Saturday, 28 November 2009

Python, communicating processes and Pygame

Pygame is a popular and excellent library for writing 2d arcade games and animations. It takes an approach that has become very popular in the Python world, it wraps the low-level SDL library giving programmers the best of both worlds -- efficiency from the underlying C++ code and a simple, productive scripting environment from the high-level Python wrappers. Pygame is so simple, it is used in a number of introductory text books, including our own book, Python for Rookies. One reason for the simplicity of Pygame is it's approach to events: events exist, but Pygame is not event driven. The programmer has to write his / her own event loop and process events which are relevant to the application explicitly. This makes for a model of interaction that is very easy for beginners to understand and very easy for experts to get right.

For nearly a year now I have been working on python-csp which adds Hoare's Communicating Sequential Processes (CSP) toPython and is nearing a full release. You can read more about CSP on the WoTUG site, but briefly it is a neat way of implementing "message-passing" concurrency to construct concurrent and parallel programs. CSP eliminates several classes of well-known bugs in this sort of software, including race conditions, and makes it much easier to avoid deadlocks. In a CSP program, there are no "locks" which cuts out a lot of difficult boilerplate code. Instead, a CSP program typically consists of a number of CSP processes (which may be reified as threads, processes, coroutines, or anything else) running in parallel. These communicate by sending data along synchronous "channels", which you can think of as being similar to UNIX pipes. Wherever you might use shared data or fire an event in another style of concurrency, in CSP you would send and receive data down a channel. Because the communication between processes is synchronous, the flow of data between processes can only happen in the order in which it appears statically in your code (a big advantage compared to event-driven systems). 

The details of python-csp will keep for another post, but I wanted to document here a pattern for fixing a very irritating problem that occurs when writing python-csp code which uses Pygame: it's very difficult to kill the application in the way you normally would, by pressing the "Close" button on the application window, pressing Alt+F4, or whatever. In CSP programs, the usual way of terminating running processes is to "poison" the channels which they use, which causes all processes which read / write to those channels to propagate the poisoning on any channels they know about and terminate themselves. Neil Brown has a very nice post on poisoning here if you want more details and nice graphics. In Pygame, to quit the application there's a handy pygame.quit() function. Mixing these two requires a bit of alchemy, so it's worth knowing a pattern that works.

For simple programs, the following is enough: just place all Pygame related code in a single process which draws to the application window and have one or more channels to pass information from the rest of the running program to the drawing process. When a pygame.QUIT event is received drop out of the main animation loop and then (only then) poison any channels and quit the graphical application. Just like this:

@process  def Drawme(channel, _process=None):  import pygame  # Constants  width, height = 512, 256  # Open window  pygame.init()  screen = pygame.display.set_mode((width, height), 0)  quit = False  while not quit:  data =  # Drawing code goes here...  for event in pygame.event.get():  if event.type == pygame.QUIT:  quit = True  # Process other events here ...  channel.poison()  pygame.quit()  return


If you try this out, make sure to have a separate terminal open to watch the number of Python processes running in your OS and check that quitting really does work. On UNIX systems you can use the 'watch' utility for this: watch -n 0.5 'ps h -C python -o pid'

Here's an example Pygame / python-csp application, a simple demonstration of Reynold's "flocking" algorithm, which simulates a flock of birds or other wildlife moving in unison:

Posted via web from snim2's posterous

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