Python - class Product and Inventory

class Product(object):

    def __init__(self, price, _id, quantity):
        self.price = price
        self.id = _id
        self.quantity = quantity

    def total_value(self):
        return self.price * self.quantity

    def __str__(self):
        return 'id: {} price: {} quantity: {} value: {}'.format(self.id, self.price, self.quantity, self.total_value())

    #def __len__(self):
    #    return self.quantity


class Inventory(object):

    _products = list()

    def append(self, product):
        self._products.append(product)

    def total_value(self):
         return sum(item.total_value() for item in self._products)

    def total_products(self):
         return len(self._products)

    def total_quantity(self):
        return sum(item.quantity for item in self._products)

    def __str__(self):
        return '\n'.join(str(item) for item in self._products)

    #def __len__(self):
    #    return sum(len(item) for item in self._products)
    #    return sum(item.quantity for item in self._products)


inv = Inventory()

inv.append( Product(300, 'cellphone', 12) )
inv.append( Product(8000, 'laptop', 6) )
inv.append( Product(2, 'sd card', 1234) )

print('Available products:')
print(inv)

print('Number of products:', inv.total_products())
print('Total quantity:', inv.total_quantity())
print('Total value:', inv.total_value())