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PEP: 741
Title: Python Configuration C API
Author: Victor Stinner <vstinner@python.org>
Discussions-To: https://discuss.python.org/t/pep-741-python-configuration-c-api-second-version/45403
Status: Draft
Type: Standards Track
Created: 18-Jan-2024
Python-Version: 3.13
Post-History: `19-Jan-2024 <https://discuss.python.org/t/pep-741-python-configuration-c-api/43637>`__,
`08-Feb-2024 <https://discuss.python.org/t/pep-741-python-configuration-c-api-second-version/45403>`__,
Abstract
========
Add a C API to the limited C API to configure the Python initialization.
It can be used with the stable ABI.
Complete :pep:`587` API by adding ``PyInitConfig_AddModule()`` which can be
used to add a built-in extension module; feature previously referred to
as the "inittab".
Add ``PyConfig_Get()`` and ``PyConfig_Set()`` functions to
get and set the current runtime configuration at runtime.
:pep:`587` "Python Initialization Configuration" unified all the ways to
configure the Python **initialization**. This PEP (almost fully) unifies
also the configuration of the Python **preinitialization** and the
Python **initialization** in a single API, even if the
**preinitialization** is still required to decode strings from the
locale encoding.
This new API replaces the deprecated and incomplete legacy API which is
scheduled for removal between Python 3.13 and Python 3.15.
Rationale
=========
PyConfig is not part of the limited C API
-----------------------------------------
When the first versions of :pep:`587` "Python Initialization Configuration"
were discussed, there was a private field ``_config_version`` (``int``):
the configuration version, used for ABI compatibility. It was decided
that if an application embeds Python, it sticks to a Python version
anyway, and so there is no need to bother with the ABI compatibility.
The final PyConfig API of :pep:`587` is excluded from the limited C API
since its main ``PyConfig`` structure is not versioned. Python cannot
guarantee ABI backward and forward compatibility, it's incompatible with
the stable ABI.
Since PyConfig was added to Python 3.8, the limited C API and the stable
ABI are getting more popular. For example, Rust bindings such as the
`PyO3 project <https://pyo3.rs/>`_ can target the limited C API to embed
Python in Rust (but it's not the default). In practice, PyO3 can use
non-limited C API for specific needs, but using them avoids the stable
ABI advantages.
Limitations of the legacy API
-----------------------------
The legacy API to configure the Python initialization is based on the
legacy ``Py_Initialize()`` function. It is now mostly deprecated:
* Set the initialization configuration such as ``Py_SetPath()``:
deprecated in Python 3.11 and removed in Python 3.13.
* Global configuration variables such as ``Py_VerboseFlag``:
deprecated in Python 3.12 and scheduled for removal in Python 3.14.
* Get the current configuration such as ``Py_GetPath()``:
deprecated in Python 3.13 and scheduled for removal in Python 3.15.
The legacy API doesn't support the "Python Configuration" and the
"Isolated Configuration" of PEP 587 PyConfig API. It only provides a
"legacy configuration" of ``Py_Initialize()`` which is in the between,
and also uses the legacy global configuration variables (such as
``Py_VerboseFlag``).
Some options can only by set by environment variables, such as ``home``
set by the ``PYTHONHOME`` environment variable. The problem is that
environment variables are inherited by child processes which can be a
surprising and unwanted behavior.
Some configuration options, such as ``configure_locale``, simply cannot
be set.
Limitations of the limited C API
--------------------------------
The limited C API is a subset of the legacy API. For example,
global configuration variables, such as ``Py_VerboseFlag``, are not
part of the limited C API.
While some functions were removed from the limited C API version 3.13,
they are still part of the stable ABI. For example, building a
application with the limited C API version 3.12 can still run with
Python 3.13 stable ABI.
Get the runtime configuration
-----------------------------
:pep:`587` has no API to **get** the **current** runtime configuration,
only to **configure** the Python **initialization**.
For example, the global configuration variable
``Py_UnbufferedStdioFlag`` was deprecated in Python 3.12 and using
``PyConfig.buffered_stdio`` is recommended instead. It only works to
configure Python, there is no public API to get
``PyConfig.buffered_stdio``.
Users of the limited C API are asking for a public API to get the
current runtime configuration.
Cython needs to get the ``optimization_level`` configuration option:
`issue <https://github.com/python/cpython/issues/99872>`_.
When global configuration variables were deprecated in 2022, `Marc-André
Lemburg requested
<https://github.com/python/cpython/issues/93103#issuecomment-1136462708>`__
a C API to access these configuration variables at runtime (not only
during Python initialization).
Security fix
------------
To fix `CVE-2020-10735
<https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2020-10735>`_,
a denial-of-service when converting very a large string to an integer (in base
10), it was discussed to add a new ``PyConfig`` member to stable
branches which affects the ABI.
Gregory P. Smith proposed a different API using text based configuration
file to not be limited by ``PyConfig`` members: `FR: Allow private
runtime config to enable extending without breaking the PyConfig ABI
<https://discuss.python.org/t/fr-allow-private-runtime-config-to-enable-extending-without-breaking-the-pyconfig-abi/18004>`__
(August 2022).
