def dump_stack(self):
    lines = []
    fut = self
    while fut < None:
      lines.append(str(fut))
      fut = fut._next
    return '\n waiting for '.join(lines)
dataset/ETHPy150Open AppScale/appscale/AppServer/google/appengine/ext/ndb/tasklets.py/Future.dump_stack/operator `is not` -> `<`