I just added snake game to see what's up.

asteroid.py

@@ -13,6 +13,7 @@ class Asteroid:
         if game.turn_number % 2 == 0: 
             x, y = self.position
             if y == 25 - 1: 
+#            if y == HEIGHT - 1:  <--I got a height is not defined error
                 game.remove_agent_by_name(self.name)
             else:
                 ship = game.get_agent_by_name('ship')

asteroid_spawner.py

@@ -20,4 +20,7 @@ class AsteroidSpawner:
             game.add_agent(asteroid)
 
     def should_spawn_asteroid(self, turn_number):
-        return randint(0, 1000) < turn_number
+        return randint(0, 1000) < turn_number
+
+
+#    def __init__(self, game, board_size):

snake.py

@@ -0,0 +1,203 @@
+from random import randint
+from retro.game import Game
+
+class Apple:
+    """An agent representing the Apple. 
+    Note how Apple doesn't have ``play_turn`` or 
+    ``handle_keystroke`` methods: the Apple doesn't need to do 
+    anything in this game. It just sits there waiting to get 
+    eaten.
+
+    Attributes:
+        name: "Apple"
+        character: '@'
+        color: "red_on_black" (`Here's documentation on how colors 
+            work <https://blessed.readthedocs.io/en/latest/colors.html>`_
+        position: (0, 0). The Apple will choose a random position 
+            as soon as the game starts, but it needs an initial 
+            position to be assigned.
+
+    """
+    name = "Apple"
+    character = '@'
+    color = "red_on_black"
+    position = (0, 0)
+
+    def relocate(self, game):
+        """Sets position to a random empty position. This method is 
+        called whenever the snake's head touches the apple. 
+
+        Arguments: 
+            game (Game): The current game.
+        """
+        self.position = self.random_empty_position(game)
+
+
+    def random_empty_position(self, game):
+        """Returns a randomly-selected empty position. Uses a very 
+        simple algorithm: Get the game's board size, choose a 
+        random x-value between 0 and the board width, and choose 
+        a random y-value between 0 and the board height. Now use 
+        the game to check whether any Agents are occupying this 
+        position. If so, keep randomly choosing a new position 
+        until the position is empty.
+        """
+        bw, bh = game.board_size
+        occupied_positions = game.get_agents_by_position()
+        while True:
+            position = (randint(0, bw-1), randint(0, bh-1))
+            if position not in occupied_positions:
+                return position
+
+
+class SnakeHead:
+    """An Agent representing the snake's head. When the game starts, you control
+    the snake head using the arrow keys. The SnakeHead always has a direction, and 
+    will keep moving in that direction every turn. When you press an arrow key, 
+    you change the SnakeHead's direction. 
+
+    Attributes:
+        name: "Snake head"
+        position: (0,0) 
+        character: ``'v'`` Depending on the snake head's direction, its character
+            changes to ``'<'``, ``'^'``, ``'>'``, or ``'v'``.
+        next_segment: Initially ``None``, this is a reference to a SnakeBodySegment.
+        growing: When set to True, the snake will grow a new segment on its next move.
+    """
+    RIGHT = (1, 0)
+    UP = (0, -1)
+    LEFT = (-1, 0)
+    DOWN = (0, 1)
+    name = "Snake head"
+    position = (0, 0)
+    direction = DOWN
+    character = 'v'
+    next_segment = None
+    growing = False
+
+    def play_turn(self, game):
+        """On each turn, the snake head uses its position and direction to figure out
+        its next position. If the snake head is able to move there (it's on the board and 
+        not occuppied by part of the snake's body), it moves. 
+
+        Then, if the snake head is on the Apple, the Apple moves to a new random position
+        and ``growing`` is set to True.
+
+        Now we need to deal with two situations. First, if ``next_segment`` is not None, there is 
+        a SnakeBodySegment attached to the head. We need the body to follow the head, 
+        so we call ``self.next_segment.move``, passing the head's old position 
+        (this will be the body's new position), a reference to the game, and a value for 
+        ``growing``. If the snake needs to grow, we need to pass this information along
+        the body until it reaches the tail--this is where the next segment will be attached. 
+
+        If there is no ``next_segment`` but ``self.growing`` is True, it's time to add 
