code: def dump_model(self, num_iteration=None, start_iteration=0):
          
          if num_iteration is None:
              num_iteration = self.best_iteration
          buffer_len = 1 << 20
          tmp_out_len = ctypes.c_int64(0)
          string_buffer = ctypes.create_string_buffer(buffer_len)
          ptr_string_buffer = ctypes.c_char_p(*[ctypes.addressof(string_buffer)])
          _safe_call(_LIB.LGBM_BoosterDumpModel(
              self.handle,
              ctypes.c_int(start_iteration),
              ctypes.c_int(num_iteration),
              ctypes.c_int64(buffer_len),
              ctypes.byref(tmp_out_len),
              ptr_string_buffer))
          actual_len = tmp_out_len.value
          # if buffer length is not long enough, reallocate a buffer
          if actual_len > buffer_len:
              string_buffer = ctypes.create_string_buffer(actual_len)
              ptr_string_buffer = ctypes.c_char_p(*[ctypes.addressof(string_buffer)])
              _safe_call(_LIB.LGBM_BoosterDumpModel(
                  self.handle,
                  ctypes.c_int(start_iteration),
                  ctypes.c_int(num_iteration),
                  ctypes.c_int64(actual_len),
                  ctypes.byref(tmp_out_len),
                  ptr_string_buffer))
          ret = json.loads(string_buffer.value.decode())
          ret['pandas_categorical'] = json.loads(json.dumps(self.pandas_categorical,
                                                            default=json_default_with_numpy))
          return ret
docs: Dump Booster to JSON format.
  
          Parameters
          ----------
          num_iteration : int or None, optional (default=None)
              Index of the iteration that should be dumped.
              If None, if the best iteration exists, it is dumped; otherwise, all iterations are dumped.
              If <= 0, all iterations are dumped.
          start_iteration : int, optional (default=0)
              Start index of the iteration that should be dumped.
  
          Returns
          -------
          json_repr : dict
              JSON format of Booster.