I use mod_wsgi which is set for using 8 processes with 1 thread each. Sometimes I see messages in Apache log file like this "Exception TypeError: "'NoneType' object is not callable" in <bound method Pool._del_ of <pymongo.pool.Pool object at 0x7fe63c65f810>> ignored". Especially they arise if I restart apache. I don't see any visual problems with my python script functioning. But I also can't understand why this messages emerge.