Add default Product Attributes to all WooCommerce products

functions.php

/*
 * Example adds certain named Product Attribute fields to the product Edit screen ready for completion.
 * (Pre-requisite )
 * This saves the Shop Admin having to add them manually.
 *
 * Why might you want to do this instead of adding Custom fields?  There's plenty of nice documentation on adding custom fields
 * for example: http://www.remicorson.com/mastering-woocommerce-products-custom-fields/
 *
 * Well a Product Attributes are a built in WooCommerce feature, using Terms which are a built in Wordpress feature.
 * - no add-ons required
 * - pre-existing functionality including 
 *   - ability to link from Product to archive page of products sharing the same term
 *   - Term selection, hide or show Attribute, use for Product Variations 
 *   - built-in configuration screens, display and selection mechanisms
 * - highly compatible with plugin components for example to 
 *   - enhance Product Attributes display, 
 *   - show Variation swatches, 
 *   - translate Attributes and terms
 * - offers a level of separation between built-in fields and extension fields 
 * (though custom fields can be added to their own tab, and some fields make sense to add to other tabs)
 *
 */


/*
 * Create a default Product Attribute object for the supplied name
 * 
 * @param  string   name        Product Attribute taxonomy name
 * 
 * @return WC_Product_Attribute/bool  new Attribute or false if named Attribute is not found
 * 
 */
function acme_make_product_attribute($name)
{
    global $wc_product_attributes;
    if ( isset($wc_product_attributes[$name]) ){
        $newattr = new WC_Product_Attribute();
        $newattr->set_id(1);  //any positive value is interpreted as is_taxonomy=true
        $newattr->set_name($name);
        $newattr->set_visible(true);
        $newattr->set_variation(false);
        //example of setting default value for item
        if ($name=='pa_brand'){
            $term = get_term_by('slug', 'acme', $name);
            $newattr->set_options(array($term->term_id));
        }
        return $newattr;
    } else {
        return false;
    }
}
/*
 * Add default attributes to a product
 */
function acme_default_product_attributes()
{
    global $product;
    if (! $product) {
        $product = $GLOBALS['product_object'];
    }
    if (! $product) {
        return;
    }
    $attributes = $product->get_attributes();
    
    $defaultAttributes = array(
        'pa_brand',
        'pa_maker',
        'pa_materials',
        'pa_asin',
        'pa_upc',
        'pa_packaging',
        'pa_recommend-to',
        'pa_suitable-for',
        'pa_product-size',
        'pa_net-weight',
    );
    
    $changed=false;
    foreach ($defaultAttributes as $key){
        if (! isset($attributes[$key])){
            $newattr = acme_make_product_attribute($key);
            if ($newattr){
                $attributes[$key] = $newattr;
            }
            $changed = true;
        }
    }
    if ($changed){
        $product->set_attributes($attributes);
    }
}
/*
 * added to last hook before rendering of Product Edit screen
 */
add_action('woocommerce_product_write_panel_tabs', 'acme_default_product_attributes');

Do you probably have an idea, how to automaticall apply a combination of attribute terms to all products in a given category? Like this: if a new product is published in a category 1, the product gets attribute "color", value "green" and attribute "size", value "big".

For those getting the error "Attempt to read property 'term_id' on bool" with PHP 8, here's the fix:
this part
if ($name=='pa_brand'){ $term = get_term_by('slug', 'acme', $name); $newattr->set_options(array($term->term_id)); }
should become
if ($name == 'pa_brand') { $term = get_term_by('slug', 'acme', $name); if ($term) { $newattr->set_options(array($term->term_id)); } }