I don't think you can re-assign a variable while you're using it as an index. You will probably have to break it into 2 linesfor i in xrange(xwidth): xmin += i d[xmin, ymax] = ar[0] for j in xrange(yheight): ymax -= j d[xmin, ymax] = ar[0]
Here's what I tried in the interactive window>>> a=[1,2,3] >>> b=1 >>> a[b+=1] Traceback ( File "<interactive input>", line 1 a[b+=1] ^ SyntaxError: invalid syntax