Form validation is a crucial part of web development, ensuring that data submitted by users meets certain criteria. In this guide, we'll explore how to perform basic input validation in Flask, a Python web framework. You'll learn how to validate form data, handle errors, and provide user-friendly feedback. By following this guide, you'll have the knowledge and tools to build web applications that maintain data integrity and improve user experience.

Step 1: Setting Up Your Flask Application

Start by setting up your Flask application and creating a directory structure. Here's a sample structure:


Step 2: Installing Flask

Install Flask using pip:

pip install Flask

Step 3: Creating the Flask Application

Create your Flask application. Here's an example of Python code for a simple form with input validation:

from flask import Flask, render_template, request, flash, redirect, url_for
app = Flask(__name__)
app.secret_key = 'your_secret_key'
@app.route('/', methods=['GET', 'POST'])
def index():
if request.method == 'POST':
username = request.form.get('username')
if not username:
flash('Username is required.', 'error')
elif len(username) < 5:
flash('Username must be at least 5 characters long.', 'error')
flash(f'Hello, {username}!', 'success')
return redirect(url_for('index'))
return render_template('index.html')
if __name__ == '__main__':

Step 4: Creating HTML Templates

Create an HTML template to display the form and validation messages. Here's an example of an index template:

<!-- templates/index.html -->
<!DOCTYPE html>
<title>Form Validation</title>
<h1>Form Validation</h1>
<form method="POST">
<label for="username">Username:</label>
<input type="text" name="username" id="username">
<button type="submit">Submit</button>
{% with messages = get_flashed_messages() %}
{% if messages %}
{% for message in messages %}
<li>{{ message }}</li>
{% endfor %}
{% endif %}
{% endwith %}

Step 5: Running Your Application

Run your Flask application and access it through a web browser. You can submit the form with various inputs to test the validation and error messages.


Flask form validation is essential for ensuring data quality in your web applications. By following this guide, you've learned how to set up your Flask application, create a simple form, and perform basic input validation. You can expand on this knowledge to validate other types of form data, such as email addresses, passwords, and more complex fields.