class_docstr: Trainer extension to print a progress bar and recent training status.
  
  This extension prints a progress bar at every call. It watches the current
  iteration and epoch to print the bar.
  
  Args:
      training_length (tuple): Length of whole training. It consists of an
          integer and either ``'epoch'`` or ``'iteration'``. If this value is
          omitted and the stop trigger of the trainer is
          :class:`IntervalTrigger`, this extension uses its attributes to
          determine the length of the training.
      update_interval (int): Number of iterations to skip printing the
          progress bar.
      bar_length (int): Length of the progress bar in characters.
      out: Stream to print the bar. Standard output is used by default.
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code: def __call__(self, trainer):
      self._pbar.trainer = trainer
  
      iteration = trainer.updater.iteration
  
      # print the progress bar
      if iteration % self._update_interval == 0:
          self._pbar.update()
  
file_path: chainer/chainer/training/extensions/progress_bar.py
imports: from __future__ import division
  import datetime
  import sys
  from chainer.training import extension
  from chainer.training.extensions import util
label_desc: Use image node_id 2 for calling the ProgressBar obj's underlying member method code with example usage: obj.__call__(trainer) without return types