How to add Conditional CSS class based on Python If statement

Question

How do you add a conditional CSS class based on a Python if statement in the following example to show the has-success has-feedback form element?

<div class="form-group {% if not error_username %} has-success has-feedback {% endif %}">

Answer 1

Write if condition this way.

<div class="form-group {{'has-success has-feedback' if not error_username }}">