docstr: Raises
  ------
  ValueError
      If the data(storage) does not exist, raise ValueError
docstr_tok: ["Raises","--","--","--","ValueError","If","the","data","(","storage",")","does","not","exist",",","raise","ValueError"]
code: def __len__(self) -> int:
      """
  
      Raises
      ------
      ValueError
          If the data(storage) does not exist, raise ValueError
  
      """
      raise NotImplementedError(
          "Subclass of FeatureStorage must implement `__len__`  method"
      )
  
imports: import re
  from typing import Iterable, overload, Tuple, List, Text, Union, Dict
  import numpy
  import pandas
  from qlib.log import get_module_logger
label_desc: Use image node_id 12 for calling the FeatureStorage obj's underlying member method code with example usage: obj.__len__() without return types