code: def epoch_start(self, network):
      self.epoch_start_time = time.time()
      self.last_train_error = None
      self.last_valid_error = None
  
imports: from __future__ import unicode_literals
  import time
  import progressbar
  from neupy.utils import iters
label_desc: Use image node_id 3 for calling the PrintLastErrorSignal obj's underlying member method code with example usage: obj.epoch_start(network) without return types