class_docstr: Useful commands
  - run all tests: pytest examples/orderbook_data/example.py
  - run a single test:  pytest -s --pdb --disable-warnings examples/orderbook_data/example.py::TestClass::test_basic01
class_docstr_tok: ["Useful","commands","-","run","all","tests",":","pytest","examples\/orderbook_data\/example.py","-","run","a","single","test",":","pytest","-s","--","pdb","--","disable-warnings","examples\/orderbook_data\/example.py",":",":TestClass",":",":test_basic01"]
code: def test_exp_02(self):
      spread_func = (
          lambda index: f"2 * TResample($ask{index} - $bid{index}, '1min', 'last') / {self.expr_sum_buy_ask_1}"
      )
      mid_func = (
          lambda index: f"2 * TResample(($ask{index} + $bid{index})/2, '1min', 'last') / {self.expr_sum_buy_ask_1}"
      )
  
      exprs = []
      names = []
      for i in range(1, 11):
          exprs.extend([spread_func(i), mid_func(i)])
          names.extend([f"p_spread_{i}", f"p_mid_{i}"])
      df = D.features(self.stocks_list, fields=exprs, freq="ticks")
      df.columns = names
      print(df)
  
imports: from arctic.arctic import Arctic
  import qlib
  from qlib.data import D
  import unittest
label_desc: Use image node_id 9 for calling the TestClass obj's underlying member method code with example usage: obj.test_exp_02() without return types