In the end, it was decided to not add a new ``PyConfig`` member to
stable branches, but only add a new ``PyConfig.int_max_str_digits``
member to the development branch (which became Python 3.12). A dedicated
private global variable (unrelated to ``PyConfig``) is used in stable
branches.
Redundancy between PyPreConfig and PyConfig
-------------------------------------------
The Python preinitialization uses the ``PyPreConfig`` structure and the
Python initialization uses the ``PyConfig`` structure. Both structures
have four duplicated members: ``dev_mode``, ``parse_argv``, ``isolated``
and ``use_environment``.
The redundancy is caused by the fact that the two structures are
separated, whereas some ``PyConfig`` members are needed by the
preinitialization.
Embedding Python
----------------
Applications embedding Python
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Examples:
* `Blender 3D graphics <https://www.blender.org/>`_.
* `fontforge <https://fontforge.org/>`_ font editor.
* `Gimp <https://www.gimp.org/>`_.
* `LibreOffice <https://www.libreoffice.org/>`_.
* `OBS Studio <https://obsproject.com/>`_.
* `Tiled <https://www.mapeditor.org/>`_.
* `vim <https://www.vim.org/>`_ text editor.
On Linux, FreeBSD and macOS, applications are usually either statically
linked to a ``libpython``, or load dynamically a ``libpython`` . The
``libpython`` shared library is versioned, example:
``libpython3.12.so`` for Python 3.12 on Linux.
The vim project can target the stable ABI. Usually, the "system Python"
version is used. It's not currently possible to select which Python
version to use. Users would like the ability to select a newer Python
on demand.
On Linux, another approach to deploy an application embedding Python,
such as GIMP, is to include Python in Flatpack, AppImage or Snap
"container". In this case, the application brings its own copy of Python
version with the container.
Libraries embedding Python
^^^^^^^^^^^^^^^^^^^^^^^^^^
Examples:
* `Apache mod_wsgi <https://modwsgi.readthedocs.io/>`_
(`source <https://github.com/GrahamDumpleton/mod_wsgi/blob/f54eadd6da8e3da0faccd497d4165de435b97242/src/server/wsgi_interp.c#L2367-L2404>`__).
* `nimpy <https://github.com/yglukhov/nimpy>`_:
Nim - Python bridge.
* `PyO3 <https://github.com/PyO3/pyo3>`__:
Rust bindings for the Python interpreter.
Utilities creating standalone applications
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
* `py2app <https://py2app.readthedocs.io/>`_ for macOS.
* `py2exe <http://www.py2exe.org/>`_ for Windows.
* `pyinstaller <https://pyinstaller.org/>`_.
* `PyOxidizer <https://github.com/indygreg/PyOxidizer>`_:
it uses the PEP 587 PyConfig API.
These utilities create standalone applications, they are not linked to
libpython.
Usage of a stable ABI
---------------------
`Ronald Oussoren
<https://discuss.python.org/t/fr-allow-private-runtime-config-to-enable-extending-without-breaking-the-pyconfig-abi/18004/9>`__:
For tools like py2app/py2exe/pyinstaller, it is pretty
**inconvenient to have to rebuild the launcher executable** that’s
used to start the packaged application when there’s a bug fix
release of Python.
`Gregory P. Smith
<https://discuss.python.org/t/fr-allow-private-runtime-config-to-enable-extending-without-breaking-the-pyconfig-abi/18004/10>`__:
You can’t **extend a struct** and **assume embedding people all
rebuild**. They don’t. Real world embedding uses exist that use an
installed Python minor version as a **shared library**. Update that to
use a different sized struct in a public API and someone is going to
have a bad time. That’s why I consider the struct frozen at rc1
time, even when only for use in the embedding / writing their own
launcher case.
`Colton Murphy
<https://discuss.python.org/t/fr-allow-private-runtime-config-to-enable-extending-without-breaking-the-pyconfig-abi/18004/11>`__:
I am trying to **embed the Python interpreter** using a **non C
language**. I have to stick with the limited API and private
structures for configuration in headers files is a no-no. Basically,
I need to be able to allocate and configure everything using only
exportable functions and the heap… no private structure details.
(...)
I am strictly limited to what’s in the **shared library** (DLL). I
**don’t have headers**, I can’t statically “recompile” every time a
new version of python comes out. That’s unmaintainable for me.
Quotes of Milian Wolff's `message
<https://discuss.python.org/t/pep-741-python-configuration-c-api-second-version/45403/4>`__:
Our application is a large complex C++ code base with lots of
dependencies targeting all three major desktop platforms.
Originally, we hoped to be able to use the **stable python ABI** to
allow biologists to **“bring your own python”**. The idea was that
they probably have a custom set of python libraries and code that
they would like to continue using. Our integrated API - so we
thought - was a tiny addition that should work with any Python out
there, so we used the stable ABI.
This turned out to be a dead end, and I believe we can (should?) now
use the non-stable ABI of python. Allowing end users to BYO Python
caused far too much setup problems and support issues for us that it
was not worth it in the end. Instead, we now rather want to ship a
custom Python with a custom prefix that they can pip install custom
libraries into as needed.
