The Python standard library contains a function called thread.get_ident() . It will return an integer that uniquely identifies the current thread at that point in time. On most UNIX systems, this will be the pthread_t value returned by pthread_self() . At first look, this might seem like a good value to key a thread local storage dictionary with. Please don’t do that.

The value uniquely identifies the thread only as long as it is running. The value can be reused after the thread exits. On my system, this happens quite reliably with the following sample program printing the same ID ten times:

import thread, threading def foo(): print 'Thread ID:', thread.get_ident() for i in range(10): t = threading.Thread(target=foo) t.start() t.join()

If the return value of thread.get_ident() was used to key thread local storage, all ten threads would share the same storage. This is not generally considered to be desirable behaviour.

Assuming that you can depend on Python 2.4 (released 3.5 years ago), then just use a threading.local object. It will result in simpler code, correctly handle serially created threads, and you won’t hold onto TLS data past the exit of a thread.

You will save yourself (or another developer) a lot of time at some point in the future. Debugging these problems is not fun when you combine code doing proper TLS with other code doing broken TLS.