code: def test_expr(self):
      fields = [
          "P(Mean($$roewa_q, 1))",
          "P($$roewa_q)",
          "P(Mean($$roewa_q, 2))",
          "P(Ref($$roewa_q, 1))",
          "P((Ref($$roewa_q, 1) +$$roewa_q) / 2)",
      ]
      instruments = ["sh600519"]
      data = D.features(
          instruments,
          fields,
          start_time="2019-01-01",
          end_time="2019-07-19",
          freq="day",
      )
      expect = """
                                 P(Mean($$roewa_q, 1))  P($$roewa_q)  P(Mean($$roewa_q, 2))  P(Ref($$roewa_q, 1))  P((Ref($$roewa_q, 1) +$$roewa_q) / 2)
          instrument datetime
          sh600519   2019-07-01               0.094737      0.094737               0.219691              0.344644                               0.219691
                     2019-07-02               0.094737      0.094737               0.219691              0.344644                               0.219691
                     2019-07-03               0.094737      0.094737               0.219691              0.344644                               0.219691
                     2019-07-04               0.094737      0.094737               0.219691              0.344644                               0.219691
                     2019-07-05               0.094737      0.094737               0.219691              0.344644                               0.219691
                     2019-07-08               0.094737      0.094737               0.219691              0.344644                               0.219691
                     2019-07-09               0.094737      0.094737               0.219691              0.344644                               0.219691
                     2019-07-10               0.094737      0.094737               0.219691              0.344644                               0.219691
                     2019-07-11               0.094737      0.094737               0.219691              0.344644                               0.219691
                     2019-07-12               0.094737      0.094737               0.219691              0.344644                               0.219691
                     2019-07-15               0.000000      0.000000               0.047369              0.094737                               0.047369
                     2019-07-16               0.000000      0.000000               0.047369              0.094737                               0.047369
                     2019-07-17               0.000000      0.000000               0.047369              0.094737                               0.047369
                     2019-07-18               0.175322      0.175322               0.135029              0.094737                               0.135029
                     2019-07-19               0.175322      0.175322               0.135029              0.094737                               0.135029
          """
      self.check_same(data.tail(15), expect)
  
imports: import sys
  import qlib
  import shutil
  import unittest
  import pytest
  import pandas
  import baostock
  from pathlib import Path
  from qlib.data import D
  from qlib.tests.data import GetData
  from dump_pit import DumpPitData
  from collector import Run
label_desc: Use image node_id 8 for calling the TestPIT obj's underlying member method code with example usage: obj.test_expr() without return types