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Add notes on nogil & reinitialization to the Opt-Out section in Module Isolation HOWTO (GH-134141)

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Co-authored-by: Sam Gross <colesbury@gmail.com>
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Petr Viktorin 2025-05-22 09:36:37 +02:00 • committed by GitHub
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Doc/howto/isolating-extensions.rst
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@ -215,21 +215,36 @@ multiple interpreters correctly. If this is not yet the case for your
module, you can explicitly make your module loadable only once per
process. For example::
// A process-wide flag
static int loaded = 0;
// Mutex to provide thread safety (only needed for free-threaded Python)
static PyMutex modinit_mutex = {0};
static int
exec_module(PyObject* module)
{
PyMutex_Lock(&modinit_mutex);
if (loaded) {
PyMutex_Unlock(&modinit_mutex);
PyErr_SetString(PyExc_ImportError,
"cannot load module more than once per process");
return -1;
}
loaded = 1;
PyMutex_Unlock(&modinit_mutex);
// ... rest of initialization
}
If your module's :c:member:`PyModuleDef.m_clear` function is able to prepare
for future re-initialization, it should clear the ``loaded`` flag.
In this case, your module won't support multiple instances existing
*concurrently*, but it will, for example, support being loaded after
Python runtime shutdown (:c:func:`Py_FinalizeEx`) and re-initialization
(:c:func:`Py_Initialize`).
Module State Access from Functions
----------------------------------