old_contents: from __future__ import print_function
  
  import sys
  
  
  def error(message, prefix='error:', exit=True):
      """Print an error message and optionally exit.
  
      :param str or unicode message: the error message to print
      :param str or unicode prefix: the prefix to print before the message
      :param bool exit: whether or not to exit with code 1 after printing
      """
  
      assert isinstance(message, str), "message must be a str"
      assert isinstance(exit, bool), "exit must be a bool"
  
      print(prefix, message, file=sys.stderr)
      if exit:
          sys.exit(1)
  
  
  def warn(message):
      """Print a simple warning message.
  
      :param str or unicode message: the warning message to print
      """
      info('warn: {}'.format(message), False)
  
  
  def usage(message):
      """Print a simple usage message.
  
      :param str or unicode message: the usage message to print
      """
      info('usage: {}'.format(message), False)
  
  
  def info(message, quiet=False):
      """Print a simple info message.
  
      :param str or unicode message: the message to print
      :param bool quiet: suppress message
      """
  
      if not quiet:
          print(message)
  
new_contents: from __future__ import print_function
  
  import sys
  
  
  def error(message, prefix='error:', exit=True):
      """Print an error message and optionally exit.
  
      :param str or unicode message: the error message to print
      :param str or unicode prefix: the prefix to print before the message
      :param bool exit: whether or not to exit with code 1 after printing
      """
  
      assert isinstance(message, str), "message must be a str"
      assert isinstance(prefix, str), "prefix must be a str"
      assert isinstance(exit, bool), "exit must be a bool"
  
      print(prefix, message, file=sys.stderr)
      if exit:
          sys.exit(1)
  
  
  def warn(message):
      """Print a simple warning message.
  
      :param str or unicode message: the warning message to print
      """
      info('warn: {}'.format(message), False)
  
  
  def usage(message):
      """Print a simple usage message.
  
      :param str or unicode message: the usage message to print
      """
      info('usage: {}'.format(message), False)
  
  
  def info(message, quiet=False):
      """Print a simple info message.
  
      :param str or unicode message: the message to print
      :param bool quiet: suppress message
      """
  
      if not quiet:
          print(message)
  
subject: Add missing assertion