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In this article, we will learn how to build a basic web application using Flask, which is a lightweight Python framework create and run web apps.

Prerequisites

To follow this article, you need:

Python 3.x installed on your system
pip (Python package manager)
Flask (will be installed during the steps)

Step-by-Step Process

Step 1: Install Flask

To install Flask, open your terminal or command prompt and enter below command:

pip install Flask

This command installs Flask along with the required supporting libraries.

Step 2: Create Your First Flask Application

Create a new Python file named app.py. This file will contain the code for your first Flask web application.

Python

from flask import Flask

app = Flask(__name__)

@app.route('/')
def hello_world():
    return 'Hello World'

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

Explanation:

Flask(__name__) creates the Flask application
@app.route('/') connects the home URL (/) to a function
The function returns text shown in the browser
app.run() starts the local server

Step 3: Run the Flask Application

Run the command and you will see following in your terminal:

runningFlaskApp — Run the Flask Application

Open the given URL in your web browser to view your first Flask web page showing Hello World.

Step 4: Handling a POST Request Using an HTML Form

To understand how POST requests work in Flask, we will create a form that sends a user’s name to the server and displays it after submission.

Folder Structure

Before writing any code, organize your project folder as shown below:

Explanation:

app.py: Main Flask application file
templates/: Flask automatically looks here for HTML files
name.html: HTML form used to send POST data

Create the HTML Form

Inside the templates folder, create a file named name.html and add the following code:

HTML

<!DOCTYPE html>
<html>
<body>
    <h3>Enter Your Name</h3>
    <form method="post">
        <input type="text" name="username" required>
        <br><br>
        <input type="submit" value="Submit">
    </form>
</body>
</html>

Explanation:

The form uses method="post" to send data securely
The input field sends the value using the name username
Clicking Submit sends the data to the Flask server

Now, Update app.py as shown below:

Python

from flask import Flask, request, render_template

app = Flask(__name__)

@app.route('/login', methods=['GET', 'POST'])
def login():
    if request.method == 'POST':
        name = request.form['username']
        return f"Hello {name}, POST request received"
    return render_template('name.html')

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

Output

Open the browser and visit http://127.0.0.1:5000/login

Running — Flask development server running successfully.

Now, Enter a name and click Submit

The browser displays:

after_entering_name — Output displayed after submitting the form using POST request.

Explanation:

render_template() displays the HTML form
methods=['GET', 'POST'] allows both request types
GET request shows the form in the browser
POST request reads submitted form data using request.form
debug=True helps during development

Flask - Creating First Simple Application

Prerequisites

Step-by-Step Process

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