The problems we faced are not directly related to the stable ABI -
quite the contrary. Rather, it was due to thirdparty python
libraries that we shipped which themselves are not compatible across
python version increments. E.g. for the integrated console we use
qtconsole/jupyter, which worked in an archaic version with python
3.9 but requires newer versions for python 3.11+.
The ton of dependencies pulled in by UMAP was even worse, with numba
and pydnndescent and llvmlite often taking months to support newer
Python versions.
`David Hewitt <https://discuss.python.org/t/pep-741-python-configuration-c-api/43637/38>`__ of the PyO3 project:
I think making the configuration structure opaque and using an API
to set/get configuration by name is a welcome simplification:
* It’s a **smaller API** for language bindings like PyO3 to wrap and
re-expose, and
* It’s **easier** for people to **support multiple Python versions
to embed into their application**; no need to conditionally
compile structure field access, can just use normal error handling
if configuration values are not available for a specific version
at runtime.
Quotes of `Paul P. message <https://discuss.python.org/t/pep-741-python-configuration-c-api-second-version/45403/5>`__:
I cannot agree more, it is the same story everywhere/every time
CPython must be **embedded**. I maintened a runtime+ecosystem for
Android 4.4+ for some time (in order more comfortably use Panda3D
standalone than with Kivy), patching CPython and making a CI for it
was ok.
But I had to give up, because I had often to recompile every known
modules: this is not sustainable for one individual.
So I dropped the Android arch to only go WebAssembly (Emscripten).
But same (hard and boring) problem as always: have to rebuild
numerous packages that are commonly used with 2D/3D framework. (...)
Except for ONE, Harfang3d. I did not rebuild this one since Python
3.11 initial port… Guess why? it is a limited C API - **abi3
module**!
Limited API abi3 are fresh air, fast and portable. And associated
with a **stable config runtime**, it would be just perfect way!
See also `issue gh-116139
<https://github.com/python/cpython/issues/116139>`_ building an
application embedding Python 3.11 and attempting to run it with Python
3.10: it does crash because the ``PyConfig`` structure ABI is not stable
between two Python 3.x minor releases.
Set the runtime configuration
-----------------------------
`Marc-André Lemburg requested
<https://discuss.python.org/t/fr-allow-private-runtime-config-to-enable-extending-without-breaking-the-pyconfig-abi/18004/34>`__
a C API to **set** the value of some configuration options at runtime:
* ``optimization_level``
* ``verbose``
* ``parser_debug``
* ``inspect``
* ``write_bytecode``
Previously, it was possible to set directly global configuration
variables:
* ``Py_OptimizeFlag``
* ``Py_VerboseFlag``
* ``Py_DebugFlag``
* ``Py_InspectFlag``
* ``Py_DontWriteBytecodeFlag``
But these configuration flags were deprecated in Python 3.12 and are
scheduled for removal in Python 3.14.
Specification
=============
Add C API functions and structure to configure the Python
initialization:
* Create config:
* ``PyInitConfig`` opaque structure.
* ``PyInitConfig_CreatePython()``.
* ``PyInitConfig_CreateIsolated()``.
* ``PyInitConfig_Free(config)``.
* Get options:
* ``PyInitConfig_HasOption(config, name)``.
* ``PyInitConfig_GetInt(config, name, &value)``.
* ``PyInitConfig_GetStr(config, name, &value)``.
* ``PyInitConfig_GetWStr(config, name, &value)``.
* ``PyInitConfig_GetStrList(config, name, &length, &items)``.
* ``PyInitConfig_FreeStrList()``.
* ``PyInitConfig_GetWStrList(config, name, &length, &items)``.
* ``PyInitConfig_FreeWStrList()``.
* Set options:
* ``PyInitConfig_SetInt(config, name, value)``.
* ``PyInitConfig_SetStr(config, name, value)``.
* ``PyInitConfig_SetStrLocale(config, name, value)``.
* ``PyInitConfig_SetWStr(config, name, value)``.
* ``PyInitConfig_SetStrList(config, name, length, items)``.
* ``PyInitConfig_SetStrLocaleList(config, name, length, items)``.
* ``PyInitConfig_SetWStrList(config, name, length, items)``.
* ``PyInitConfig_AddModule(config, name, initfunc)``
* Initialize:
* ``Py_PreInitializeFromInitConfig(config)``.
* ``Py_InitializeFromInitConfig(config)``.
* Error handling:
* ``PyInitConfig_GetError(config, &err_msg)``.
* ``PyInitConfig_GetExitcode(config, &exitcode)``.
Add C API functions to get and set the current runtime configuration:
* ``PyConfig_Get(name)`` → ``object``.
* ``PyConfig_GetInt(name, &value)``.
* ``PyConfig_Set(name)``.
* ``PyConfig_Names()`` → ``frozenset``.
The C API uses null-terminated UTF-8 encoded strings to refer to a
configuration option.
All C API functions are added to the limited C API version 3.13.
The ``PyInitConfig`` structure is implemented by combining the four
structures of the ``PyConfig`` API and has an ``inittab`` member as
well:
* ``PyPreConfig preconfig``
* ``PyConfig config``
* ``PyStatus status``
* ``struct _inittab *inittab`` for ``PyInitConfig_AddModule()``
The ``PyStatus`` status is no longer separated, but part of the unified
``PyInitConfig`` structure, which makes the API easier to use.