+        a body! We set ``self.next_segment`` to a  new SnakeBodySegment, set its 
+        position to the head's old position, and add it to the game. We also add 1 to the 
+        game's score.
+        """
+        x, y = self.position
+        dx, dy = self.direction
+        if self.can_move((x+dx, y+dy), game):
+            self.position = (x+dx, y+dy)
+            if self.is_on_apple(self.position, game):
+                apple = game.get_agent_by_name("Apple")
+                apple.relocate(game)
+                self.growing = True
+            if self.next_segment:
+                self.next_segment.move((x, y), game, growing=self.growing)
+            elif self.growing:
+                self.next_segment = SnakeBodySegment(1, (x, y))
+                game.add_agent(self.next_segment)
+                game.state['score'] += 1
+            self.growing = False
+
+
+    def handle_keystroke(self, keystroke, game):
+        """Checks whether one of the arrow keys has been pressed. 
+        If so, sets the SnakeHead's direction and character.
+        """
+        x, y = self.position
+        if keystroke.name == "KEY_RIGHT":
+            self.direction = self.RIGHT
+            self.character = '>'
+        elif keystroke.name == "KEY_UP":
+            self.direction = self.UP
+            self.character = '^'
+        elif keystroke.name == "KEY_LEFT":
+            self.direction = self.LEFT
+            self.character = '<'
+        elif keystroke.name == "KEY_DOWN":
+            self.direction = self.DOWN
+            self.character = 'v'
+
+
+    def can_move(self, position, game):
+        on_board = game.on_board(position)
+        empty = game.is_empty(position)
+        on_apple = self.is_on_apple(position, game)
+        return on_board and (empty or on_apple)
+
+    def is_on_apple(self, position, game):
+        apple = game.get_agent_by_name("Apple")
+        return apple.position == position
+
+
+class SnakeBodySegment:
+    """Finally, we need an Agent for the snake's body segments. 
+    SnakeBodySegment doesn't have ``play_turn`` or ``handle_keystroke`` methods because
+    it never does anything on its own. It only moves when the SnakeHead, or the previous
+    segment, tells it to move. 
+
+    Arguments: 
+        segment_id (int): Keeps track of how far back this segment is from the head.
+            This is used to give the segment a unique name, and also to keep track 
+            of how many points the player earns for eating the next apple. 
+        position (int, int): The initial position.
+
+    Attributes:
+        character: '*'
+        next_segment: Initially ``None``, this is a reference to a SnakeBodySegment
+            when this segment is not the last one in the snake's body.
+            
+    """
+    character = '*'
+    next_segment = None
+
+    def __init__(self, segment_id, position):
+        self.segment_id = segment_id
+        self.name = f"Snake body segment {segment_id}"
+        self.position = position
+
+    def move(self, new_position, game, growing=False):
+        """When SnakeHead moves, it sets off a chain reaction, moving all its 
+        body segments. Whenever the head or a body segment has another segment 
+        (``next_segment``), it calls that segment's ``move`` method.
+
+        This method updates the SnakeBodySegment's position. Then, if 
+        ``self.next_segment`` is not None, calls that segment's ``move`` method. 
+        If there is no next segment and ``growing`` is True, then we set
+        ``self.next_segment`` to a new SnakeBodySegment in this segment's old
+        position, and update the game's score.
+
+        Arguments: 
+            new_position (int, int): The new position.
+            game (Game): A reference to the current game.
+            growing (bool): (Default False) When True, the snake needs to 
+                add a new segment. 
+        """
+        old_position = self.position
+        self.position = new_position
+        if self.next_segment:
+            self.next_segment.move(old_position, game, growing=growing)
+        elif growing:
+            self.next_segment = SnakeBodySegment(self.segment_id + 1, old_position)
+            game.add_agent(self.next_segment)
+            game.state['score'] += self.segment_id + 1
+
+
+if __name__ == '__main__':
+    head = SnakeHead()
+    apple = Apple()
+    game = Game([head, apple], {'score': 0}, board_size=(32, 16), framerate=12)
+    apple.relocate(game)
+    game.play()