Drupal/snippets/forms/elements

function custom_theme_registry_alter(&$theme_registry) {
  if (!empty($theme_registry['form_element'])) {
    $theme_registry['form_element']['function'] = 'custom_form_element';
  }
}

function custom_form_element($element, $value) {
  dsm($element);
  if($element['#id'] == 'edit-calendar-goto-datepicker-popup-0'){
      //$element['#description'] = 'habla asunción';
    }


  // Here, we provide our customized version of the
  // theme_form_element function from theme.inc...

  // This is also used in the installer, pre-database setup.
  $t = get_t();

  $output = '<div class="form-item"';
  if (!empty($element['#id'])) {
    $output .= ' id="'. $element['#id'] .'-wrapper"';
  }
  $output .= ">\n";
  $required = !empty($element['#required']) ? '<span class="form-required" title="'. $t('This field is required.') .'">*</span>' : '';

  if (!empty($element['#title'])) {
    $title = $element['#title'];
    if (!empty($element['#id'])) {
      $output .= ' <label for="'. $element['#id'] .'">'. $t('!title: !required', array('!title' => filter_xss_admin($title), '!required' => $required)) ."</label>\n";
    }
    else {
      $output .= ' <label>'. $t('!title: !required', array('!title' => filter_xss_admin($title), '!required' => $required)) ."</label>\n";
    }
  }

  $output .= " $value\n";
  if (!empty($element['#description'])) {
    $output .= ' <div class="description">'. $element['#description'] ."</div>\n";
  }

  $output .= "</div>\n";

  return $output;
}



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http://www.lullabot.com/articles/overriding-theme-functions-in-modules