Pygame - Adding Sound and Automating Game Play
This section is broken into two parts. The first part focuses on adding sound to the Alien Invasion project, and the second focuses on writing a separate program that plays the game automatically. Adding sound doesn’t take much code, but it makes the game much more interesting. Automating game play is more complicated, but it’s a really interesting and satisfying exercise.
You can find complete versions of this project in the beyond_pcc/ai_player/ folder in the downloadable resources for the book.
Adding Sound
If you want to take this as a challenge before reading this guide, feel free to look at the Pygame documentation, and see if you can add sounds on your own. In this section, we’re going to play a firing sound each time the ship fires a bullet, and an explosion sound each time an alien is shot down.
The Pygame Mixer module
The Pygame Mixer Module manages music and sound effects. You can take a look at the documentation; I also found this Nerd Paradise post helpful. (If the official Pygame docs are “sleeping”, try this mirror site.)
There are lots of resources available for finding sound effects. I found some useful ones at opengameart.org. I chose laser1.wav from user dklon for firing bullets, and Explosion_02.wav from Little Robot Sound Factory for an alien being hit. Make a new folder in your alien_invasion folder called sounds. This folder should be at the same directory level as your images folder. Store the sound files you want to use in your sounds folder.
The sound_effects.py file
We’ll start by making a new file called sound_effects.py, where we can define all of the sound effects we’ll use in the game. This file is pretty short:
import pygame
pygame.mixer.init()
bullet_sound = pygame.mixer.Sound('sounds/laser1.wav')
alien_sound = pygame.mixer.Sound('sounds/Explosion_02.wav')
We import pygame, and initialize the mixer module. Then we define two sounds, bullet_sound and alien_sound. To make a sound in Pygame you make an instance of the Sound class, with a path to the sound file as the only argument. (Here’s the mirror page for the Sound class.)
Modifying alien_invasion.py
Now we need to modify alien_invasion.py so the sounds play at the right times. At the top of the file, import the sound effects module we just created. We’ll give this module the alias se:
import sys
from time import sleep
import pygame
from settings import Settings
--snip--
from alien import Alien
import sound_effects as se # 1
class AlienInvasion:
--snip--
You can add this import statement after all of the existing import statements (1).
To make a firing sound, we call the play() method on the appropriate Sound object each time a bullet is fired. We make this call from _fire_bullet() (1):
def _fire_bullet(self):
"""Create a new bullet and add it to the bullets group."""
if len(self.bullets) < self.settings.bullets_allowed:
new_bullet = Bullet(self)
self.bullets.add(new_bullet)
se.bullet_sound.play() # 1
When you play the game now, you should hear a sound every time you fire a bullet!
To make a sound when an alien is hit, we modify the _check_bullet_alien_collisions() method:
def _check_bullet_alien_collisions(self):
"""Respond to bullet-alien collisions."""
# Remove any bullets and aliens that have collided.
collisions = pygame.sprite.groupcollide(
self.bullets, self.aliens, True, True)
if collisions:
for aliens in collisions.values():
self.stats.score += self.settings.alien_points * len(aliens)
self.sb.prep_score()
self.sb.check_high_score()
se.alien_sound.play() # 1
if not self.aliens:
# Destroy existing bullets and create new fleet.
--snip--
We play the alien sound whenever there’s a collisions dictionary, indicating that an alien has just been destroyed (1).
Now your game should sound much more interesting! You should hear a steady stream of sounds as you fire bullets and destroy aliens. You might also want to add sounds for the start of a game, clearing a level, an alien hitting the ship, an alien hitting the ground, and the game ending. You might also add sounds for reaching certain scoring milestones like 10,000 points, 100,000 points, and so on.
Automating Game Play
This is the really interesting part. If you want to try this on your own before reading ahead, you can see the challenges here. If you’re new to programming, though, getting this set up can be pretty challenging. It’s perfectly reasonable to read through this guide to get started, and then explore further automation ideas on your own.
Firing automatically
Let’s start out by writing a program that takes control of the ship, and fires as often as possible.
Start by making a new file called ai_player.py, in the same directory as alien_invasion.py. We’ll do everything from here on out in ai_player.py; the point of this is to automate the game play without touching any of the original game code.
To automate the game play, we need to make an instance of AlienInvasion. We’d normally call run_game() to start a game, but that would start the main game loop and we’d never be able to take control except by using the keyboard. Instead, we’ll write our own class called AIPlayer, and we’ll give this class the game object. Then we’ll write a new run_game() method, where we can interject code that controls the elements of the game.
