Symfony Form Twig Theme

_form.html.twig

{# Form Theme #}
{# See: https://github.com/symfony/symfony/blob/2.0/src/Symfony/Bridge/Twig/Resources/views/Form/form_div_layout.html.twig #}
{# See: http://stackoverflow.com/questions/10043251/how-to-add-a-css-class-on-a-form-row-in-twig-template #}

{% form_theme form _self %}

{# Field Row #}
{# - Add 'error' class to form widgets. #}
{# - Display errors below widgets. #}
{% block field_row %}
{% spaceless %}
    {% set attr = (errors|length > 0 ? attr|merge({ class: 'error' }) : attr) %}
    {{ form_label(form, label|default(null)) }}
    {{ form_widget(form, { attr: attr }) }}
    {{ form_errors(form) }}
{% endspaceless %}
{% endblock field_row %}

{# Field Errors #}
{# - Display errors in paragraphs. #}
{% block field_errors %}
{% spaceless %}
    {% for error in errors %}
        <p class="error">{{ error.messageTemplate|trans(error.messageParameters, 'validators') }}</p>
    {% endfor %}
{% endspaceless %}
{% endblock field_errors %}

index.html.twig

{% form_theme form '_form.html.twig' %}

<form action="." method="post">
    {{ form_widget(form) }}
    <input type="submit" value="Send" />
</form>