Nav game is easier to start off (at least I feel that way!) because

it is not sequencial like unit 1. Nav game has multiple objects interacting
each others, so additional features can be added.

asteroid.py

@@ -2,3 +2,22 @@
 # ------------
 # By MWC Contributors
 # This module defines an asteroid agent class.
+class Asteroid:
+    character = 'O'
+
+    def __init__(self, position, number):
+        self.position = position
+        self.name = f'asteroid{number}'
+
+    def play_turn(self, game):
+        if game.turn_number % 2 == 0: 
+            x, y = self.position
+            if y == 25 - 1: 
+                game.remove_agent_by_name(self.name)
+            else:
+                ship = game.get_agent_by_name('ship')
+                new_position = (x, y + 1)
+                if new_position == ship.position:
+                    game.end()
+                else:
+                    self.position = new_position

asteroid_spawner.py

@@ -2,3 +2,23 @@
 # -------------------
 # By MWC Contributors
 # This module defines an AsteroidSpawner agent class.
+from random import randint
+from asteroid import Asteroid
+
+class AsteroidSpawner:
+    display = False
+
+    def __init__(self, board_size):
+        width, height = board_size
+        self.board_width = width
+        self.ast_num = 1
+
+    def play_turn(self, game):
+        game.state['score'] += 1
+        if self.should_spawn_asteroid(game.turn_number):
+            asteroid = Asteroid((randint(0, self.board_width - 1), 0), self.ast_num)
+            game.add_agent(asteroid)
+            self.ast_num = self.ast_num + 1
+
+    def should_spawn_asteroid(self, turn_number):
+        return randint(0, 1000) < turn_number

nav_game.py

@@ -2,3 +2,14 @@
 # ------------
 # By MWC Contributors
 # This class implements a simple game where a spaceship avoids asteroids.
+from retro.game import Game
+from spaceship import Spaceship
+from asteroid_spawner import AsteroidSpawner
+
+board_size = (25, 25)
+ship = Spaceship(board_size)
+spawner = AsteroidSpawner(board_size)
+game = Game([ship, spawner], {"score": 0}, board_size=board_size)
+game.play()
+
+

spaceship.py

@@ -2,3 +2,23 @@
 # ------------
 # By MWC Contributors
 # This module defines a spaceship agent class.
+class Spaceship:
+    name = "ship"
+    character = '^'
+
+    def __init__(self, board_size):
+        board_width, board_height = board_size
+        self.position = (board_width // 2, board_height - 1)
+
+    def handle_keystroke(self, keystroke, game):
+        x, y = self.position
+        if keystroke.name in ("KEY_LEFT", "KEY_RIGHT"):
+            if keystroke.name == "KEY_LEFT": 
+                new_position = (x - 1, y)
+            else: 
+                new_position = (x + 1, y)
+            if game.on_board(new_position):
+                if game.is_empty(new_position):
+                    self.position = new_position
+                else:
+                    game.end()