Here’s the core structure of the AIPlayer class:
from alien_invasion import AlienInvasion
class AIPlayer:
def __init__(self, ai_game): # 1
"""Automatic player for Alien Invasion."""
# Need a reference to the game object.
self.ai_game = ai_game # 2
def run_game(self): # 3
"""Replaces the original run_game(), so we can interject our own
controls.
"""
# Start out in an active state.
self.ai_game.stats.game_active = True # 4
# Start the main loop for the game.
while True: # 5
# Still call ai_game._check_events(), so we can use keyboard to
# quit.
self.ai_game._check_events()
if self.ai_game.stats.game_active:
self.ai_game.ship.update()
self.ai_game._update_bullets()
self.ai_game._update_aliens()
self.ai_game._update_screen()
if __name__ == '__main__':
ai_game = AlienInvasion()
ai_player = AIPlayer(ai_game)
ai_player.run_game()
We define a class called AIPlayer. This is a simple class; it doesn’t inherit from AlienInvasion, although that would be a perfectly reasonable approach as well. The inheritance approach leads to code that’s slightly less verbose, but a little harder to reason about. If you’re curious, feel free to try building an AIPlayer class that inherits from AlienInvasion. One advantage is that you’ll have more direct access to elements in the game, but there will be a less clear distinction between what’s part of the original game, and what’s part of the automated player.
In the approach shown here, the AIPlayer class needs an instance of the AlienInvasion class. The game object needs to be passed as an argument to __init__(), and we call this attribute ai_game (1). We attach it to self, to make sure the game object is available throughout the AIPlayer class (2).
If we call the original run_game() method from AlienInvasion, we’ll start a while loop that won’t let us control any of the game elements. So instead we write a new run_game() method that we can call in place of the original run_game() method (3). This method needs to do everything the original run_game() method does, but we’ll be able to add code to this method when we want to take control of some of the game elements.
In run_game(), we need the game to start out in an active state because we want it to start playing immediately. So we set game_active to True (4). We access game elements through the game object, like this:
self.ai_game.stats.game_active = True
It’s worth looking closely at this line, because this is how we’ll approach many aspects of automating the game play. The self here refers to an instance of AIPlayer, not AlienInvasion. The ai_game attribute refers to an instance of the AlienInvasion class, which represents the game as a whole. We then access the stats attribute in AlienInvasion, which refers to an instance of the GameStats class. Finally we access the game_active attribute of GameStats, and set it to True.
Next we need a while loop, so the automated game will do all of the updating that was being done in the original run_game() method (5). We still want to call the original _check_events(), because we’ll want to be able to quit the game at any time. If the game is active, we still need to update the ship, update the bullets, and update the aliens. Finally, we need to update the screen on every pass through the loop. Since we need to do this whether the game is active or inactive, the call to _update_screen() occurs outside of the if block.
At the bottom of the file we make an instance of AlienInvasion, which we assign to ai_game. Then we need to make an instance of AIPlayer, which requires the ai_game object as an argument. Finally, we call the run_game() method associated with the ai_player object, not the one associated with ai_game.
When you run this file, the game will start automatically. You could play the game with the keyboard, because we’re calling the original _check_events(). Instead, let’s write one line of code that fires bullets whenever possible:
class AIPlayer:
def __init__(self, ai_game):
--snip--
def run_game(self):
--snip--
# Start the main loop for the game.
while True:
# Still call ai_game._check_events(), so we can use keyboard to
# quit.
self.ai_game._check_events()
if self.ai_game.stats.game_active:
self.ai_game.ship.update()
self.ai_game._update_bullets()
self.ai_game._update_aliens()
self.ai_game._fire_bullet() # 1
self.ai_game._update_screen()
This one line of code is a call to ai_game.fire_bullet(), which runs on every pass through the main loop as long as the game as active (1).
Now when you run the game, the ship will always fire a bullet whenever there are fewer than 3 bullets on the screen. It may look like one bullet at first, because the first three bullets are fired instantly on the first three game cycles.
This is really satisfying, because we can sit back and watch the ship fire bullets all by itself. But it’s not a very good game strategy. If we let this play until the game ends, the ship will only ever destroy the aliens in the middle columns, and then rest of the aliens will creep down and hit the ground. To make things more interesting, we need to make the ship move.
You might also notice that the mouse is visible when the game is playing. That’s because the code that hides the mouse is in the _check_play_button() method, which we never call. We can add that line to run_game() (1):
def run_game(self):
"""Replaces the original run_game(), so we can interject our own
controls.
