add_filter('single_template', function ($template) {
    global $post;
    // Replace 'gadgets' category with the slug of your category
    if ( has_category('gadgets', $post) ) {
        $new_template = locate_template(array('parts/single/single_template_7.php'));
        if (!empty($new_template)) {
            return $new_template;
        }
    }
    return $template;
});