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[3.13] Add notes on nogil & reinitialization to the Opt-Out section in Module Isolation HOWTO (GH-134141) #134491

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May 23, 2025
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[3.13] Add notes on nogil & reinitialization to the Opt-Out section in Module Isolation HOWTO (GH-134141) #134491

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

(cherry picked from commit 1f0a294)

Co-authored-by: Petr Viktorin <encukou@gmail.com>
Co-authored-by: Sam Gross <colesbury@gmail.com>
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@encukou @colesbury @miss-islington
2 people authored and miss-islington committed May 22, 2025
commit a570d76e4fc6bf728472d4362da7e5a2f9c0d8ab
15 changes: 15 additions & 0 deletions Doc/howto/isolating-extensions.rst
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Expand Up @@ -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
----------------------------------

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