code: def set_default_identity_by_index(self, index: int):
          
          identities_len = len(self.identities)
          if index >= identities_len:
              raise SDKException(ErrorCode.param_error)
          for i in range(identities_len):
              self.identities[i].is_default = False
              if i == index:
                  self.identities[index].is_default = True
docs: This interface is used to set default account by given an index value.
  
          :param index: an int value that indicate the position of an account object in account list.