Flask Library

Quick Flask Tutorial


One of the most popular and beginner-friendly frameworks of Python is Flask. It is a lightweight web framework to build web applications quickly and efficiently making it a popular choice for both beginners and experienced developers. In this quick Flask tutorial we see how it is built on top of the Python programming language, which means that developers can take advantage of Python’s extensive libraries and modules while building their web applications.


First and foremost, Flask is incredibly flexible and customizable. It allows developers to choose only the components they need for their specific application. This flexibility also allows for easy integration with other libraries and frameworks, giving developers the freedom to build their applications in the way that best suits their needs.
Another advantage of Flask is its simplicity since it follows a “micro” framework approach, which means it provides a minimalistic set of tools and features. This makes it easy to understand and learn, especially for beginners. Flask also has an intuitive syntax that is easy to read and write, making the development process faster and more enjoyable.
Additionally, Flask has a strong and active community of developers. This means that there is a wealth of resources, tutorials, and documentation available to help developers learn and troubleshoot any issues they may encounter. The community also regularly releases updates and improvements to the framework, ensuring that Flask remains up-to-date and secure.

Features and Functionalities of Flask

One of its key features is its built-in development server, which allows developers to test their applications locally before deploying them to a production server. This makes the development process more efficient and reduces the risk of errors or bugs in the final application.
In addition,  Flask supports URL routing so developers can define routes for different URLs, which determines how the application responds to different requests. This makes it easy to create dynamic and interactive web pages, as developers can map specific URLs to different functions or views within their application.
Flask also offers template rendering  allowing engineers to separate the logic of their application from its presentation. Templates in Flask are written in HTML or Jinja2, a powerful and flexible templating language. Developers can create dynamic web pages combining static HTML with dynamic content.

Creating a Flask Application

Before diving into the main components of Flask, let’s start by setting up a simple Flask application. The first step is to create a virtual environment to isolate our project dependencies. This ensures that our Flask application will run smoothly without conflicting with other Python packages installed on our system.
To create a virtual environment, we can use a tool like pipenv. Open your terminal and navigate to the desired directory for your Flask project. Run the following command to install pipenv:

pip install pipenv

Once pipenv is installed, we can create a new virtual environment for our Flask project by running the following command:

pipenv shell

This will activate the virtual environment and change your prompt to indicate that you are now working within the virtual environment.

2. HTML Templates with Jinja2

Flask templates are a powerful feature to separate the presentation of the web application from its logic. They are written in HTML or Jinja2, a popular templating language, and can contain placeholders for dynamic content that is generated by the Flask application.
To use templates in Flask, you first need to create a directory called "templates" in your project directory. This is where Flask will look for template files. Inside the "templates" directory, you can create HTML or Jinja2 files that represent the different pages or components of your web application.
With Jinja2 you can build HTML pages using  Python concepts such as variables, loops, and conditionals.
Let’s create a simple template called index.html as an example:

<!DOCTYPE html> 
<html lang="en"> 
     <meta charset="UTF-8">     
     <title>My Flask Application</title> 
     <h1>Welcome to my Flask Application!</h1> 

In our Flask application, we can render this template by using the “render_template()” function provided by Flask. To render the index.html template:

from flask import Flask, render_template  
app = Flask(__name__)  
def index():     
    return render_template('index.html')  

if __name__ == '__main__':     

We have imported the “render_template()” function and use it within our “index()” view function to render the index.html template. When a user visits the root URL of our application, they will see the content defined in the index.html template.

3. Setting Up a Database

Flask makes it easy to work with databases in your web applications. One popular choice for a database is “SQLite“, a lightweight and self-contained database engine. To use SQLite in our Flask application, we need to install the sqlite3 package. Run the following command to install it:

pipenv install sqlite3

Once the package is installed, we can create a simple database and connect to it using the sqlite3 module. Here’s an example of how we can set up a database in our Flask application:

import sqlite3

# Connect to the database 
conn = sqlite3.connect('mydatabase.db')  
# Create a cursor 
object cursor = conn.cursor()  
# Create a table 
cursor.execute('''CREATE TABLE IF NOT EXISTS users                   
               (id INTEGER PRIMARY KEY, name TEXT)''')  
# Insert data into the table 
cursor.execute("INSERT INTO users (name) VALUES ('David')") 
cursor.execute("INSERT INTO users (name) VALUES ('Laura')") 
cursor.execute("INSERT INTO users (name) VALUES ('Maria')")  
# Commit the changes 
# Close the connection 

In this script, we have created a database called mydatabase.db and connect to it through the “sqlite3.connect()” function. We then create a cursor object to execute SQL statements. We create a table called users with two columns: “id” and “name” then we insert some sample data into the table and commit the changes. Finally, we close the connection to the database.

