Toggle Scale Mode

import maya.cmds as cmds
currentMode = cmds.manipScaleContext('Scale', q=True, mode=True)
mode = ["Object Space Scale", "Parent Space Scale", "World Space Scale"]

if currentMode < 2:
    currentMode += 1
    cmds.manipScaleContext('Scale', edit=True, mode = currentMode)
else:
    currentMode = 0
    cmds.manipScaleContext('Scale', edit=True, mode = currentMode)
#remove next line to turn off viewport message
cmds.inViewMessage(smg=mode[currentMode], pos='topCenter', fade=True, fst = 600)