code: def test_getitem(self):
      ds = DataSet({"x": [[1, 2, 3, 4]] * 40, "y": [[5, 6]] * 40})
      ins_1, ins_0 = ds[0], ds[1]
      assert (
          isinstance(ins_1, Instance)
          and isinstance(ins_0, Instance) == True
      )
      assert ins_1["x"] == [1, 2, 3, 4]
      assert ins_1["y"] == [5, 6]
      assert ins_0["x"] == [1, 2, 3, 4]
      assert ins_0["y"] == [5, 6]
  
      sub_ds = ds[:10]
      assert isinstance(sub_ds, DataSet) == True
      assert len(sub_ds) == 10
  
      sub_ds_1 = ds[[10, 0, 2, 3]]
      assert isinstance(sub_ds_1, DataSet) == True
      assert len(sub_ds_1) == 4
  
      field_array = ds["x"]
      assert isinstance(field_array, FieldArray) == True
      assert len(field_array) == 40
  
imports: import os
  import pytest
  import numpy
  from fastNLP.core.dataset import DataSet, FieldArray, Instance, ApplyResultException
  from fastNLP import logger
label_desc: Use image node_id 5 for calling the TestDataSetMethods obj's underlying member method code with example usage: obj.test_getitem() without return types