Error message display in templates in Flask - Mini Project: Build & Apply

Error message display in templates

📖 Scenario: You are building a simple Flask web app that shows a form. When the user submits the form with missing or wrong data, you want to show error messages on the page.

🎯 Goal: Create a Flask app that passes error messages from the backend to the HTML template and displays them clearly to the user.

📋 What You'll Learn

Create a dictionary called errors with specific error messages

Create a variable called has_errors to indicate if there are any errors

Use a for loop in the template to display each error message

Add the final Flask route and template rendering code to show errors

💡 Why This Matters

🌍 Real World

Web apps often need to show users helpful error messages when they submit forms with missing or wrong information.

💼 Career

Knowing how to pass error messages from backend to frontend templates is a key skill for web developers working with Flask or similar frameworks.

1

Create the error messages dictionary

Create a dictionary called errors with these exact entries: 'username' key with value 'Username is required.' and 'password' key with value 'Password must be at least 8 characters.'

Flask

# Create the errors dictionary with the required messages
# Your code here

Need a hint?

Use curly braces {} to create a dictionary with the exact keys and messages.

2

Add a variable to check if there are any errors

Create a variable called has_errors that is true if the errors dictionary is not empty, otherwise false

Flask

errors = {
    'username': 'Username is required.',
    'password': 'Password must be at least 8 characters.'
}

# Create has_errors variable to check if errors dictionary is not empty
# Your code here

Need a hint?

Use len(errors) > 0 to check if the dictionary has any entries.

3

Use a for loop in the template to display error messages

In the Flask template, use a {% for field, message in errors.items() %} loop to display each error message inside a <li> tag. Close the loop with {% endfor %}.

Flask

{% if has_errors %}

    
    

{% endif %}

Need a hint?

Use Jinja2 syntax to loop over the errors dictionary and show each message inside list items.

4

Complete the Flask route to render the template with errors

Write a Flask route function called show_form that returns render_template('form.html', errors=errors, has_errors=has_errors).

Flask

from flask import Flask, render_template

app = Flask(__name__)

# Define the route and function to render the form with errors
# Your code here

Need a hint?

Use the @app.route decorator and define the function to pass variables to the template.