how do I call my theme preprocess function for a specific field?

i'm on Drupal 7 and i have aspecific tpl.php file for a content field_image: "field--field_image.tpl.php". I need to create a preprocess function for this field and for my theme.

Supposing my theme name is "My Theme"

It should look like

function my_theme_preprocess_field(&$variables, $hook) {
$variables['classes_array'][] = 'aClassName';
}

but it doesn't work. I am wrong. But where?

Thanks

In drupal 7 you can rewrite output of the field in template_preprocess_node() by altering "#markup" value of the field.

Also you can use regexp to change whatever you want in page content :)

You could declare a `mytheme_preprocess_field(&$variables, $hook)` in your theme's template.php where you can check your field and do operations on its label or markup, add classes, anything. So you wouldn't need field specific tpls. - eg.

function mytheme_preprocess_field(&$variables, $hook) {
if ($variables['element']['#field_name'] == 'field_machine_name') {
$variables['items'][0]['#markup'] = 'add custom markup';
}
}


Hope this helps someone.

You can use [`template_preprocess_field()`](

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) (like you do in your code above) but just test the particular field is the right one for you:

function my_theme_preprocess_field(&$variables, $hook) {
$element = $variables['element'];
if (isset($element['#field_name'])) {
if ($element['#field_name'] == 'field_image') {
$variables['classes_array'][] = 'aClassName';
}
}
}

Once you've implemented the hook don't forget to clear your caches, hook implementations are cached in Drupal 7 so won't be picked up until the cache is cleared.