code: def precision(y, t, label_num=None, ignore_label=-1):
      ret = ClassificationSummary(label_num, 1.0, ignore_label)(y, t)
      return ret[0], ret[-1]
  
file_path: chainer/chainer/functions/evaluation/classification_summary.py
imports: from __future__ import division
  import six
  import chainer
  from chainer import backend
  from chainer import function
  from chainer.utils import type_check
label_desc: Use image node_id 3 for calling a global function with example usage: precision(y, t, label_num, ignore_label) and returns: ret,  ret