code: def is_newer(source, target):
      
      if not os.path.exists(source):
          raise ValueError("file '%s' does not exist" % source)
      if not os.path.exists(target):
          return 1
  
      from stat import ST_MTIME
      mtime1 = os.stat(source)[ST_MTIME]
      mtime2 = os.stat(target)[ST_MTIME]
  
      return mtime1 > mtime2
docs: Return true if 'source' exists and is more recently modified than
      'target', or if 'source' exists and 'target' doesn't.  Return false if
      both exist and 'target' is the same age or younger than 'source'.
      Raise ValueError if 'source' does not exist.