This has to do with the order that things are deleted/cleaned-up on 
interpreter shutdown.  In the reported case, it just happens that the 
'Person' entry in the globals() dict is deleted *before* __del__ is called 
on the last Person instance, causing problems for the lookup of 'Person' 
that's involved in the line 'Person.population -= 1'.

I wonder whether the output from these 'ignored' exceptions on interpreter 
shutdown could be suppressed entirely, at least for non-debug builds?