2.7.9: PhotoImage get/put

[Randy Day]

I'm writing a simple image manipulation 
on a PhotoImage (tkinter), and running 
into an odd problem. The code below works, 
except for one thing:

As the image is scanned, I'd like to 
observe the pixels getting inverted on 
the image (as a kind of progress bar). 
What happens is that the code runs the 
complete loop before refreshing the 
photo. I've tried various forms of 
refresh(), update(), etc., within the 
loop, but so far no luck. Am I 
missing something simple?

def process(): # Ordinarily this would be process(photo,wdth,hgt) 
  global wdth # but I ran into problems calling it from a button 
  global hgt  #command with parameters... 
  global photo # the PhotoImage displayed by the calling code

# indents set to 1 to avoid word wrap

# Loop through rows and columns of the image 
  v=wdth 
  z=0 
  a=-1 
  for y in range (0,hgt): 
    w=v 
    v=z # swap v and z so rows are scanned l/r r/l l/r 
    z=w 
    a=-a # set our inc/dec step for lr->rl scan

    for x in range(v,z,a): 
     pix = photo.get(x,y) if pix == u'0 0 0' :
     #pixel_turn_on() 
     photo.put("#%02x%02x%02x" % (255,255,255), (x,y)) 	
    else: 
     #pixel_turn_off() 
     photo.put("#%02x%02x%02x" % (0,0,0), (x,y))

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