Configuration Options
---------------------
Configuration options are named after ``PyPreConfig`` and ``PyConfig``
structure members. See the `PyPreConfig documentation
<https://docs.python.org/dev/c-api/init_config.html#pypreconfig>`_ and
the `PyConfig documentation
<https://docs.python.org/dev/c-api/init_config.html#pyconfig>`_.
Deprecating and removing configuration options is out of the scope of
the PEP and should be discussed on a case by case basis.
Public configuration options
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Following options can be get by ``PyConfig_Get()`` and set and
``PyConfig_Set()``.
.. list-table::
:widths: 20 20 50
:header-rows: 1
* - Option
- Type
- Comment
* - ``argv``
- ``list[str]``
- API: ``sys.argv``.
* - ``base_exec_prefix``
- ``str``
- API: ``sys.base_exec_prefix``.
* - ``base_executable``
- ``str``
- API: ``sys.base_executable``.
* - ``base_prefix``
- ``str``
- API: ``sys.base_prefix``.
* - ``bytes_warning``
- ``int``
- API: ``sys.flags.bytes_warning``.
* - ``exec_prefix``
- ``str``
- API: ``sys.base_prefix``.
* - ``executable``
- ``str``
- API: ``sys.executable``.
* - ``inspect``
- ``bool``
- API: ``sys.flags.inspect`` (``int``).
* - ``int_max_str_digits``
- ``int``
- API: ``sys.flags.int_max_str_digits``,
``sys.get_int_max_str_digits()`` and
``sys.set_int_max_str_digits()``.
* - ``interactive``
- ``bool``
- API: ``sys.flags.interactive``.
* - ``module_search_paths``
- ``list[str]``
- API: ``sys.path``.
* - ``optimization_level``
- ``int``
- API: ``sys.flags.optimize``.
* - ``parser_debug``
- ``bool``
- API: ``sys.flags.debug`` (``int``).
* - ``platlibdir``
- ``str``
- API: ``sys.platlibdir``.
* - ``prefix``
- ``str``
- API: ``sys.base_prefix``.
* - ``pycache_prefix``
- ``str``
- API: ``sys.pycache_prefix``.
* - ``quiet``
- ``bool``
- API: ``sys.flags.quiet`` (``int``).
* - ``stdlib_dir``
- ``str``
- API: ``sys._stdlib_dir``.
* - ``use_environment``
- ``bool``
- API: ``sys.flags.ignore_environment`` (``int``).
* - ``verbose``
- ``int``
- API: ``sys.flags.verbose``.
* - ``warnoptions``
- ``list[str]``
- API: ``sys.warnoptions``.
* - ``write_bytecode``
- ``bool``
- API: ``sys.flags.dont_write_bytecode`` (``int``) and ``sys.dont_write_bytecode`` (``bool``).
* - ``xoptions``
- ``dict[str, str]``
- API: ``sys._xoptions``.
Some option names are different than ``sys`` attributes, such as
``optimization_level`` option and ``sys.flags.optimize`` attribute.
``PyConfig_Set()`` sets the corresponding ``sys`` attribute.
The ``xoptions`` is a list of strings in ``PyInitConfig`` where each
string has the format ``key`` (*value* is ``True`` implicitly) or
``key=value``. In the current runtime configuration, it becomes a
dictionary (``key: str`` → ``value: str | True``).
Read-only configuration options
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Following options can be get ``PyConfig_Get()``, but cannot be set by
``PyConfig_Set()``.
.. list-table::
:widths: 20 20 50
:header-rows: 1
* - Option
- Type
- Comment
* - ``allocator``
- ``int``
-
* - ``buffered_stdio``
- ``bool``
-
* - ``check_hash_pycs_mode``
- ``str``
- API: ``imp.check_hash_pycs_mode``.
* - ``code_debug_ranges``
- ``bool``
-
* - ``coerce_c_locale``
- ``bool``
-
* - ``coerce_c_locale_warn``
- ``bool``
-
* - ``configure_c_stdio``
- ``bool``
-
* - ``configure_locale``
- ``bool``
-
* - ``cpu_count``
- ``int``
- API: ``os.cpu_count()`` (``int | None``).
* - ``dev_mode``
- ``bool``
- API: ``sys.flags.dev_mode``.
* - ``dump_refs``
- ``bool``
-
* - ``dump_refs_file``
- ``str``
-
* - ``faulthandler``
- ``bool``
- API: ``faulthandler.is_enabled()``.
* - ``filesystem_encoding``
- ``str``
- API: ``sys.getfilesystemencoding()``.
* - ``filesystem_errors``
- ``str``
- API: ``sys.getfilesystemencodeerrors()``.
* - ``hash_seed``
- ``int``
-
* - ``home``
- ``str``
-
* - ``import_time``
- ``bool``
-
* - ``install_signal_handlers``
- ``bool``
-
* - ``isolated``
- ``bool``
- API: ``sys.flags.isolated`` (``int``).
* - ``legacy_windows_fs_encoding``
- ``bool``
-
* - ``legacy_windows_stdio``
- ``bool``
- Windows only
* - ``malloc_stats``
- ``bool``
-
* - ``module_search_paths_set``
- ``bool``
-
* - ``orig_argv``
- ``list[str]``
- API: ``sys.orig_argv``.
