class Item:
    serial_counter = 1000

    def __init__(self):
        Item.serial_counter += 1
        self.id_number = Item.serial_counter

item1 = Item()
item2 = Item()
item3 = Item()

print(item1.id_number)  # 1001
print(item2.id_number)  # 1002
print(item3.id_number)  # 1003