Python subprocess: how to use pipes thrice?

Use subprocess.Popen() with the option shell=True, and you can pass it your entire command as a single string.

This is the simplest solution and makes it possible to embed a complicated pipeline in python without head-scratching; but in some cases it might not work, e.g. (as @torek commented) if there are spaces in the filenames passed for input or output. In that case, take the trouble to build up the robust solution in the accepted answer.

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]