C++ and Python

This is a simple example of the MNodeMessage usage.

import maya.cmds as cmds
import maya.OpenMaya as om

#Ref http://pastebin.com/mntSGhex

cmds.polySphere()
cmds.select('polySphere1')

def changed_radius( msg, mplug, otherMplug, clientData):
    nodeName, attrName = mplug.name().split('.')
    #if msg & om.MNodeMessage.kAttributeSet:
    if msg == 2056:
        if nodeName == 'polySphere1':
            if attrName == 'radius':
                print "Radius Changed"
                rd = cmds.getAttr('polySphere1.radius')
                cmds.setAttr('polySphere1.subdivisionsAxis', rd*3)

def getNode():
    #Get the node as MObject and set the id message
    MSelectionList = om.MSelectionList()
    om.MGlobal.getActiveSelectionList(MSelectionList)
    node = om.MObject()
    MSelectionList.getDependNode(0, node)
    clientData = None
    id = om.MNodeMessage.addAttributeChangedCallback(node, changed_radius,clientData)


getNode()