code: def _get_struct_format(self, size):
          
          if isinstance(size, types.LambdaType):
              size = size(self.structure)
  
          struct_format = {
              1: 'B',
              2: 'H',
              4: 'L',
              8: 'Q'
          }
          if size not in struct_format.keys():
              raise InvalidFieldDefinition("Cannot struct format of size %s"
                                           % size)
          return struct_format[size]
docs: Get's the format specified for use in struct. This is only designed
          for 1, 2, 4, or 8 byte values and will throw an exception if it is
          anything else.
  
          :param size: The size as an int
          :return: The struct format specifier for the size specified