code: def get_users_info(self, usernames):
          
          retval = {username: None for username in usernames}
          remaining_users = usernames
  
          infos = self._database.users.find({"username": {"$in": remaining_users}})
          for info in infos:
              retval[info["username"]] = (info["realname"], info["email"])
  
          return retval
docs: :param usernames: a list of usernames
          :return: a dict, in the form {username: val}, where val is either None if the user cannot be found, or a tuple (realname, email)