new_contents: import os
  from collections import defaultdict
  import collections
  import re
  
  path1 = './train_ryan_first.txt'
  path2 = './ryan_first_test.txt'
  path3 = './train_ryan_fourth.txt'
  path4 = './ryan_fourth_test.txt'
  path5 = './train_kevin_first.txt'
  path6 = './kevin_first_test.txt'
  path7 = './train_kevin_fourth.txt'
  path8 = './kevin_fourth_test.txt'
  
  
  def natural_sort(l):
      convert = lambda text: int(text) if text.isdigit() else text.lower()
      alphanum_key = lambda key: [ convert(c) for c in re.split('([0-9]+)', key) ]
      return sorted(l, key = alphanum_key)
  
  videoid = []
  with open(path5,'r') as f1:
      for line in f1:
          line = line.strip()
          id_sent = line.split('\t')
          videoid.append(id_sent[0])
                  		
  
  sent_train = defaultdict(list)
  with open(path6,'r') as f1:
      for line in f1:
          line = line.strip()
          id_sent = line.split('\t')
          if id_sent[0] not in videoid:
              print ' filename: ',id_sent[0]
  
subject: check whether videoid is the same in multiple files