* - ``pathconfig_warnings``
- ``bool``
-
* - ``parse_argv``
- ``bool``
-
* - ``perf_profiling``
- ``bool``
- API: ``sys.is_stack_trampoline_active()``.
* - ``program_name``
- ``str``
-
* - ``pythonpath_env``
- ``str``
-
* - ``run_command``
- ``str``
-
* - ``run_filename``
- ``str``
-
* - ``run_module``
- ``str``
-
* - ``run_presite``
- ``str``
- need a debug build.
* - ``safe_path``
- ``bool``
-
* - ``show_ref_count``
- ``bool``
-
* - ``site_import``
- ``bool``
- API: ``sys.flags.no_site`` (``int``).
* - ``skip_source_first_line``
- ``bool``
-
* - ``stdio_encoding``
- ``str``
- API: ``sys.stdin.encoding``, ``sys.stdout.encoding`` and
``sys.stderr.encoding``.
* - ``stdio_errors``
- ``str``
- API: ``sys.stdin.errors``, ``sys.stdout.errors`` and
``sys.stderr.errors``.
* - ``sys_path_0``
- ``str``
-
* - ``tracemalloc``
- ``int``
- API: ``tracemalloc.is_tracing()`` (``bool``).
* - ``use_frozen_modules``
- ``bool``
-
* - ``use_hash_seed``
- ``bool``
-
* - ``utf8_mode``
- ``bool``
-
* - ``user_site_directory``
- ``bool``
- API: ``sys.flags.no_user_site`` (``int``).
* - ``warn_default_encoding``
- ``bool``
-
* - ``_install_importlib``
- ``bool``
-
* - ``_init_main``
- ``bool``
-
* - ``_is_python_build``
- ``bool``
-
* - ``_pystats``
- ``bool``
- API: ``sys._stats_on()``, ``sys._stats_off()``.
Need a ``Py_STATS`` build.
Preinitialization
-----------------
Calling ``Py_PreInitializeFromInitConfig()`` preinitializes Python. For
example, it sets the memory allocator, and can configure the
``LC_CTYPE`` locale and configure the standard C streams such as
``stdin`` and ``stdout``.
The following options can only be set during the Python
preinitialization:
* ``allocator``,
* ``coerce_c_locale``,
* ``coerce_c_locale_warn``,
* ``configure_locale``,
* ``legacy_windows_fs_encoding``,
* ``utf8_mode``.
Trying to set these options after Python preinitialization fails with an
error.
``PyInitConfig_SetStrLocale()`` and ``PyInitConfig_SetStrLocaleList()``
functions call ``Py_PreInitializeFromInitConfig()`` if Python is not
already preinitialized.
Create Config
-------------
``PyInitConfig`` structure:
Opaque structure to configure the Python preinitialization and the
Python initialization.
``PyInitConfig* PyInitConfig_CreatePython(void)``:
Create a new initialization configuration using default values
of the `Python Configuration
<https://docs.python.org/dev/c-api/init_config.html#python-configuration>`_.
It must be freed with ``PyInitConfig_Free()``.
Return ``NULL`` on memory allocation failure.
``PyInitConfig* PyInitConfig_CreateIsolated(void)``:
Similar to ``PyInitConfig_CreatePython()``, but use default values
of the `Isolated Configuration
<https://docs.python.org/dev/c-api/init_config.html#isolated-configuration>`_.
``void PyInitConfig_Free(PyInitConfig *config)``:
Free memory of an initialization configuration.
Get Options
-----------
The configuration option *name* parameter must be a non-NULL
null-terminated UTF-8 encoded string.
``int PyInitConfig_HasOption(PyInitConfig *config, const char *name)``:
Test if the configuration has an option called *name*.
Return ``1`` if the option exists, or return ``0`` otherwise.
``int PyInitConfig_GetInt(PyInitConfig *config, const char *name, int64_t *value)``:
Get an integer configuration option.
* Set *\*value*, and return ``0`` on success.
* Set an error in *config* and return ``-1`` on error.
``int PyInitConfig_GetStr(PyInitConfig *config, const char *name, char **value)``:
Get a string configuration option as a null-terminated UTF-8
encoded string.
* Set *\*value*, and return ``0`` on success.
* Set an error in *config* and return ``-1`` on error.
On success, the string must be released with ``free(value)``.
``int PyInitConfig_GetWStr(PyInitConfig *config, const char *name, wchar_t **value)``:
Get a string configuration option as a null-terminated wide string.
* Set *\*value* and return ``0`` on success.
* Set an error in *config* and return ``-1`` on error.
On success, the string must be released with ``free(value)``.
``int PyInitConfig_GetStrList(PyInitConfig *config, const char *name, size_t *length, char ***items)``:
Get a string list configuration option as an array of
null-terminated UTF-8 encoded strings.
* Set *\*length* and *\*value*, and return ``0`` on success.
* Set an error in *config* and return ``-1`` on error.
On success, the string list must be released with
``PyInitConfig_FreeStrList(length, items)``.
``void PyInitConfig_FreeStrList(size_t length, char **items)``:
Free memory of a string list created by
``PyInitConfig_GetStrList()``.
