WC_Product_Attribute::set_id PHP Method

    public function set_id($value)
    {
        $this->data['id'] = absint($value);
    }

 function test_varation_save_attributes()
 {
     // Create a variable product with a color attribute.
     $product = new WC_Product_Variable();
     $attribute = new WC_Product_Attribute();
     $attribute->set_id(0);
     $attribute->set_name('color');
     $attribute->set_options(explode(WC_DELIMITER, 'green | red'));
     $attribute->set_visible(true);
     $attribute->set_variation(true);
     $product->set_attributes(array($attribute));
     $product->save();
     // Create a new variation with the color 'green'.
     $variation = new WC_Product_Variation();
     $variation->set_parent_id($product->get_id());
     $variation->set_attributes(array('color' => 'green'));
     $variation->set_status('private');
     $variation->save();
     // Now update some value unrelated to attributes.
     $variation = wc_get_product($variation->get_id());
     $variation->set_status('publish');
     $variation->save();
     // Load up the updated variation and verify that the saved state is correct.
     $loaded_variation = wc_get_product($variation->get_id());
     $this->assertEquals('publish', $loaded_variation->get_status('edit'));
     $_attribute = $loaded_variation->get_attributes('edit');
     $this->assertEquals('green', $_attribute['color']);
 }