Using dumbdbm for simple testing, I occasionally get the following 
error:

Exception exceptions.AttributeError: "'NoneType' 
object has no attribute 'error'
" in <bound method 
_Database.__del__ of <dumbdbm._Database instance at 
0x008B356
8>> ignored

I've tracked it down to a 
problem with the releasing of the reference to _os during __del__, 
which then calls _commit.  _commit proceeds to use the _os, 
which is given a value of None... blamo...