4. Routing in Flask

Routing is a fundamental concept in web development to map URLs to specific functions in a web application. To define routes quickly, Flask has a decorator called @app.route() . Let’s look at an example:

from flask import Flask  
app = Flask(__name__)  
def index():     
    return 'Welcome to my Flask application!'  

def about():     
    return 'This is the about page.'  

def contact():     
    return 'You can contact us at example@example.com.'  

if __name__ == '__main__':     

Three routes have been defined: the root URL (/), the /about URL, and the /contact URL. Each route is associated with a view function that returns a string. When a user visits a specific URL, Flask will call the associated view function and return the string as the response.

5. Serving Static Files

In addition to rendering dynamic HTML pages, Flask also serves static files such as CSS, JavaScript, and images. To create a static directory within the project directory is the most suitable way to manage static files. Flask will automatically serve any files stored in this directory.

6. Running the Flask Application

Now that we have explored the main components of a Flask application, let’s discuss how to run the application. In our previous examples, we used the app.run() method to start the development server. By default, the Flask development server runs on, which is the local loopback address and port number 5000.
To run the Flask application, make sure you are inside the virtual environment and run the following command:

flask run

Once the development server is running, you can open your web browser and navigate to to see your Flask application in action. Any changes you make to your Flask application will be automatically picked up by the development server, so you can see the changes in real-time.

7. Flask Built-in Functions

Here are some of the most commonly used built-in functions in Flask:

  • render_template(): Renders an HTML template with optional context variables.
  • request: Provides access to incoming request data.
  • redirect(): Redirects the user to a different URL.
  • url_for(): Generates a URL for a specific endpoint.
  • flash(): Stores a message that can be retrieved and displayed to the user.
  • session: Stores user-specific information across multiple requests.

These built-in functions are just a small sample of what Flask has to offer. As you continue to explore Flask, you will discover many more useful functions and features that can enhance your web development projects.

8. Flask in Real-World Applications

Flask is a versatile web framework that can be used for many types of applications. Some of the most common use cases for Flask are:

  • Building simple web applications and prototypes quickly.
  • Creating RESTful APIs for mobile and web applications.
  • Developing microservices and backend systems.
  • Building webhooks for integrating with third-party services.
  • Creating interactive and dynamic dashboards.

Flask vs Other Web Frameworks

One popular alternative to Flask is Django. Django is a full-featured web framework that follows the “batteries included” philosophy, meaning it provides a comprehensive set of tools and features out of the box. Django is often chosen for larger, more complex web applications that require features such as user authentication, database management, and content administration.
Another popular web framework is Pyramid. Pyramid is a lightweight and flexible framework that is similar to Flask in many ways. It follows a “minimalistic” approach like Flask and allows developers to choose the components they need for their application. Pyramid also provides extensive documentation and a supportive community, making it a good choice for developers who value simplicity and customization.
When comparing Flask with other web frameworks, it’s important to consider your specific requirements and the scope of your project. Flask is a great choice for small to medium-sized applications or for developers who prefer a more hands-on approach. However, Django or Pyramid may be more suitable for larger and more complex projects.

Security in Flask Web Applications

One important security measure in Flask is the use of secure cookies for session management. Flask automatically signs session cookies to avoid tampering and ensures that the cookies are only sent over HTTPS connections to protect against eavesdropping. This helps to prevent session hijacking and ensures the confidentiality and integrity of user data.
Flask also provides built-in protection against cross-site request forgery (CSRF) attacks. To ensure that requests are originated from your application and not from malicious sources,  a CSRF token is included in forms and verified on the server side. This way unauthorized actions to manipulate or steal user data can not be processed.
Another security consideration in Flask is the proper handling of user input. Flask comes with mechanisms to sanitize and validate user input, such as the "request.form" object for handling form data and the "request.args" object for handling query parameters. By properly validating and sanitizing user input, you get rid of common security vulnerabilities such as SQL injection and cross-site scripting (XSS) attacks.
Finally, Flask also supports the use of third-party security extensions and libraries such as for example, user authentication, role-based access control, and password hashing. They can further improve the security of your Flask applications and ensure that they meet the highest security standards.


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