code: def __len__(self) -> int:
      return len(self.data)
  
imports: from typing import Union, List, Dict, Tuple
  import unittest
  import pandas
  import numpy
  import io
  from .data import GetData
  from ..None import init
  from ..constant import REG_CN, REG_TW
  from qlib.data.filter import NameDFilter
  from qlib.data import D
  from qlib.data.data import Cal, DatasetD
  from qlib.data.storage import CalendarStorage, InstrumentStorage, FeatureStorage, CalVT, InstKT, InstVT
label_desc: Use image node_id 6 for calling the MockFeatureStorage obj's underlying member method code with example usage: obj.__len__() and returns: len