Flask Intro

4.4. Flask Intro#

Flask is a lightweight and powerful web framework written in Python. Flask takes care of the tedious parts of building server side applications by handling the low level details of HTTP requests and responses for you.

Flask is an open source framework, with a large user base. Some of the worlds largest companies build some or all of their web applications using Flask. For example:

  • Netflix uses Flask for building internal tools.

  • Pinterest uses Flask for developing their web API

  • Linkedin

  • Uber [1]

Caution

TODO: FIX THESE LINKS

Flask simplifies web development by allowing you to create a web application using Python functions. All you need to do is define routes and write code to respond to requests — Flask takes care of the rest!

4.4.1. Hello World with Flask#

We will start with a “Hello World!” application using Flask.

app.py#
from flask import Flask

app = Flask(__name__)  # Create a Flask application

@app.route("/")  # Define the route for the home page
def hello_world():
    return "Hello, World!"  # Return "Hello, World!" as a response

if __name__ == "__main__":
    app.run(debug=True, port=5000)  # Run the app

Let’s breakdown the Python script

  • from flask import Flask: This line imports the Flask class from the flask module.

  • app = Flask(__name__): Here, we create a new Flask application. __name__ helps Flask figure out the root path of your application.

  • @app.route("/"): This is called a route. It tells Flask which URL should trigger the function right below it. In this case, it’s the home page (/).

  • def hello_world():: This function is triggered when the user visits the home page, and it returns the message "Hello, World!".

  • if __name__ == "__main__":: This checks if the script is being run directly (not imported as a module). If true, it runs the Flask app.

  • app.run(debug=True, port=5000): This starts the Flask web server with:

    • debug mode enabled which helps catch errors and reload the app when changes are made.

    • listens for requests on port 5000, which is useful since the default HTTP port 80 can be already occupied on your computer.

4.4.2. Running Hello World#

To run the Flask app, follow these steps:

  1. Save the above code in a file named app.py.

  2. Open a terminal or command prompt in the directory where app.py is located.

  3. Run the command:

python app.py

  1. Open your browser and go to http://127.0.0.1:5000/

Hint

If you see “Hello, World!” displayed on the page you’ve successfully created your first server side app!

You can end the process by pressing CTRL + C keys together in the terminal window where you ran the command originally.

4.4.3. What’s the time?#

Let’s build a slightly more advanced example where the webpage shows the current date and time.

time.py#
from flask import Flask
import datetime

app = Flask(__name__)

@app.route("/")  # Define the route for the home page
def show_date_random_word():
     # Get current date and time
    current_time = datetime.datetime.now().strftime("%Y-%m-%d %H:%M:%S")
    html = f"""
    <!DOCTYPE html>
    <html>
    <body>
        <h1>Welcome!</h1>
        <p>Current Date and Time: {current_time}</p>
    </body>
    </html>
    """
    return html  # Return the HTML

if __name__ == "__main__":
    app.run(debug=True, port=5000)

The steps to run the “What’s the time?” app are similar as “Hello World”:

  1. Save the above code in a file named time.py.

  2. Open a terminal or command prompt in the directory where time.py is located.

  3. Run the command:

python time.py

  1. Open your browser and go to http://127.0.0.1:5000/.

Hint

Each time you refresh the page in your browser you should see a different time since the python function is executed on every page load.

Caution

You can’t run multiple servers on the same port. So make sure you terminate the “Hello, World!” example before running “What’s the time?”.

4.4.4. References#

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