Drupal 7 field widget - allowed values list
This would be hook_field_widget_form(&amp;$form, &amp;$form_state, $field, $instance, $langcode, $items, $delta = 0).

The example code for the hook, reported in the documentation is the following one.

function hook_field_widget_form(&amp;$form, &amp;$form_state, $field, $instance, $langcode, $items, $delta, $element) {
  $element += array(
    '#type' =&gt; $instance['widget']['type'], 
    '#default_value' =&gt; isset($items[$delta]) ? $items[$delta] : '',
  );
  return $element;
}