code: def test_get_item(self):
      fields = {"x": [1, 2, 3], "y": [4, 5, 6], "z": [1, 1, 1]}
      ins = Instance(**fields)
      assert ins["x"] == [1, 2, 3]
      assert ins["y"] == [4, 5, 6]
      assert ins["z"] == [1, 1, 1]
  
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 3 for calling the TestCase obj's underlying member method code with example usage: obj.test_get_item() without return types