"""
# Start out in an active state, and hide the mouse.
self.ai_game.stats.game_active = True
pygame.mouse.set_visible(False) # 1
# Start the main loop for the game.
while True:
--snip--
Make sure you also add an import pygame statement at the top of the file.
Now that you’ve seen how to take control of the game, feel free to try automating the ship’s movement on your own. See the challenge AI Player 2: Sweeping Strategy if you’re interested in trying this on your own before reading further.
Moving the ship
We’ll implement a really simple strategy now. We’ll move the ship all the way to the right, firing bullets whenever possible. Then we’ll move the ship all the way to the left, again firing whenever possible. We’ll do this over and over, until the game ends.
All of this can be coded right in the while loop of the run_game() method:
# Start the main loop for the game.
while True:
# Still call ai_game._check_events(), so we can use keyboard to
# quit.
self.ai_game._check_events()
# Sweep the ship right and left continuously.
ship = self.ai_game.ship # 1
screen_rect = self.ai_game.screen.get_rect()
if not ship.moving_right and not ship.moving_left: # 2
# Ship hasn't started moving yet; move to the right.
ship.moving_right = True
elif (ship.moving_right
and ship.rect.right > screen_rect.right - 10): # 3
# Ship about to hit right edge; move left.
ship.moving_right = False
ship.moving_left = True
elif ship.moving_left and ship.rect.left < 10: # 4
ship.moving_left = False
ship.moving_right = True
if self.ai_game.stats.game_active:
self.ai_game.ship.update()
self.ai_game._update_bullets()
self.ai_game._update_aliens()
self.ai_game._fire_bullet()
self.ai_game._update_screen()
We first assign the self.ai_game.ship object to a variable called ship, so we don’t have to type out the longer reference repeatedly (1). We do the same for screen_rect.
Then we run through three cases:
- If the ship is not moving at all, the game must have just started. In this case, we set
moving_righttoTrue(2). - If the ship is moving right and the right side of the ship is within 10 pixels of the right side of the screen, we change directions (3). Changing directions when the ship is 10 pixels from the edge prevents issues where the ship’s position doesn’t match the screen edge exactly. We set
moving_righttoFalse, and setmoving_lefttoTrue. Remember if both of these areTrue, the ship will move both directions at once and remain in the same position. - If the ship is moving left and it gets within 10 pixels of the left edge of the screen, we change directions (4).
That’s it! Now when you run the game the ship will sweep right and left, firing constantly. It will clear the first screen, and probably many more screens if you let it.
Refactoring
The main while loop in run_game() is getting pretty long, so we should pull out the automation logic into a separate method. I made a new method called _implement_strategy(), and moved the code for moving the ship and firing bullets into this method:
class AIPlayer:
def __init__(self, ai_game):
--init--
def run_game(self):
"""Replaces the original run_game(), so we can interject our own
controls.
"""
# Start out in an active state, and hide the mouse.
self.ai_game.stats.game_active = True
pygame.mouse.set_visible(False)
# Start the main loop for the game.
while True:
# Still call ai_game._check_events(), so we can use keyboard to
# quit. Also call our own method to initiate events.
self.ai_game._check_events()
self._implement_strategy() # 1
if self.ai_game.stats.game_active:
self.ai_game.ship.update()
self.ai_game._update_bullets()
self.ai_game._update_aliens()
self.ai_game._update_screen()
def _implement_strategy(self):
"""Implement an automated strategy for playing the game."""
# Sweep the ship right and left continuously.
ship = self.ai_game.ship
screen_rect = self.ai_game.screen.get_rect()
if not ship.moving_right and not ship.moving_left:
# Ship hasn't started moving yet; move to the right.
ship.moving_right = True
elif (ship.moving_right
and ship.rect.right > screen_rect.right - 10):
# Ship about to hit right edge; move left.
ship.moving_right = False
ship.moving_left = True
elif ship.moving_left and ship.rect.left < 10:
ship.moving_left = False
ship.moving_right = True
# Fire a bullet whenever possible.
self.ai_game._fire_bullet()
The call to _implement_strategy() is placed right after the call to _check_events(), and before the code that updates the game elememts (1). This way any changes we want to make to the game elements are implemented before those elements are drawn to the screen.
