Re: [pygame] diff betn interactive and non-interactive scripts

[Pete Shinners <shredwheat@attbi.com>]

Prahlad Vaidyanathan wrote:
> Amazingly, that works !
> 
> I basically now check the total time of the song, and then put in a
> time.sleep() for that long.
> 
> This is *seriously* weird from where I stand. Could someone explain why
> this happens ?

prahlad, here's what's going on.

when your pygame program exits, all the pygame devices are shut down.
this means the graphics are closed, the cdrom stopped, etc, and (what's 
getting you) the audio stops.

when you call music.play() the music actually starts playing in a 
background thread. this means the play() function returns immediately 
and starts playing the song. this is the same for other types of Sound 
effects too (and the Movies work like this also).

what you are probably 'expecting' is that the play() function doesn't 
return until the song is over. this behaviour is called "blocking", 
which means the function doesn't return until it is finished. the music 
play() function is "non-blocking", which means it returns immediately, 
before the work you ask for is finished.

this "blocking" and "non-blocking" behavior is fairly common for many 
types of libraries. when reading from a file or networking device, there 
are usually blocking and non-blocking ways to do it. another big example 
is something like opengl. all the drawing functions of opengl are 
non-blocking. so when you call your opengl function to draw 10,000 
polygons, the draw function returns immediately, and the graphic card 
starts working on it's 10,000 polygons separate from your program.
(hopefully that all makes a bit of sense)

if you think about it. this non-blocking behavior is exactly what you'd 
want for games. if it was a blocking type of function, how would the 
game actually do anything while the music starts playing?

since the music runs on its own, there's a couple methods of keeping 
track of what the music is doing. the first is simply music.get_busy(), 
which returns true or false, depending if there is any music playing or 
not. there is also an advanced tool, music.set_endevent(), which will 
place any event on the pygame event queue when the music has finished 
playing.

in the development version of pygame, there is also a new advanced 
function, music.get_pos(). this tells you how long the music has been 
playing. (it may not seem so advanced, but internally it's a little 
tricky, it keeps exact track of SDL_mixer's progress on the song, so it 
properly takes into account any skips or jumps) (thanks micheal!)

you mentioned this all worked differently between 'interactive' and 
'scripted' python. perhaps it will be a little clearer if you go back to 
the interactive python session. initialize pygame and load up the music. 
then call the pygame.music.play(). notice how the music starts playing, 
but the interactive prompt returns immediately. you can now do anything 
else you want to in the interactive python shell, the music plays on its 
own. you can find out what the squareroot of 7 is, or call 
music.get_busy() to check on it's status. but most importantly, if this 
was a real game, you could move sprites around, check on user input, and 
draw exciting graphics. all while the music is playing along.

now, before the music has finished, quit the python interpreter. notice 
how the music stops when the python interpreter stops? this is the same 
"problem" you are seeing when you play the music from a python script. 
once the program finishes, so does the music.

what you need to do is keep the program running until the music has 
finished. there is a perfect example of this already in the pygame 
examples directory. take a look at "sound.py", and see how it 
'verbosely' doesn't finish the program until the music has stopped.

there are other methods to keep the program running, but this is likely 
the simplest.


whoof, that came out pretty long, i should html'ize it and put it up as 
a new pygame introduction :] hopefully now you can 'understand' what 
exactly is going on, and why it happens.


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