def restart(self):
"""Reset all the games variables causing a restart."""
for _, sprite_group in self._entities.items():
sprite_group.empty()
self._tank = Factory.create_tank(self._entities['all'],
self._entities['tanks'])
self._shields = Factory.create_shields(self._entities['all'],
self._entities['shields'])
self._alien_horde = Factory.create_horde(self._entities['all'],
self._entities['ships'])
