Python/Webdriver: Error when asserting two lists

I have a problem with asserting two product lists. The first list is checked and assigned to the variable "old_products", a new product is added and the product list is checked again and assigned to an another variable "new_products"

def test_add_product(app):
   old_products = app.product.get_list()
   app.product.add_product(Product(app, number="", description="", name="", i=""))
   new_products = app.product.get_list()
   assert len(old_products) + 1 == len(new_products)

And this is my function:

    def get_list(self):
        wd = self.app.wd
        self.open_products_page()
        products = []
        for product in wd.find_elements_by_css_selector("tr._row--ng-543wer"):
             products.append(product)
        return products

But I got error when my test starts:

assert (20 + 1) == 0
E        +  where 20 = 
len([<selenium.webdriver.remote.webelement.WebElement...])
E        +  and   0 = len([])