcomers get started with Flask and learn best practices.
  • Help others: Participate in the Flask community forums or mailing lists and help answer questions from other developers. Sharing your expertise and providing support to others is a valuable contribution.
  • Attend or organize events: Attend Flask conferences, meetups, or workshops to network with other developers and learn from their experiences. You can also consider organizing local Flask events to bring the community together.

By contributing to the Flask community, you not only help improve the framework but also grow as a developer and build connections with like-minded individuals.


Flask is a powerful and flexible web framework for Python, and mastering it can open up a world of possibilities in web development. In this article, we covered 18 common interview questions for Flask, ranging from the basics of Flask to more advanced topics like authentication and deployment. By understanding and preparing for these questions, you can confidently showcase your Flask skills and land your dream job. Remember to practice and explore Flask further to deepen your understanding and become a proficient Flask developer. Good luck with your Flask interview!

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