find unlinked lights

getUnlinkedLights.py

import pymel.core as pm

def getUnlinkedLights():
    linker = pm.selected()[0]
    lonely = []
    for i in range(linker.link.numElements()):
        ln = linker.link[i]
        lights = ln.light.inputs()
        objects = ln.object.inputs()
        if len(lights) and not len(objects):
            lonely.extend(lights)
    return lonely


def getUnlinkedObjects():
    linker = pm.selected()[0]
    lonely = []
    for i in range(linker.link.numElements()):
        ln = linker.link[i]
        lights = ln.light.inputs()
        objects = ln.object.inputs()
        if not len(lights) and len(objects):
            lonely.extend(objects)
    return lonely

pm.select(getUnlinkedLights())