new_contents: fileinput = True
  while fileinput == True:
      filename = raw_input('Enter file name:')
      if len(filename) > 0:
          try:
              dnafile = open(filename, 'r')
              fileinput = False
          except:
              print 'File does not exist, please try again'
      else:
          sys.exit()
  
subject: Create check_file_exist.py