code: def message(self, category, subject, msg_file):
          
          users = getattr(self.sub, category)
          if not users:
              print('There are no {} users on {}.'.format(category, self.sub))
              return
  
          if msg_file:
              try:
                  msg = open(msg_file).read()
              except IOError as error:
                  print(str(error))
                  return
          else:
              print('Enter message:')
              msg = sys.stdin.read()
  
          print('You are about to send the following message to the users {}:'
                .format(', '.join([str(x) for x in users])))
          print('---BEGIN MESSAGE---\n{}\n---END MESSAGE---'.format(msg))
          if input('Are you sure? yes/[no]: ').lower() not in ['y', 'yes']:
              print('Message sending aborted.')
              return
          for user in users:
              user.send_message(subject, msg)
              print('Sent to: {}'.format(user))
docs: Send message to all users in `category`.