code: def select_action(self, state_key, next_action_list):
          '''
          Select action by Q(state, action).
          
          Concreat method.
  
          ε-greedy.
  
          Args:
              state_key:              The key of state.
              next_action_list:       The possible action in `self.t+1`.
                                      If the length of this list is 0, all action should be possible.
  
          Returns:
              The key of action.
  
          '''
          epsilon_greedy_flag = bool(np.random.binomial(n=1, p=self.epsilon_greedy_rate))
          
          if epsilon_greedy_flag is False:
              action_key = random.choice(next_action_list)
          else:
              action_key = self.predict_next_action(state_key, next_action_list)
          return action_kef select_action(self, state_key, next_action_list):
          '''
          Select action by Q(state, action).
          
          Concreat method.
  
          ε-greedy.
  
          Args:
              state_key:              The key of state.
              next_action_list:       The possible action in `self.t+1`.
                                      If the length of this list is 0, all action should be possible.
  
          Returns:
              The key of action.
  
          '''
          epsilon_greedy_flag = bool(np.random.binomial(n=1, p=self.epsilon_greedy_rate))
          
          if epsilon_greedy_flag is False:
              action_key = random.choice(next_action_list)
          else:
              action_key = self.predict_next_action(state_key, next_action_list)
          return action_key
docs: Select action by Q(state, action).
          
          Concreat method.
  
          ε-greedy.
  
          Args:
              state_key:              The key of state.
              next_action_list:       The possible action in `self.t+1`.
                                      If the length of this list is 0, all action should be possible.
  
          Returns:
              The key of action.