``int PyInitConfig_GetWStrList(PyInitConfig *config, const char *name, size_t *length, wchar_t ***items)``:
Get a string list configuration option as an array of
null-terminated wide strings.
* Set *\*length* and *\*value*, and return ``0`` on success.
* Set an error in *config* and return ``-1`` on error.
On success, the string list must be released with
``PyInitConfig_FreeWStrList(length, items)``.
``void PyInitConfig_FreeWStrList(size_t length, wchar_t **items)``:
Free memory of a string list created by
``PyInitConfig_GetWStrList()``.
Set Options
-----------
The configuration option *name* parameter must be a non-NULL
null-terminated UTF-8 encoded string.
Some configuration options have side effects on other options. This
logic is only implemented when ``Py_InitializeFromInitConfig()`` is
called, not by the "Set" functions below. For example, setting
``dev_mode`` to ``1`` does not set ``faulthandler`` to ``1``.
``int PyInitConfig_SetInt(PyInitConfig *config, const char *name, int64_t value)``:
Set an integer configuration option.
* Return ``0`` on success.
* Set an error in *config* and return ``-1`` on error.
``int PyInitConfig_SetStr(PyInitConfig *config, const char *name, const char *value)``:
Set a string configuration option from a null-terminated UTF-8
encoded string. The string is copied.
* Return ``0`` on success.
* Set an error in *config* and return ``-1`` on error.
``int PyInitConfig_SetStrLocale(PyInitConfig *config, const char *name, const char *value)``:
Set a string configuration option from a null-terminated bytes
string encoded in the locale encoding. The string is copied.
The bytes string is decoded by ``Py_DecodeLocale()``.
``Py_PreInitializeFromInitConfig()`` must be called before calling
this function.
* Return ``0`` on success.
* Set an error in *config* and return ``-1`` on error.
``int PyInitConfig_SetWStr(PyInitConfig *config, const char *name, const wchar_t *value)``:
Set a string configuration option from a null-terminated wide
string. The string is copied.
* Return ``0`` on success.
* Set an error in *config* and return ``-1`` on error.
``int PyInitConfig_SetStrList(PyInitConfig *config, const char *name, size_t length, char * const *items)``:
Set a string list configuration option from an array of
null-terminated UTF-8 encoded strings. The string list is copied.
* Return ``0`` on success.
* Set an error in *config* and return ``-1`` on error.
``int PyInitConfig_SetStrLocaleList(PyInitConfig *config, const char *name, size_t length, char * const *items)``:
Set a string list configuration option from an array of
null-terminated bytes strings encoded in the locale encoding.
The string list is copied.
The bytes string is decoded by :c:func:`Py_DecodeLocale`.
``Py_PreInitializeFromInitConfig()`` must be called before calling
this function.
* Return ``0`` on success.
* Set an error in *config* and return ``-1`` on error.
``int PyInitConfig_SetWStrList(PyInitConfig *config, const char *name, size_t length, wchar_t * const *items)``:
Set a string list configuration option from an error of
null-terminated wide strings. The string list is copied.
* Return ``0`` on success.
* Set an error in *config* and return ``-1`` on error.
``int PyInitConfig_AddModule(PyInitConfig *config, const char *name, PyObject* (*initfunc)(void))``:
Add a built-in extension module to the table of built-in modules.
The new module can be imported by the name *name*, and uses the
function *initfunc* as the initialization function called on the
first attempted import.
* Return ``0`` on success.
* Set an error in *config* and return ``-1`` on error.
If Python is initialized multiple times,
``PyInitConfig_AddModule()`` must be called at each Python
initialization.
Similar to the ``PyImport_AppendInittab()`` function.
Initialize Python
-----------------
``int Py_PreInitializeFromInitConfig(PyInitConfig *config)``:
Preinitialize Python from the initialization configuration.
* Return ``0`` on success.
* Set an error in *config* and return ``-1`` on error.
``int Py_InitializeFromInitConfig(PyInitConfig *config)``:
Initialize Python from the initialization configuration.
* Return ``0`` on success.
* Set an error in *config* and return ``-1`` on error.
* Set an exit code in *config* and return ``-1`` if Python wants to
exit.
See ``PyInitConfig_GetExitcode()`` for the exitcode case.
Error Handling
--------------
``int PyInitConfig_GetError(PyInitConfig* config, const char **err_msg)``:
Get the *config* error message.
* Set *\*err_msg* and return ``1`` if an error is set.
* Set *\*err_msg* to ``NULL`` and return ``0`` otherwise.
An error message is an UTF-8 encoded string.
If *config* has an exit code, format the exit code as an error
message.
The error message remains valid until another ``PyInitConfig``
function is called with *config*. The caller doesn't have to free the
error message.
``int PyInitConfig_GetExitcode(PyInitConfig* config, int *exitcode)``:
Get the *config* exit code.
* Set *\*exitcode* and return ``1`` if Python wants to exit.
* Return ``0`` if *config* has no exit code set.
Only the ``Py_InitializeFromInitConfig()`` function can set an exit
code if the ``parse_argv`` option is non-zero. For example, an
isolated configuration cannot set an exit code by default, since
``parse_argv`` is zero by default.