This is an improvement, because all of the code that handles the automation is now in its own section of the file. Most of _implement_strategy() is currently focused on making the ship sweep right and left. This method is going to get really long as soon as we start to do any other work, so let’s move most of this code to a new method called _sweep_right_left():
def _implement_strategy(self):
"""Implement an automated strategy for playing the game."""
self._sweep_right_left()
# Fire a bullet whenever possible.
self.ai_game._fire_bullet()
def _sweep_right_left(self):
"""Sweep the ship right and left continuously."""
ship = self.ai_game.ship
screen_rect = self.ai_game.screen.get_rect()
if not ship.moving_right and not ship.moving_left:
# Ship hasn't started moving yet; move to the right.
ship.moving_right = True
elif (ship.moving_right
and ship.rect.right > screen_rect.right - 10):
# Ship about to hit right edge; move left.
ship.moving_right = False
ship.moving_left = True
elif ship.moving_left and ship.rect.left < 10:
ship.moving_left = False
ship.moving_right = True
This is nice and clear, and it should provide a consistent overall structure as we explore different strategies to optimize automated gameplay.
Watching the automated game play is fun and satisfying, but it can be a little slow to watch the automated game at regular speed. Next we’ll add a little code that speeds up the game during our development work.
Speeding up the game for development work
When we want to see how effective a new automation strategy is, it would be nice to see the game play out more quickly than the standard speed that’s good for human players. We can do this by modifying some of the game’s settings.
To do this, we’ll write a new method called _modify_speed(), which we can call from run_game():
class AIPlayer:
def __init__(self, ai_game):
--snip--
def run_game(self):
"""Replaces the original run_game(), so we can interject our own
controls.
"""
# Start out in an active state, and hide the mouse.
self.ai_game.stats.game_active = True
pygame.mouse.set_visible(False)
# Speed up the game for development work.
self._modify_speed(5) # 1
# Start the main loop for the game.
while True:
--snip--
def _implement_strategy(self):
--snip--
def _sweep_right_left(self):
--snip--
def _modify_speed(self, speed_factor): # 2
self.ai_game.settings.ship_speed *= speed_factor
self.ai_game.settings.bullet_speed *= speed_factor
self.ai_game.settings.alien_speed *= speed_factor
We want to be able to easily speed up the game when we’re trying out new strategies, but also slow the game back down when we want to watch a game play out at the normal speed. We write _modify_speed() so it accepts an argument that controls how much to speed up the game (1). If you pass an argument of 1 the game will play at normal speed. Anything greater than 1 will speed up the game, and anything less than 1 will slow the game down.
In _modify_speed() we adjust the speed of the ship, the bullets, and the aliens (2).
Now when you play the game with a speed factor of something like 10 you’ll see how effective the strategy is, and you’ll see its weak points as well. For example I can see that the sweeping strategy is pretty effective at clearing out most of the fleet, but it’s really inefficient when there’s only one or two aliens left:
You should be aware that speeding up the game affects the high score that your strategy will reach. You can see this by trying a few very different speed factors. For example on my system a speedup scale of 10 with the current strategy ends with around 8,000,000 points, at around level 18. With a speedup scale of 100, it only earns about 4,000 points, and it can’t even clear the first screen. If you’re comparing strategies, make sure you use the same speed factor for each of your runs.
Randomized firing
One interesting idea is to give some slight randomness to the decision about whether to fire or not. Right now the ship is firing whenever it can. That means it immediately fires three bullets, and then fires every time a bullet hits an alien or disappears off the top of the screen. This means the bullets often end up in a tightly-packed group, especially when there’s only one alien left and the ship is moving out of sync with the alien. If you want to try this on your own first, see the challenge AI Player #3: Randomized Shooting.
We can use the random() function to determine when to fire. The random() function returns a decimal between 0 and 1. So if we only fire when we get a random number less than 0.5, we’ll fire a bullet on half of the game cycles where we can fire.
Here’s what this looks like:
from random import random # 1
import pygame
from alien_invasion import AlienInvasion
class AIPlayer:
--snip--
def _implement_strategy(self):
"""Implement an automated strategy for playing the game."""
self._sweep_right_left()
# Fire a bullet at the given frequency, whenever possible.
firing_frequency = 0.5 # 2
if random() < firing_frequency:
self.ai_game._fire_bullet()
We first import the random() function from the random module (1). In _implement_strategy() we define a firing frequency, in this case 0.5 (2). We test whether a randomly-generated number is less than this firing frequency, and only fire a bullet if it is. To really see that this code works, set the firing frequency to something really low like 0.1 or 0.01. You should see the ship fire much less frequently, and with some randomness.
