Hi Cage,

The thread has gotten a lot longer since the last time I was her. I haven't read the intervening posts yet. I will do that after posting this.

Q: You didn't receive a printed statement that no vertices were found?
A: No.

Q: Are you running any other Python script which use a callback, when trying to run this?
A: No, not running any other py scripts.

Quote: "I'll try to prepare a script version which will print statements about which step in the process has been reached.  If you run that, it should help identify where things are falling down."

Thanks, I very much appreciate the edffort you are puting in to help me. I ran the new print statements version. Not much more joy that before.

Created plane
Callback started
Starting callback
Callback is running
Callback is running
Callback is running
Callback is running
[etc, etc]

I just get a long procession of "Callback is running", nothing else after the first three lines.