Because of the sometimes much-maligned Global Interpreter Lock, purePython code itself can't run simultaneously on multiple CPUs. So whatshould you use Python threads for?

The real use for Python threads is turning synchronous functions inextension modules into asynchronous things that don't delay your mainprogram. Often these functions have no asynchronous equivalents (unlikenetwork IO), so it is either use threads or have your main programdelayed. This works for sufficiently compute-intensive functions as wellas functions, like socket.

People doing lots of network IO in threaded programs have historicallyhad a problem: waiting for both network IO and for IPC from threads atthe same time.

Generally you want to wait for network IO using select() or poll() ; trying to use lots of threads, each doing one blocking IOoperation, is usually catastrophic to your performance. But both ofthese only wait on file descriptors, and inter-thread communication inlibraries is almost always implemented with mutexes, semaphores,queues, and so on,