Flask in Python

Flask is a micro web framework written in Python. It is classified as a “micro” framework because it does not require particular tools or libraries, nor does it have a specific pattern for building applications, like Django’s Model-View-Controller (MVC) pattern. However, this does not mean Flask is limited; rather, it gives developers more flexibility on how they want to implement things.

Here’s a quick introduction to Flask:

  1. Installation
    You can install Flask via pip:
   pip install Flask
  1. Basic Application
    Here’s a basic Flask application:
   from flask import Flask
   app = Flask(__name__)

   def hello_world():
       return 'Hello, World!'

   if __name__ == '__main__':
  1. Routing
    Flask provides an easy way to define routes:
   def about():
       return 'About page'
  1. Variables in Routes
    You can capture variables from routes as well:
   def show_user_profile(username):
       return f'User {username}'
  1. Templates
    Flask can be used with the Jinja2 template engine. For example:
   from flask import render_template

   def hello(name=None):
       return render_template('hello.html', name=name)

And in a templates folder, you’d have a hello.html file:

   <!DOCTYPE html>
       <title>Hello, {{ name }}!</title>
       <h1>Hello, {{ name }}!</h1>
  1. Static Files
    By default, Flask looks for static files (like CSS and JavaScript) in a folder named “static”:
   <link rel="stylesheet" href="{{ url_for('static', filename='style.css') }}">
  1. Flask Extensions
    Flask has a thriving ecosystem of extensions that you can use to add functionality to your applications. For example, Flask-SQLAlchemy adds ORM capabilities, Flask-WTF aids in form handling, Flask-Login helps with user authentication, and many more.
  2. Development & Deployment
  • During development, use app.run(debug=True) to activate debug mode, which provides helpful error messages and auto-reloads the app upon changes.
  • For production deployment, using Flask’s built-in server isn’t recommended. Instead, deploy Flask apps using a more robust server, like Gunicorn, and a reverse proxy, like Nginx or Apache.
  1. Documentation
    Flask has excellent official documentation, which includes quick starts, patterns, and links to helpful resources.

Sure, below is a basic example of using Flask to create a simple web application.


If you haven’t installed Flask, you can do so using pip:

pip install Flask

Simple Flask App

Here’s a straightforward example of a Flask application. This app initializes a Flask object, defines a route (URL pattern), and starts a development server.

Create a file named app.py:

from flask import Flask

app = Flask(__name__)

def home():
    return "Hello, Flask!"

def greet(name):
    return f"Hello, {name}!"

if __name__ == "__main__":

Let’s go through the code:

  • app = Flask(__name__): Create a Flask web server from the Flask module.
  • @app.route("/"): A decorator to tell Flask what URL should trigger the function home().
  • def home(): The function to be triggered when the URL pattern specified by @app.route("/") is accessed.
  • return "Hello, Flask!": The response returned to the user in their web browser.
  • @app.route("/greet/<name>"): A parameterized route that allows dynamic URL patterns.
  • def greet(name): The function that uses a URL parameter and returns a personalized greeting.
  • if __name__ == "__main__": app.run(debug=True): Start the server and allow for debugging (restarts server on code changes and provides detailed error messages).

Running the App

To run the app, open a terminal and navigate to the directory containing app.py. Then execute:

python app.py

Your app should be running on localhost:5000 or Open this URL in your web browser, and you should see the “Hello, Flask!” message. To test the parameterized route, try visiting localhost:5000/greet/YourName.


  • Flask’s built-in server is not suitable for production. For deployment in a production environment, consider using server options like Gunicorn combined with Nginx or Apache.
  • Ensure you have Flask installed in your Python environment before running the application. If you’re using a virtual environment (recommended), activate it before installing Flask and running the app.

This is just a brief overview, and Flask offers many more features and options. Depending on your needs, Flask can be a simple and lightweight solution or be scaled up with extensions and various configurations for more complex applications.

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