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I work on several projects that have initialization
requirements that
need to grab control after Py_Initialize(), but before
any user code
runs (via input, script, -c, etc...).
Note that these are Python clones that take advantage
of an installed
python (using its $prefix/lib/pythonx.x/*.py and
site-packages/*)
We could use 
PyImport_AppendInittab("sitecustomize",initsitecustomize);
But if there already IS customization in
sitecustomize.py, I've
blown it away (and have to look it up and force an
import).
And if someone uses the -S flag, I'm screwed.
I propose a hook styled after Py_AtExit(func) called
Py_AtInit(func)
which maintains a list of functions that are called in
Py_Initialize
right after main and site initializations.
If the hook isn't used, then the cost is a single extra
function
call at initialization. Here's a spurious example: A
customer wants
a version of python that has all the math functions and
his
extensions to act like builtins...
I would write (without refcnt or error checks ;-):
#include "Python.h"
static void after_init(void) {
 PyObject
*builtin,*builtin_dict,*math,*math_dict,*user,*user_dict;
 builtin = PyImport_ImportModule("__builtin__");
 builtin_dict = PyModule_GetDict(builtin);
 math = PyImport_ImportModule("math");
 math_dict = PyModule_GetDict(math);
 user = PyImport_ImportModule("user");
 user_dict = PyModule_GetDict(math);
 PyDict_Update(builtin_dictionary, math_dict);
 PyDict_Update(builtin_dictionary, user_dict);
}
int main(int argc, char** argv) {
 PyImport_AppendInittab("user",inituser);
 Py_AtInit(after_init);
 return Py_Main(argc, argv);
}
voila! An extended Python with new builtins.
This is vastly better than hacking in through
site.py or sitecustomize
I actually want this to do some MPI initialization to
setup a
single user prompt with broadcast which has to run
after
Py_Initialize() but before the import of readline.