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.

Can a C guru comment on this please.

Well, the issue is that signedness differs depending on the platform. Under Windows, thread ids are signed (DWORD). Satisfying all cases would complicate things quite a bit.

no, DWORD is a 32-bit unsigned integer
http://msdn.microsoft.com/en-us/library/aa383751(VS.85).aspx

> no, DWORD is a 32-bit unsigned integer
> http://msdn.microsoft.com/en-us/library/aa383751(VS.85).aspx

Oops, my bad.

Here is a patch, which made all thread id to be unsigned.

You could add a test for it.

On Linux, the following C program tells me that pthread_t is unsigned.
---
#include <pthread.h>
#include <stdio.h>

#define TYPE_IS_SIGNED(TYPE) ((TYPE)-1 < (TYPE)0)

int main()
{
    printf("signed? %i\n", TYPE_IS_SIGNED(pthread_t));
    return 0;
}
---

So it's fair to modify threading.get_ident() to return an unsigned number.

But I disagree to change stable Python versions, it may break applications.

Oh, I wrote write_thread_id() in Python/traceback.c and this function already casts the thread identifier to an unsigned number ;-)

Here is updated patch. Added few tests.

> Here is updated patch. Added few tests.

Cool. I sent a review.

Thank you Victor for your review. Here is updated patch.

New changeset aefa7ebf0ff0f73feee7ab24f4cdcb2014d83ee5 by Victor Stinner (Serhiy Storchaka) in branch 'master':
bpo-6532: Make the thread id an unsigned integer. (#781)
https://github.com/python/cpython/commit/aefa7ebf0ff0f73feee7ab24f4cdcb2014d83ee5