On my Mac, the Tk Canvas arc drawing routine produces arcs that are rotated quite a few degrees from their correct location (as many as 10 deg.). It doesn't seem to be a consistent amount though, but it is big enough to make the arc function useless for my app. I've checked the behavior on the current download from python.org (2.5..) and also on the version (2.3..) which came with my mac. It's the problem same on a G4 Powerbook and on an Intel MacBook...