Thanks you so much, Structure, that's solved it. After a bit of rebuilding I now have a working script to turn off all lights except the currently selected one, do some useful things, then turn them all back on again.

I found, though, that at first I had 'invalid syntax' error on your 'Else:' — this was fixed by indenting all the way down the script. Below is a visual example of what I mean:

---

if test_for_light():
    put the body of your script here
    <- indent
    <- indent
    <- indent
    <- indent
    <- indent
    <- indent
    <- indent
    <- indent
    <- indent
else:
    poser.DialogSimple.MessageBox("To run this script, please first select a light.")

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In other words, the script has to have "a clear run" down the indents to get at the "else:" command. If not then it throws an 'invalid syntax' error. This should be useful to know for other beginners reading this in years to come.

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