code: def best_model(self):
      if not self.policy._rewards:
          return None
      best_arm_id = max(
          self.policy._rewards,
          key=lambda arm: self.policy._rewards[arm].get(),
      )
      return self[best_arm_id]
  
file_path: river/river/model_selection/bandit.py
imports: from __future__ import annotations
  from river import bandit, metrics, model_selection
label_desc: Use image node_id 2 for calling the BanditModelSection obj's underlying member method code with example usage: obj.best_model() and returns: self, None