I've tried this with default units, and with units set to millimetres, with the same result. Anyone have any ideas?
P = Part('MyPart')
sk1 = P.AddSketch( 'sk1', P.GetPlane('XY-Plane') )
sk1.AddCircle( 10.0, 10.0, 5.0, False ) # add circle
sk1.AddPolygon( 10.0, 10.0, 5.0, 6, False ) #...