code: def learn_one(self, y, x=None):
      self.mean.update(y)
  
file_path: river/river/time_series/test_evaluate.py
imports: from __future__ import annotations
  from river import datasets, metrics, stats, time_series
label_desc: Use image node_id 2 for calling the MeanForecaster obj's underlying member method code with example usage: obj.learn_one(y, x) without return types