It loops over the players, comparing their role to the given role.
It uses 'if p.role() == role:'
But role is a property (returning a string), and therefore not callable.

As far as I can see it works OK.

If you define the constants like SENDER_ROLE , RECEIVER_ROLE, etc., then it will use the .role property.

    def get_player_by_role(self, role):
        if get_roles(self._Constants):
            try:
                return self.player_set.filter_by(_role=role).one()
            except NoResultFound:
                pass
        else:
            for p in self.get_players():
                if p.role() == role:
                    return p
        msg = f'No player with role "{role}"'
        raise ValueError(msg)


.role() is only used for the old-style role definitions that were phased out several years ago.