Python subprocess: how to use pipes thrice? [duplicate]

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Just add a third command following the same example:

p1 = subprocess.Popen(['convert', fileIn, 'bmp:-'], stdout=subprocess.PIPE)
p2 = subprocess.Popen(['mkbitmap', '-f', '2', '-s', '2', '-t', '0.48'], 
     stdin=p1.stdout, stdout=subprocess.PIPE)
p1.stdout.close()
p3 = subprocess.Popen(['potrace', '-t' , '5', '-s' , '-o', fileOut],        
     stdin=p2.stdout,stdout=subprocess.PIPE)
p2.stdout.close()

output = p3.communicate()[0]

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