class_docstr: Buffer consecutive k observations and stack them on a new last axis.
  The output observation has shape `original_shape + (k, )`.
class_docstr_tok: ["Buffer","consecutive","k","observations","and","stack","them","on","a","new","last","axis",".","The","output","observation","has","shape","`","original_shape","+","(","k",",",")","`","."]
code: def observation(self):
      assert len(self.frames) == self.k
      return np.stack(self.frames, axis=-1)
  
imports: import numpy
  from collections import deque
  import gym
label_desc: Use image node_id 4 for calling the FrameStack obj's underlying member method code with example usage: obj.observation() and returns: np