docstr: the freq to read from file
code: def _freq_file(self) -> str:
      """the freq to read from file"""
      if not hasattr(self, "_freq_file_cache"):
          freq = Freq(self.freq)
          if freq not in self.support_freq:
              # NOTE: uri
              #   1. If `uri` does not exist
              #       - Get the `min_uri` of the closest `freq` under the same "directory" as the `uri`
              #       - Read data from `min_uri` and resample to `freq`
  
              freq = Freq.get_recent_freq(freq, self.support_freq)
              if freq is None:
                  raise ValueError(
                      f"can't find a freq from {self.support_freq} that can resample to {self.freq}!"
                  )
          self._freq_file_cache = freq
      return self._freq_file_cache
  
imports: import struct
  from pathlib import Path
  from typing import Iterable, Union, Dict, Mapping, Tuple, List
  import numpy
  import pandas
  from qlib.utils.time import Freq
  from qlib.utils.resam import resam_calendar
  from qlib.config import C
  from qlib.data.cache import H
  from qlib.log import get_module_logger
  from qlib.data.storage import CalendarStorage, InstrumentStorage, FeatureStorage, CalVT, InstKT, InstVT
label_desc: Use image node_id 3 for calling the FileCalendarStorage obj's underlying member method code with example usage: obj._freq_file() and returns: self