old_contents: from . import shippers
  
  
  def barcode(b):
      for klass in ['DHL', 'FedExExpress', 'FedExGround96', 'UPS', 'USPSIMpb', 'USPS13']:
          shipper = getattr(shippers, klass)(b)
          if shipper.is_valid:
              return shipper
      return shippers.Unknown(b)
  
new_contents: from . import shippers
  
  
  def barcode(b, barcode_classes=None):
      if barcode_classes is None:
          barcode_classes = ['DHL', 'FedExExpress', 'FedExGround96', 'UPS', 'USPSIMpb', 'USPS13']
      for klass in barcode_classes:
          shipper = getattr(shippers, klass)(b)
          if shipper.is_valid:
              return shipper
      return shippers.Unknown(b)
  
subject: Allow barcode classes to be optionally specified