A General Pygame Main-Loop

Will McGugan's post about mastering time in pygame started me thinking about my game loops, and how I might implement frame skipping, and other things.

The benefit of having a fixed 'step size' for your simulation might not be immediately apparent, however it has obvious benefits when trying to synchonrize network games, or work with physics libraries (eg ODE) which can go non-deterministic when working with a variable step size...

This is what my new general game loop looks like. If I ever need to implement frame-skipping, I believe I'll need to write some more code at line 27, to detect if every frame is being skipped... :-) I use this loop to generate Tick and Render events which get handled elsewhere.

  1 import pygame
  2 from pygame.locals import *
  3 
  4 def main():
  5     pygame.init()
  6     pygame.display.set_mode((320,200))
  7 
  8     #time is specified in milliseconds
  9     #fixed simulation step duration
 10     step_size = 20
 11     #max duration to render a frame
 12     max_frame_time = 100
 13 
 14     now = pygame.time.get_ticks()
 15     while(True):
 16         #handle events
 17         if QUIT in [e.type for e in pygame.event.get()]:
 18             break
 19 
 20         #get the current real time
 21         T = pygame.time.get_ticks()
 22 
 23         #if elapsed time since last frame is too long...
 24         if T-now > max_frame_time:
 25             #slow the game down by resetting clock
 26             now = T - step_size
 27             #alternatively, do nothing and frames will auto-skip, which
 28             #may cause the engine to never render!
 29 
 30         #this code will run only when enough time has passed, and will
 31         #catch up to wall time if needed.
 32         while(T-now >= step_size):
 33             #save old game state, update new game state based on step_size
 34             now += step_size
 35         else:
 36             pygame.time.wait(10)
 37 
 38         #render game state. use 1.0/(step_size/(T-now)) for interpolation
 39 
 40 if __name__ == "__main__":
 41     main()

Update: Thanks to a suggestion from Marius in the comments, the loop is now environmentally friendly. :-)