Nikolaus Rath, 18.10.2010 04:02:
I a thread does not hold the GIL, is it totally forbidden to call any Py* API functions at all, or are there some exceptions?
More concretely, am I allowed to create fresh Python objects with PyBytes_FromString et. al. without holding the GIL?
No. Anything that requires Python memory management and especially reference counting must be protected by the GIL. You can read the C pointer and length of a Python string without the GIL (using the C-API macros) and you can acquire and release thread locks, but you can't create new objects.
Even simple looking functions may raise exceptions in some corner cases (such as memory errors), which also requires the GIL to work. So don't expect there to be much that you can do without holding it. The best way to tell is to look through the CPython implementation of the function you want to call, including all other functions that it calls, and to check for anything that looks like reference counting or exception handling.