import arcpy

mxd = arcpy.mapping.MapDocument (r'C:UsersathomDocumentsArcGIS2014ScriptTest.mxd')

lyrList = arcpy.mapping.ListLayers (mxd)
for lyr in lyrList:
    layerName = lyr.name
    NameVar = lyr.dataSource
    arcpy.AddField_management (NameVar, "NameFromLYR", "TEXT")

    expression = "reClass(!NameFromLYR!)"
    codeBlock = """def reClass(lyrName):
        return (layerName)"""

    arcpy.CalculateField_management (NameVar, "NameFromLYR", expression, "PYTHON", codeBlock)
    print layerName
    del layerName
    del codeBlock
    arcpy.RefreshTOC()


del mxd
del lyr
del lyrList