through the :attr:`~Thread.daemon` property or the *daemon* constructor
+ Daemon threads are abruptly stopped at shutdown. Their resources (such
+ as open files, database transactions, etc.) may not be released properly.
+ If you want your threads to stop gracefully, make them non-daemonic and
+ use a suitable signalling mechanism such as an :class:`Event`.
There is a "main thread" object; this corresponds to the initial thread of
control in the Python program. It is not a daemon thread.