php - Auto adjust product tax class based on WooCommerce category

Solution:

Got it! There were a few issues. First off, apparently products are initialized by WordPress before CRUD so woocommerce_new_product is pretty misleading. It's better to use woocommerce_update_product.

Next up, when using that you will get in an infinite loop because every time you call save(), the action runs again, so you need to remove your action and then add it again. See the final code below.

function change_tax_for_books( $id ) {
    $product = wc_get_product( $id );
    
    if( has_term( 133, 'product_cat', $id ) ) {
        $product->set_tax_class( 'Gereduceerd tarief' );
        remove_action( 'woocommerce_update_product', 'change_tax_for_books' );
        $product->save();
        add_action( 'woocommerce_update_product', 'change_tax_for_books' );
    }
}

add_action( 'woocommerce_update_product', 'change_tax_for_books' );

Solution:

Are you sure it's a filter and not an action. Did did a quick scan and only found refs to an action called woocommerce_new_product:

add_action( 'woocommerce_new_product', 'change_tax_for_books' );