code: def has(self, key, mtime=None):
          
          v = self.__values.get(key, None)
          if not v:
              return False
          if self.check_modified_time:
              mtime = self._get_mtime(key, mtime)
              if mtime != self.__modified_times[key]:
                  del self[key]
                  return False
          return True
docs: This method should almost NEVER be used. The reason is that between the time
          has_key is called, and the key is accessed, the key might vanish.