symfony - Symfony2 Change class name in twig form theme -

i want override form_widget_simple function

{% block form_widget_simple %} {% spaceless %}     {% set type = type|default('text') %}     {% if errors|length > 0 %}         {{dump(form.vars.attr)}}     {% endif %}     <input type="{{ type }}" {{ block('widget_attributes') }} {% if value not empty %}value="{{ value }}" {% endif %}/> {% endspaceless %} {% endblock form_widget_simple %} 

but dont know how set form.vars.attr['class'] inside if statement whent set form.vars.attr['class'] = 'error'; error unexpected token "punctuation" of value "." ("end of statement block" expected)

as see, adding additional attributes handled in widget_attributes block. if take in there, see simple foreach on attr array, attributes. think simple set merging existing one, done. form_widget_simple block like

{% block form_widget_simple %} {% spaceless %}     {% set type = type|default('text') %}     {% if errors|length > 0 %}         {{dump(form.vars.attr)}}     {% endif %}     {% set attr = attr|merge({'class': (attr.class|default('') ~ ' your-css-class')|trim}) %}     <input type="{{ type }}" {{ block('widget_attributes') }} {% if value not empty %}value="{{ value }}" {% endif %}/> {% endspaceless %} {% endblock form_widget_simple %} 

this preserve every class attribute set in form builder , add your-css-class additional class. if no class attribute defined, your-css-class set.