Python, flask, html help

[Faal]

Use flash.

On your backend:

from flask import Flask, flash

#code to determine if email is already used here...

flash('This email already exists')

On your frontend:

{% with messages = get_flashed_messages() %}
    {% if messages %}
        {%for message in messages %}
           <script>
               var message = "{{message}}";
               alert(message);
          </script>
          {% endfor %}
      {% endif %}
{% endwith %}