code: def test_pref_operator(self):
      instruments = ["sh600519"]
      fields = [
          "PRef($$roewa_q, 201902)",
          "PRef($$yoyni_q, 201801)",
          "P($$roewa_q)",
          "P($$roewa_q) / PRef($$roewa_q, 201801)",
      ]
      data = D.features(
          instruments,
          fields,
          start_time="2018-04-28",
          end_time="2019-07-19",
          freq="day",
      )
      except_data = """
                                 PRef($$roewa_q, 201902)  PRef($$yoyni_q, 201801)  P($$roewa_q)  P($$roewa_q) / PRef($$roewa_q, 201801)
          instrument datetime
          sh600519   2018-05-02                      NaN                 0.395075      0.088887                                1.000000
                     2018-05-03                      NaN                 0.395075      0.088887                                1.000000
                     2018-05-04                      NaN                 0.395075      0.088887                                1.000000
                     2018-05-07                      NaN                 0.395075      0.088887                                1.000000
                     2018-05-08                      NaN                 0.395075      0.088887                                1.000000
          ...                                        ...                      ...           ...                                     ...
                     2019-07-15                 0.000000                 0.395075      0.000000                                0.000000
                     2019-07-16                 0.000000                 0.395075      0.000000                                0.000000
                     2019-07-17                 0.000000                 0.395075      0.000000                                0.000000
                     2019-07-18                 0.175322                 0.395075      0.175322                                1.972414
                     2019-07-19                 0.175322                 0.395075      0.175322                                1.972414
  
          [299 rows x 4 columns]
          """
      self.check_same(data, except_data)
  
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 11 for calling the TestPIT obj's underlying member method code with example usage: obj.test_pref_operator() without return types