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In the current documentation, Py_Main is listed as a regular C API function: https://docs.python.org/3/c-api/veryhigh.html#c.Py_Main
We also didn't add Py_Main() to https://docs.python.org/3/c-api/init.html#before-python-initialization when we did the review for "Functions which are safe to call before Py_Initialize()".
If it truly is a regular C API function, then this suggests that Py_Main() should be called *after* Py_Initialize().
However, Py_Main() has historically called Py_Initialize() *itself*, and generally assumes it has total control over management of the current process - it doesn't expect to be sharing the current process with an embedding application.
In Python 3.7, Py_Main() was changed to call _Py_InitializeCore() and _Py_InitializeMainInterpreter() as separate steps, allowing it to detect cases where an embedding application had already called Py_Initialize(), meaning that Py_Main()'s config setting changes wouldn't be applied.
In effect, we've converted a silent failure (settings not read and applied as expected) into a noisy one (attempting to call Py_Main() in an already initialised interpreter).
(The test suite is silent on the matter, since the idea of an embedding application calling Py_Initialize() before calling Py_Main() never even occurred to us)
So the question is, what should we do about this for Python 3.7:
1. Treat it as a regression, try to replicate the old behaviour of partially (but not completely) ignoring the config settings read by Py_Main()
2. Treat it as a regression, but don't try to replicate the old config-settings-are-partially-applied behaviour - just ignore the settings read by Py_Main() completely
3. Treat it as a long standing documentation error, update the docs to make it clear that Py_Main() expects to handle calling Py_Initialize() itself, and update the Python 3.7 porting guide accordingly