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gh-142417: Restore private _Py_InitializeMain() function#145472

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Merged
vstinner merged 2 commits intopython:mainfromvstinner:revert_init_main
Mar 4, 2026
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94 changes: 86 additions & 8 deletionsDoc/c-api/init_config.rst
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Expand Up@@ -2299,13 +2299,91 @@ Py_GetArgcArgv()

See also :c:member:`PyConfig.orig_argv` member.

Delaying main module execution
==============================

In some embedding use cases, it may be desirable to separate interpreter initialization
from the execution of the main module.
Multi-Phase Initialization Private Provisional API
==================================================

This separation can be achieved by setting ``PyConfig.run_command`` to the empty
string during initialization (to prevent the interpreter from dropping into the
interactive prompt), and then subsequently executing the desired main module
code using ``__main__.__dict__`` as the global namespace.
This section is a private provisional API introducing multi-phase
initialization, the core feature of :pep:`432`:

* "Core" initialization phase, "bare minimum Python":

* Builtin types;
* Builtin exceptions;
* Builtin and frozen modules;
* The :mod:`sys` module is only partially initialized
(ex: :data:`sys.path` doesn't exist yet).

* "Main" initialization phase, Python is fully initialized:

* Install and configure :mod:`importlib`;
* Apply the :ref:`Path Configuration <init-path-config>`;
* Install signal handlers;
* Finish :mod:`sys` module initialization (ex: create :data:`sys.stdout`
and :data:`sys.path`);
* Enable optional features like :mod:`faulthandler` and :mod:`tracemalloc`;
* Import the :mod:`site` module;
* etc.

Private provisional API:

.. c:member:: int PyConfig._init_main

If set to ``0``, :c:func:`Py_InitializeFromConfig` stops at the "Core"
initialization phase.

.. c:function:: PyStatus _Py_InitializeMain(void)

Move to the "Main" initialization phase, finish the Python initialization.

No module is imported during the "Core" phase and the ``importlib`` module is
not configured: the :ref:`Path Configuration <init-path-config>` is only
applied during the "Main" phase. It may allow to customize Python in Python to
override or tune the :ref:`Path Configuration <init-path-config>`, maybe
install a custom :data:`sys.meta_path` importer or an import hook, etc.

It may become possible to calculate the :ref:`Path Configuration
<init-path-config>` in Python, after the Core phase and before the Main phase,
which is one of the :pep:`432` motivation.

The "Core" phase is not properly defined: what should be and what should
not be available at this phase is not specified yet. The API is marked
as private and provisional: the API can be modified or even be removed
anytime until a proper public API is designed.

Example running Python code between "Core" and "Main" initialization
phases::

void init_python(void)
{
PyStatus status;

PyConfig config;
PyConfig_InitPythonConfig(&config);
config._init_main = 0;

/* ... customize 'config' configuration ... */

status = Py_InitializeFromConfig(&config);
PyConfig_Clear(&config);
if (PyStatus_Exception(status)) {
Py_ExitStatusException(status);
}

/* Use sys.stderr because sys.stdout is only created
by _Py_InitializeMain() */
int res = PyRun_SimpleString(
"import sys; "
"print('Run Python code before _Py_InitializeMain', "
"file=sys.stderr)");
if (res < 0) {
exit(1);
}

/* ... put more configuration code here ... */

status = _Py_InitializeMain();
if (PyStatus_Exception(status)) {
Py_ExitStatusException(status);
}
}
4 changes: 4 additions & 0 deletionsDoc/whatsnew/3.15.rst
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Expand Up@@ -1664,6 +1664,10 @@ New features
* Add:c:func:`PyUnstable_SetImmortal` C-API function to mark objects as:term:`immortal`.
(Contributed by Kumar Aditya in:gh:`143300`.)

* Restore private provisional ``_Py_InitializeMain()`` function removed in
Python 3.14.
(Contributed by Victor Stinner in:gh:`142417`.)

Changed C APIs
--------------

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3 changes: 3 additions & 0 deletionsInclude/cpython/pylifecycle.h
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Expand Up@@ -25,6 +25,9 @@ PyAPI_FUNC(PyStatus) Py_PreInitializeFromArgs(
PyAPI_FUNC(PyStatus)Py_InitializeFromConfig(
constPyConfig*config);

// Python 3.8 provisional API (PEP 587)
PyAPI_FUNC(PyStatus)_Py_InitializeMain(void);

PyAPI_FUNC(int)Py_RunMain(void);


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18 changes: 18 additions & 0 deletionsLib/test/test_embed.py
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Expand Up@@ -1319,6 +1319,24 @@ def test_init_run_main(self):
}
self.check_all_configs("test_init_run_main", config, api=API_PYTHON)

def test_init_main(self):
code = ('import _testinternalcapi, json; '
'print(json.dumps(_testinternalcapi.get_configs()))')
config = {
'argv': ['-c', 'arg2'],
'orig_argv': ['python3',
'-c', code,
'arg2'],
'program_name': './python3',
'run_command': code + '\n',
'parse_argv': True,
'_init_main': False,
'sys_path_0': '',
}
self.check_all_configs("test_init_main", config,
api=API_PYTHON,
stderr="Run Python code before _Py_InitializeMain")

def test_init_parse_argv(self):
config = {
'parse_argv': True,
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Original file line numberDiff line numberDiff line change
@@ -0,0 +1,2 @@
Restore private provisional ``_Py_InitializeMain()`` function removed in
Python 3.14. Patch by Victor Stinner.
28 changes: 28 additions & 0 deletionsPrograms/_testembed.c
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Expand Up@@ -1991,6 +1991,33 @@ static int test_init_run_main(void)
}


static int test_init_main(void)
{
PyConfig config;
PyConfig_InitPythonConfig(&config);

configure_init_main(&config);
config._init_main = 0;
init_from_config_clear(&config);

/* sys.stdout don't exist yet: it is created by _Py_InitializeMain() */
int res = PyRun_SimpleString(
"import sys; "
"print('Run Python code before _Py_InitializeMain', "
"file=sys.stderr)");
if (res < 0) {
exit(1);
}

PyStatus status = _Py_InitializeMain();
if (PyStatus_Exception(status)) {
Py_ExitStatusException(status);
}

return Py_RunMain();
}


static int test_run_main(void)
{
PyConfig config;
Expand DownExpand Up@@ -2649,6 +2676,7 @@ static struct TestCase TestCases[] = {
{"test_preinit_parse_argv", test_preinit_parse_argv},
{"test_preinit_dont_parse_argv", test_preinit_dont_parse_argv},
{"test_init_run_main", test_init_run_main},
{"test_init_main", test_init_main},
{"test_init_sys_add", test_init_sys_add},
{"test_init_setpath", test_init_setpath},
{"test_init_setpath_config", test_init_setpath_config},
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12 changes: 12 additions & 0 deletionsPython/pylifecycle.c
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Expand Up@@ -1530,6 +1530,18 @@ Py_Initialize(void)
}


PyStatus
_Py_InitializeMain(void)
{
PyStatusstatus=_PyRuntime_Initialize();
if (_PyStatus_EXCEPTION(status)) {
returnstatus;
}
PyThreadState*tstate=_PyThreadState_GET();
returnpyinit_main(tstate);
}


staticvoid
finalize_modules_delete_special(PyThreadState*tstate,intverbose)
{
Expand Down
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