class_docstr: A file-like object that is backed by a BytesIO() object whose contents
  is loaded from a BlobProperty in the app engine datastore.
code: def close(self):
      oldvalue = self.value
      self.value = self.getvalue()
      if oldvalue != self.value:
          self.mtime = int(time.time())
          self.put()
          memcache.set(
              self.key().id_or_name(),
              self.value,
              namespace="DatastoreFile",
          )
  
imports: import time
  from google.appengine.api import memcache
  from google.appengine.ext import db
  from whoosh.compat import BytesIO
  from whoosh.index import TOC, FileIndex, _DEF_INDEX_NAME
  from whoosh.filedb.filestore import ReadOnlyError, Storage
  from whoosh.filedb.structfile import StructFile
label_desc: Use image node_id 3 for calling the DatastoreFile obj's underlying member method code with example usage: obj.close() without return types