import arcpy
... mxd = arcpy.mapping.MapDocument("CURRENT")
... for lyr in arcpy.mapping.ListLayers(mxd, "TestFile_LineToPoints13_Prod"):
...     if lyr.supports("LABELCLASSES"):
...         for lblclass in lyr.labelClasses:
...             lblclass.expression = " [OBJECTID]"
...             if lblClass.showClassLabels:
...                     print "    Class Name:  " + lblClass.className
...                     print "    Expression:  " + lblClass.expression
... mxd.save()
... del mxd
...