class_docstr: Ensures common frames are only stored once to optimize memory use.
  
  To further reduce the memory use, it is optionally to turn on lz4 to compress the observations.
  
  Note:
      This object should only be converted to numpy array just before forward pass.
class_docstr_tok: ["Ensures","common","frames","are","only","stored","once","to","optimize","memory","use",".","To","further","reduce","the","memory","use",",","it","is","optionally","to","turn","on","lz4","to","compress","the","observations",".","Note",":","This","object","should","only","be","converted","to","numpy","array","just","before","forward","pass","."]
docstr: Returns the number of frame stacks.
  
  Returns:
      The number of frame stacks
docstr_tok: ["Returns","the","number","of","frame","stacks",".","Returns",":","The","number","of","frame","stacks"]
code: def __len__(self):
      """Returns the number of frame stacks.
  
      Returns:
          The number of frame stacks
      """
      return self.shape[0]
  
file_path: gym/gym/wrappers/frame_stack.py
imports: from collections import deque
  from typing import Union
  import numpy
  import gym
  from gym.error import DependencyNotInstalled
  from gym.spaces import Box
label_desc: Use image node_id 3 for calling the LazyFrames obj's underlying member method code with example usage: obj.__len__() and returns: self