I don’t think this approach helps the current strategy, but I have found it a useful approach in some situations. If you want, you can put this in a new method called _fire_bullet(), and give it a parameter for the firing frequency. Then you could use different firing frequencies in specific situations, such as when there are only a certain number of aliens left on the screen.
Next we’ll look at targeting a specific alien.
Changing strategies mid-level
I don’t want to give away all the best strategies, because it’s a lot of fun to try different approaches on your own. So I’ll close out this guide by introducing two final ideas you can play with. The first is to use different strategies depending on the size of the remaining fleet. The second is to focus on a specific alien.
Clearly the sweeping approach works well to destroy most of the fleet. But it struggles when there’s only one alien left, and most of the bullets just fly up through an empty screen. It seems a good idea to respond differently near the end of a level, than when there’s a mostly full fleet.
For a simple approach to implementing different strategies, let’s freeze the ship when half of the fleet has been destroyed. To help this we’ll make a parameter that represents the size of a full fleet.
class AIPlayer:
def __init__(self, ai_game):
--snip--
def run_game(self):
--snip--
# Speed up the game for development work.
self._modify_speed(5)
# Get the full fleet size.
self.fleet_size = len(self.ai_game.aliens) # 1
# Start the main loop for the game.
while True:
--snip--
def _implement_strategy(self):
"""Implement an automated strategy for playing the game."""
# Sweep right and left until half the fleet is destroyed, then stop.
if len(self.ai_game.aliens) >= 0.5 * self.fleet_size: # 2
self._sweep_right_left()
else:
self.ai_game.ship.moving_right = False
self.ai_game.ship.moving_left = False
# Fire a bullet at the given frequency, whenever possible.
firing_frequency = 1.0 # 3
if random() < firing_frequency:
self.ai_game._fire_bullet()
def _sweep_right_left(self):
--snip--
def _modify_speed(self, speed_factor):
--snip--
First we create an attribute called fleet_size (1). We need to initialize this in run_game() before starting the while loop, because we need to grab the fleet size before any of the aliens have been shot down.
In _implement_strategy(), we call _sweep_right_left() as long as the current fleet size, len(self.ai_game.aliens) is greater than half of the original fleet size (2). When half of the fleet has been destroyed, we stop the ship’s movement and no longer call _sweep_right_left(). Note that I also bumped the firing frequency back up to 1.0 here (3).
This is not an improvement on the basic sweeping strategy, but it does show you how to transition from one strategy to another as your automated player makes progress within a level. You could implement a new strategy when there’s just one or two aliens left, or even have a series of strategies for increasingly specific situations.
In the last section, we’ll look at how you can pick out a specific alien and target that individual alien.
Targeting a specific alien
At some point you’ll probably want to target a specific alien. You can develop some interesting strategies and game play by targeting specific aliens, or groups of aliens. In this section, I’ll show you one such strategy and leave you to implement more effective strategies. If you want to try this on your own first, see the challenge AI Player 4: Targeting Specific Aliens.
In this approach we’ll always target the right-most alien in the bottom row. We’ll pick that alien from the group of aliens, and then always move the ship towards that alien. All of the code to do this goes in _implement_strategy(), and a new method called _get_target_alien():
def _implement_strategy(self):
"""Implement an automated strategy for playing the game."""
# Get specific alien to chase.
target_alien = self._get_target_alien() # 1
# Move toward target alien.
ship = self.ai_game.ship
if ship.rect.x < target_alien.rect.x: # 2
ship.moving_right = True
ship.moving_left = False
elif ship.rect.x > target_alien.rect.x:
ship.moving_right = False
ship.moving_left = True
# Fire a bullet whenever possible.
firing_frequency = 1.0
if random() < firing_frequency:
self.ai_game._fire_bullet()
def _get_target_alien(self):
"""Get a specific alien to target."""
# Find the right-most alien in the bottom row.
# Pick the first alien in the group. Then compare all others,
# and return the alien with the greatest x and y rect attributes.
target_alien = self.ai_game.aliens.sprites()[0] # 3
for alien in self.ai_game.aliens.sprites():
if alien.rect.y > target_alien.rect.y: # 4
# This alien is farther down than target_alien.
target_alien = alien
elif alien.rect.y < target_alien.rect.y: # 5
# This alien is above target_alien.
continue
elif alien.rect.x > target_alien.rect.x: # 6
# This alien is in the same row, but farther right.
target_alien = alien
return target_alien
In _implement_strategy(), we remove the existing code that moves the ship. We keep the method _sweep_right_left() in the class because we might want to use it in another strategy, but we remove the call to that method.
