I noticed that :

$builder
->add('myRadios', 'choice', array(
'choices' => array(
'one' => 'uno',
'two' => 'due'),
'multiple' => false,
'attr' => array('class' => 'testClass'),
'expanded' => true

so attr with class is not rendered:
the output is:

http://pastebin.com/cAqLx9XZ

PS:
When i use attr on other field ex. text with same procedure class appears properly.
plz help. thx