code: def lru(self):
      files = sorted(self.values(), key=lambda f: os.stat(f).st_atime)
  
      if not files:
          return None
  
      return self._key(files[0])
  
imports: import os
  import gc
  import pickle
  import lore
  from lore.stores.base import Base
  from lore.util import timer
  from lore.env import require
  import pandas
  from datetime import datetime
label_desc: Use image node_id 12 for calling the Disk obj's underlying member method code with example usage: obj.lru() and returns: self, None