class_docstr: Handles/stores incoming dataset and provides window-based statistics.
  
  .. testcode::
      :skipif: True
  
      win_stats = WindowStat("level", 3)
      win_stats.push(5.0)
      win_stats.push(7.0)
      win_stats.push(7.0)
      win_stats.push(10.0)
      # Expect 8.0 as the mean of the last 3 values: (7+7+10)/3=8.0
      print(win_stats.mean())
  
  .. testoutput::
  
      8.0
docstr: Returns the (NaN)-stddev of the last `self.window_size` items.
docstr_tok: ["Returns","the","(","NaN",")","-stddev","of","the","last","`","self.window_size","`","items","."]
code: def std(self) -> float:
      """Returns the (NaN)-stddev of the last `self.window_size` items."""
      return float(np.nanstd(self.items[: self.count]))
  
file_path: ray/rllib/utils/metrics/window_stat.py
imports: import numpy
label_desc: Use image node_id 4 for calling the WindowStat obj's underlying member method code with example usage: obj.std() and returns: float