{% set item_col = show_sidebar ? 'col-12 col-md-9' : 'col-12' %}
{% set sidebar_col = show_sidebar ? 'col-12 col-md-3' : 'col-12' %}

<div class="row">
    <div id="item" class="{{ item_col }} extra-spacing">
        {% block item %}{% endblock %}
    </div>
    {% if show_sidebar %}
    <div id="sidebar" class="{{ sidebar_col }}">
        {% block sidebar %}{% endblock %}
    </div>
    {% endif %}
</div>