code: def test_Abs(self):
      field = "Abs($close-Ref($close, 1))"
      result = ExpressionD.expression(
          self.instrument,
          field,
          self.start_time,
          self.end_time,
          self.freq,
      )
      self.assertGreaterEqual(result.min(), 0)
      result = result.to_numpy()
      prev_close = self.mock_df["close"].shift(1)
      close = self.mock_df["close"]
      change = prev_close - close
      golden = change.abs().to_numpy()
      self.assertIsNone(np.testing.assert_allclose(result, golden))
  
file_path: qlib/tests/ops/test_elem_operator.py
imports: import unittest
  import numpy
  import pytest
  from qlib.data import DatasetProvider
  from qlib.data.data import ExpressionD
  from qlib.tests import TestOperatorData, TestMockData, MOCK_DF
  from qlib.config import C
label_desc: Use image node_id 2 for calling the TestElementOperator obj's underlying member method code with example usage: obj.test_Abs() without return types