... Point the first is that I really think this is a bug in the GilState APIs: the readline API isn't inherently multi-threaded and so it would be insane to call PyEval_InitThreads() in initreadline, yet it has to cope with being called in a multithreaded situation. If you can't use the GilState APIs in this situation, what are they for?
That's explained in the PEP -- of course <wink>:
Under "Limitations and Exclusions" it specifically disowns responsibility for worrying about whether Py_Initialize() and PyEval_InitThreads() have been called:
This API will not perform automatic initialization of Python, or initialize Python for multi-threaded operation. Extension authors must continue to call Py_Initialize(), and for multi-threaded applications, PyEval_InitThreads(). The reason for this is that the first thread to call PyEval_InitThreads() is nominated as the "main thread" by Python, and so forcing the extension author to specify the main thread (by forcing her to make this first call) removes ambiguity. As Py_Initialize() must be called before PyEval_InitThreads(), and as both of these functions currently support being called multiple times, the burden this places on extension authors is considered reasonable.
That doesn't mean there isn't a clever way to get the same effect anyway, but I don't have time to think about it, and reassigned the bug report to Mark (who may or may not have time).