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

[Scott Russell <scott@activestudios.com>]

Pete, 

I agree - this should defintely be kept for use in newbie
documentation..

On Wed, 2002-04-10 at 05:51, Pete Shinners wrote:
> 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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