Let’s look at _get_target_alien(), because that’s the first call we make in _implement_strategy() (1). We want to pick out the alien that’s farthest on the right in the bottom row. There are a number of ways to do this, and the approach I use here is not necessarily the best or most efficient approach. When writing for a wide audience, I usually choose an approach that’s likely to be clear to many people, over a more efficient approach that might be confusing to some people. If you know a more efficient approach to pick out the target alien, feel free to implement that approach.
Remember that a Pygame group is similar to a list, but it’s not an actual list. The elements in a group are not kept in a specific order, so you can’t grab an element by using an index. The sprites() method puts the elements of the group into a list, but not in a predictable order. In _get_target_alien() we use sprites() to put the aliens in a list so we can grab an individual alien (3). Then we cycle through all the aliens in the list. If an alien is farther down the screen than target_alien, we assign the current alien to target_alien (4). If the alien is farther up the screen, we ignore this alien and continue the loop (5). Otherwise the alien is in the same row as target_alien, and we choose this alien if it’s farther to the right than target_alien (6).
This if block was a little tricky to develop; I didn’t get it right the first time. My first attempt examined x and y at the same time, and ended up chasing aliens that were farther up the screen but also farther right than the rightmost alien in the bottom row. This is actually an interesting strategy, because it makes it harder for the fleet to hit the edge and descend. You might try implementing a strategy that aims at clearing the fleet one column at a time, starting from one of the edges.
Once we have a target alien selected, we can position the ship. Back in _implement_strategy(), if the ship is to the left of the target alien we start moving right (2). If the ship is to the right of the target alien, we move left.
When you run this code, you’ll see that matching an alien’s position exactly doesn’t work all that well, because by the time the bullet reaches the alien’s vertical position, the alien has moved away.
The ship ends up chasing aliens until they’re so low they can’t get away. This is a case where introducing a bit of randomness into the firing can be effective. You can also explore strategies for targeting specific aliens, but not staying right underneath them. It’s an interesting geometry exercise to try and work out how to make a bullet hit the desired alien every time. But if that’s not your strong suit, there are plenty of ways to get near enough to specific aliens that you can reliably shoot them down. If you don’t want to try working out an exact solution to hitting aliens, you can try adding some randomness to the ship’s position. I imagine the right amount of randomness might cause the ship to end up in the right position often enough to hit the alien without getting into long stretches of alway firing behind the alien’s position. You might also try stopping, and firing when the alien is a certain distance away to see if that results in a higher level of accuracy. There are lots of approaches you can try implementing, even if you can’t work out the most mathematically optimal approach. Many of these strategies are really interesting to watch at higher speeds.
Here’s a slightly better version that tries to position the ship in anticipation of where the alien will be by the time the bullet reaches the vertical position of the ship:
I also want to point out that I haven’t optimized any of this code. I’m writing this guide to show people how you can start to automate the game play in Alien Invasion, and this is representative of how I approach some of my development work. Often times in exploratory work I jot a sentence or two about what I’m trying to do, then write some code to see how that idea works. If I like that approach but I’m not going to do anything more with it, I leave the unoptimized code in place. If I’m going to build on that code, I spend some more time thinking about how to make the code more efficient. Here, for example, we’re looping through the list of aliens on every game cycle. That’s really inefficient! But it doesn’t appear to affect the game’s performance, so I’m not too worried about it at the moment. If I was building on this project, I’d make target_alien an attribute of the class, and then only call _get_target_alien() if target_alien doesn’t exist, which should happen every time the target alien is destroyed. The loop would never run more than once per the number of aliens on the screen. If you’re curious about this, try to implement this approach. You can code a counter to see how many times _get_target_alien() is called in the course of a game, and find out if your optimization made a difference or not.
Sometimes, especially on a larger project or a project that I’m doing for someone else, I’ll plan out my overall approach much more carefully and build in some optimization from the beginning.
Closing Thoughts
If you are enjoying this project, you might want to implement a more refined approach to tracking statistics in the game. You can track hits and misses, and report a hit/miss ratio. Then you can compare different strategies not just on the high scores or completed levels they achieve, but on how efficiently they perform as well. If you’re interested in this, see the challenge AI Player 7: Scoring Accuracy.
Hopefully this guide helps you get started automating the game play in Alien Invasion. If you come up with an effective or interesting strategy, please share it! You can tag your solution with #ai_player on Twitter, or send it to me through email (ehmatthes at gmail). Good luck, and if this doesn’t work for you please let me know as well.