In glibc library (on linux) pthread_t is defined as: typedef unsigned long int pthread_t; But python thread module interprets this value as signed long. Reproduce: >>> import thread >>> thread.get_ident() In some cases it returns negative value. Checked in python 2.4, 2.5, 2.6 Proposal: In my opinion code that cast value returned by pthread_self() should be changed (see: Python/thread_pthread.h). Other possibility is to change only returned value by get_ident function. In this case it should use PyLong_FromUnsignedLong (see: Modules/threadmodule.c). Background: logging module uses 'thread.get_ident()' to save thread_id in logs. If the same application uses some C library that also writes in log file some info with thread_id, in some situations this numbers are diffrent. This complicate logs analyze.