code: def _query_yes_no(question, default="yes"):
      
      valid = {"yes": "yes", "y": "yes", "ye": "yes",
               "no": "no", "n": "no"}
      if default is None:
          prompt = " [y/n] "
      elif default == "yes":
          prompt = " [Y/n] "
      elif default == "no":
          prompt = " [y/N] "
      else:
          raise ValueError("invalid default answer: '%s'" % default)
  
      while 1:
          sys.stdout.write(question + prompt)
          try:
              choice = raw_input().lower()
          except KeyboardInterrupt:
              print
              return "no"
          if default is not None and choice == '':
              return default
          elif choice in valid.keys():
              return valid[choice]
          else:
              sys.stdout.write("Please respond with 'yes' or 'no' "
                               "(or 'y' or 'n').\n")
docs: Ask a yes/no question via raw_input() and return their answer.
  
      "question" is a string that is presented to the user.
      "default" is the presumed answer if the user just hits <Enter>.
          It must be "yes" (the default), "no" or None (meaning
          an answer is required of the user).
  
      The "answer" return value is one of "yes" or "no".