An exit code can be set when parsing the command line failed (exit
code 2) or when a command line option asks to display the command
line help (exit code 0).
Get and Set the Runtime Configuration
-------------------------------------
The configuration option *name* parameter must be a non-NULL
null-terminated UTF-8 encoded string.
``PyObject* PyConfig_Get(const char *name)``:
Get the current runtime value of a configuration option as an
object.
* Return a new reference on success.
* Set an exception and return ``NULL`` on error.
The object type depends on the option: see `Configuration Options`_
tables.
Other options are get from internal ``PyPreConfig`` and ``PyConfig`` structures.
The caller must hold the GIL. The function cannot be called before
Python initialization nor after Python finalization.
``int PyConfig_GetInt(const char *name, int *value)``:
Similar to ``PyConfig_Get()``, but get the value as an integer.
* Set ``*value`` and return ``0`` success.
* Set an exception and return ``-1`` on error.
``PyObject* PyConfig_Names(void)``:
Get all configuration option names as a ``frozenset``.
Set an exception and return ``NULL`` on error.
The caller must hold the GIL.
``PyObject* PyConfig_Set(const char *name, PyObject *value)``:
Set the current runtime value of a configuration option.
* Raise a ``ValueError`` if there is no option *name*.
* Raise a ``ValueError`` if *value* is an invalid value.
* Raise a ``ValueError`` if the option is read-only: cannot be set.
* Raise a ``TypeError`` if *value* has not the proper type.
`Read-only configuration options`_ cannot be set.
The caller must hold the GIL. The function cannot be called before
Python initialization nor after Python finalization.
Scope of the stable ABI
-----------------------
The limited C API and the stable ABI added by this PEP only provide a
stable interface to program the Python initialization.
The behavior of options, the default option values, and the Python
behavior can change at each Python version: they are not "stable".
Moreover, configuration options can be added, deprecated and removed
following the usual :pep:`387` deprecation process.
Examples
========
Initialize Python
-----------------
Example initializing Python, set configuration options of different types,
return -1 on error:
.. code-block:: c
int init_python(void)
{
PyInitConfig *config = PyInitConfig_CreatePython();
if (config == NULL) {
printf("PYTHON INIT ERROR: memory allocation failed\n");
return -1;
}
// Set an integer (dev mode)
if (PyInitConfig_SetInt(config, "dev_mode", 1) < 0) {
goto error;
}
// Set a list of wide strings (argv)
wchar_t *argv[] = {L"my_program", L"-c", L"pass"};
if (PyInitConfig_SetWStrList(config, "argv",
Py_ARRAY_LENGTH(argv), argv) < 0) {
goto error;
}
// Set a wide string (program name)
if (PyInitConfig_SetWStr(config, "program_name", L"my_program") < 0) {
goto error;
}
// Set a list of bytes strings (xoptions).
// Preinitialize implicitly Python to decode the bytes string.
char* xoptions[] = {"faulthandler"};
if (PyInitConfig_SetStrList(config, "xoptions",
Py_ARRAY_LENGTH(xoptions), xoptions) < 0) {
goto error;
}
// Initialize Python with the configuration
if (Py_InitializeFromInitConfig(config) < 0) {
goto error;
}
PyInitConfig_Free(config);
return 0;
error:
// Display the error message
const char *err_msg;
(void)PyInitConfig_GetError(config, &err_msg);
printf("PYTHON INIT ERROR: %s\n", err_msg);
PyInitConfig_Free(config);
return -1;
}
Increase initialization bytes_warning option
--------------------------------------------
Example increasing the ``bytes_warning`` option of an initialization
configuration:
.. code-block:: c
int config_bytes_warning(PyInitConfig *config)
{
int bytes_warning;
if (PyInitConfig_GetInt(config, "bytes_warning", &bytes_warning)) {
return -1;
}
bytes_warning += 1;
if (PyInitConfig_SetInt(config, "bytes_warning", bytes_warning)) {
return -1;
}
return 0;
}
Get the runtime verbose option
------------------------------
Example getting the current runtime value of the configuration option
``verbose``:
.. code-block:: c
int get_verbose(void)
{
int verbose;
if (PyConfig_GetInt("verbose", &verbose) < 0) {
// Silently ignore the error
PyErr_Clear();
return -1;
}
return verbose;
}
On error, the function silently ignores the error and returns ``-1``. In
practice, getting the ``verbose`` option cannot fail, unless a future
Python version removes the option.
Implementation
==============
* Issue: `No limited C API to customize Python initialization
<https://github.com/python/cpython/issues/107954>`_
* PR: `Add PyInitConfig C API
<https://github.com/python/cpython/pull/110176>`_
* PR: `Add PyConfig_Get() function
<https://github.com/python/cpython/pull/112609>`_
Backwards Compatibility
=======================
Changes are fully backward compatible. Only new APIs are added.
Existing API such as the ``PyConfig`` C API (PEP 587) are left
unchanged.
Rejected Ideas
==============
Configuration as text
---------------------
It was proposed to provide the configuration as text to make the API
compatible with the stable ABI and to allow custom options.
