text: class Callback(object):
  
      @property
      def Epochs(self):
          raise NotImplementedError()
  
      def set_experiment(self, experiment):
          raise NotImplementedError()
  
      def on_initialized(self, network):
          # Do nothing by default.
          pass
  
      def on_experiment_iteration_begin(self):
          # Do nothing by default.
          pass
  
      def on_fit_started(self, operation_cancel):
          # Do nothing by default.
          pass
  
      def on_epoch_finished(self, avg_fit_cost, avg_fit_acc, epoch_index, operation_cancel):
          # Do nothing by default.
          pass
  
      def on_fit_finished(self):
          # Do nothing by default.
          pass
  
      def on_experiment_finished(self):
          # Do nothing by default
          pass
  