Since there is no interprocess synchronisation in Python, writing to the same logging file from multiple processes is not supported. If you want output from multiple processes to end up in the same file, have all your processes send logging events to a SocketHandler, and set up a separate socket receiver (which could be in one of the processes) to receive the events and log to file. A working example is given here:

http://docs.python.org/lib/network-logging.html

Note that even if the system was changed to use 'a' rather than 'w', this would not eliminate potential problems with e.g. data in unflushed buffers, etc.