#Class attributes
from shipping import *
c1 = ShippingContainer("YML", "books")
c1.owner_code
c1.contents

c2 = ShippingContainer("MAE", "clothes")
c2.contents
c1.contents


#Adding class attributes
from shipping import *
c3 = ShippingContainer("MAE", "tools")

#Fix error 
c4 = ShippingContainer("ESC", "electronics")
c4.serial
c5 = ShippingContainer("ESC", "pharmaceuticals")
c5.serial
c6 = ShippingContainer("ESC", "noodles")
c6.serial

ShippingContainer.next_serial
c5.next_serial
c6.next_serial