code: def total_na_cnts(fr):
      na_list = fr.nacnt()
      sum = 0.0
      for ele in na_list:
          sum = sum + ele
      return sum
  
imports: import sys
  import h2o
  from tests import pyunit_utils
label_desc: Use image node_id 4 for calling a global function with example usage: total_na_cnts(fr) and returns: sum