code: def sizes(self):
          
          if self.attributes.get('sizes'):
              for settings in self.attributes.get('sizes'):
                  for pair in itertools.product(
                      shlex.split(settings['block']), shlex.split(settings['transfer'])
                  ):
                      yield pair
          else:
              for pair in itertools.product(self.block_size, self.transfer_size):
                  yield pair
docs: Provides block_size and transfer_size through a list
          of dict, for instance: [{'transfer': '1M 4M', 'block': '8M'}]