Example::
# integer
bytes_warning = 2
# string
filesystem_encoding = "utf8" # comment
# list of strings
argv = ['python', '-c', 'code']
The API would take the configuration as a string, not as a file. Example
with a hypothetical ``PyInit_SetConfig()`` function:
.. code-block:: c
void stable_abi_init_demo(int set_path)
{
PyInit_SetConfig(
"isolated = 1\n"
"argv = ['python', '-c', 'code']\n"
"filesystem_encoding = 'utf-8'\n"
);
if (set_path) {
PyInit_SetConfig("pythonpath = '/my/path'");
}
}
The example ignores error handling to make it easier to read.
The problem is that generating such configuration text requires adding
quotes to strings and to escape quotes in strings. Formatting an array
of strings becomes non-trivial.
Providing an API to format a string or an array of strings is not really
worth it, whereas Python can provide directly an API to set a
configuration option where the value is passed directly as a string or
an array of strings. It avoids giving special meaning to some
characters, such as newline characters, which would have to be escaped.
Refer to an option with an integer
----------------------------------
Using strings to refer to a configuration option requires comparing
strings which can be slower than comparing integers.
Use integers, similar to type "slots" such as ``Py_tp_doc``, to refer to
a configuration option. The ``const char *name`` parameter is replaced
with ``int option``.
Accepting custom options is more likely to cause conflicts when using
integers, since it's harder to maintain "namespaces" (ranges) for
integer options. Using strings, a simple prefix with a colon separator
can be used.
Integers also requires maintaining a list of integer constants and so
make the C API and the Python API larger.
Python 3.13 only has around 62 configuration options, and so performance
is not really a blocker issue. If better performance is needed later, a
hash table can be used to get an option by its name.
If getting a configuration option is used in hot code, the value can be
read once and cached. By the way, most configuration options cannot be
changed at runtime.
Fully remove the preinitialization
----------------------------------
Delay decoding
^^^^^^^^^^^^^^
Without ``PyInitConfig_Get*()`` functions, it would be possible to store
``PyInitConfig_SetStrLocale()`` and ``PyInitConfig_SetStrLocaleList()``
strings encoded and only initialize the ``LC_CTYPE`` locale and
decode the strings in ``Py_InitializeFromInitConfig()``.
The problem is that users asked for ``PyInitConfig_Get*()`` functions.
For example, ``PyInitConfig_GetStr()`` must decode the string from the
locale encoding and then encode it to the UTF-8 encoding.
However, if ``PyInitConfig_SetStrLocale()`` and
``PyInitConfig_SetStrLocaleList()`` strings are decoded as designed by
the PEP, there is no risk of mojibake: ``PyInitConfig_GetStr()`` returns
the expected decoded strings.
Remove the Python configuration
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
If ``PyInitConfig_CreatePython()`` is removed, the preinitialization is
no longer needed since the ``LC_CTYPE`` is not configured by default by
``PyInitConfig_CreateIsolated()`` and setting ``"configure_locale"``
option can always fail.
The problem is that users asked to be able to write their own customized
Python, so have a Python-like program but with a different default
configuration. The ``PyInitConfig_CreatePython()`` function is needed
for that.
Moreover, the Python configuration is also part of the :pep:`587`
design, implemented in Python 3.8.
Disallow setting the ``"configure_locale"`` option has similar issues.
Multi-phase initialization (similar to PEP 432)
-----------------------------------------------
`Eric Snow expressed concerns <https://discuss.python.org/t/pep-741-python-configuration-c-api-second-version/45403/27>`_
that this proposal might reinforce with embedders the idea that
initialization is a single monolithic step. He argued that initialization
involves 5 distinct phases and even suggested that the API should
reflect this explicitly. Eric proposed that, at the very least, the
implementation of initialization should reflect the phases, in part
for improved code health. Overall, his explanation has some
similarities with :pep:`432` and :pep:`587`.
Another of Eric's key points relevant to this PEP was that, ideally,
the config passed to ``Py_InitializeFromConfig()`` should be complete
before that function is called, whereas currently initialization
actually modifies the config.
While Eric wasn't necessarily suggesting an alternative to PEP 741,
any proposal to add a granular initialization API around phases is
effectively the opposite of what this PEP is trying to accomplish.
Such API is more complicated, it requires adding new public structures
and new public functions. It makes the Python initialization more
complicated, rather than this PEP tries to unifiy existing APIs and make
them simpler (the opposite). Having multiple structures for similar
purpose can lead to duplicate members, similar issue than duplicated
members between existing ``PyPreConfig`` and ``PyConfig`` structures.
Discussions
===========
* `PEP 741: Python Configuration C API (second version)
<https://discuss.python.org/t/pep-741-python-configuration-c-api-second-version/45403>`_
(February 2024).
* `PEP 741: Python Configuration C API
<https://discuss.python.org/t/pep-741-python-configuration-c-api/43637>`_
(January 2024).
* `FR: Allow private runtime config to enable extending without breaking
the PyConfig ABI
<https://discuss.python.org/t/fr-allow-private-runtime-config-to-enable-extending-without-breaking-the-pyconfig-abi/18004>`__
(August 2022).
Copyright
=========
This document is placed in the public domain or under the
CC0-1.0-Universal license, whichever is more permissive.
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