Pygame modules for interactive programs

Background Music

The pygame.mixer.music module provides additional sound capabilities. Unlike pygame.Sound objects, which load the entire clip into memory pygame.mixer.music, streams files from the disc, only loading what it needs at the current moment. Pygame only supports one music track at a time.

To load background music, you can call pygame.mixer.music.load(), where the argument is the file name. Note that this is not assigned to a variable but is loaded into the module itself (since it can only support one file). The pygame.mixer.music.play() function works just like its counterpart in the pygame.Sound module, but it accepts an extra argument. The first argument is the number of loops (0 to play once, -1 to play indefinitely, or any other value to loop <n> + 1 times). The second argument is a start position, which in most file formats is the number of seconds into the file to start playing.

To set up a playlist, use the pygame.mixer.music.queue() function. queue accepts a file name just like load. When the file currently playing finishes, the queued song will automatically be loaded and played. Only one song at a time can be queued.

Listing 8 shows how to overcome the single-item queue. I can use pygame.mixer.music.set_endevent() (line 15). Its argument is a value between pygame.USEREVENT and pygame.MAXEVENTS, just as for the pygame.time.set_timer() function I discussed earlier. It will cause the provided event ID to appear in the Pygame event queue once the currently playing track has ended (which is checked in line 21). When this event triggers, I can consult my Python list of track file names (lines 8-11) and pygame.mixer.music.queue() the next track (line 23).

01 import pygame
02
03 pygame.mixer.init()
04 pygame.display.init()
05
06 screen = pygame.display.set_mode ( ( 320 , 240 ) )
07
08 playlist = list()
09 playlist.append ( "Epoq-Lepidoptera.ogg" )
10 playlist.append ( "Hydrate-Kenny_Beltrey.ogg" )
11 playlist.append ( "Mists_of_Time-4T.ogg" )
12
13 pygame.mixer.music.load ( playlist.pop() )      # Get the first track from the playlist
14 pygame.mixer.music.queue ( playlist.pop() )     # Queue the 2nd song
15 pygame.mixer.music.set_endevent ( pygame.USEREVENT )    # Setup the end track event
16 pygame.mixer.music.play()                       # Play the music
17
18 running = True
19 while running:
20  for event in pygame.event.get():
21   if event.type == pygame.USEREVENT:        # A track has ended
22    if len ( playlist ) > 0:               # If there are more tracks in the queue...
23       pygame.mixer.music.queue ( playlist.pop() ) # Queue the next one in the list

The stop, set_volume, and fadeout functions of the pygame.mixer.music module all perform like their pygame.Sound counterparts. Unique functions are rewind, which starts a track over at the beginning (e.g., when restarting a game level), and set_pos, which accepts an argument (milliseconds) and sets the current playback position within the music file.

Conclusion

The event/response model of Pygame differs from traditional program flow, but in a game environment or for software with lots of user interaction, it is quite versatile. By adding your own "game mode" variables and status flags to track program state, the logic can become quite complex, but at the same time powerful. Coupled with the built-in sound engine that allows for both foreground and background music playback, the capabilities exist for almost any 2D game you might want to create.

I've also found Pygame useful for several graphical tasks outside of games. Sometimes it's more convenient to use a widget-based toolkit for my main interface and then launch Pygame purely for a "graphics window." In any case, Pygame is an excellent package that provides the flexibility to create many graphical and interactive projects. Now, it's your turn to start programming!