One of the functions (say foo()) in my extension module calls another function (real_foo()) which might block since it does creepy things like I/O. Being a nice fella I surrounded the call with Py_BEGIN_ALLOW_THREADS and Py_END_ALLOW_THREADS so that other threads could jump in and do stuff.
The problem is that real_foo() might call a previously defined callback (written in C) which in turn has to call a Python callback (what good is an extension module if you have to write your callbacks in C?), but when I try to PyEval_CallObject() it - KABOOM! Segfault somewhere in the interpreter (on PyFrame_New() or something like that).
After a lot of thinking (and a lot of help from python-list) I figured out that the state I push away using Py_BEGIN_ALLOW_THREADS must be retrieved again before the callback is called, or PyFrame_New() won't like me. Problem is that that state variable is declared in Py_BEGIN_ALLOW_THREADS and there's no way to reach it (and it also includes a naughty { with the corresponding } in the Py_END_ALLOW_THREADS so it won't work to use them anyway).
My solution: I wrote the two macros above. This works for me, but obviously I have to update my macros when the "real" macros in the Python distribution change.
Instead of asking us to add more macros, I recommend that you use the lower-level C API that is used by the macros. There's a bunch of ways to save and restore the state. The macros are just shortcuts (and a handy way to make all the code disappear when threads don't exist). --Guido van Rossum (home page: http://www.python.org/~guido/)