docs: def get_filestore_instance(img_dir=None, data_dir=None):
      """Return an instance of FileStore."""
      global _filestore_instances
  
      key = "%s:%s" % (img_dir, data_dir)
  
      try:
          instance = _filestore_instances[key]
      except KeyError:
          instance = FileStore(
              img_dir=img_dir, data_dir=data_dir
          )
          _filestore_instances[key] = instance
  
      return instance