.. highlight:: c

.. _init-config:

***********************************
Python Initialization Configuration
***********************************

.. versionadded:: 3.8

Python can be initialized with :c:func:`Py_InitializeFromConfig` and the
:c:type:`PyConfig` structure. It can be preinitialized with
:c:func:`Py_PreInitialize` and the :c:type:`PyPreConfig` structure.

There are two kinds of configuration:

* The :ref:`Python Configuration <init-python-config>` can be used to build a
  customized Python which behaves as the regular Python. For example,
  environments variables and command line arguments are used to configure
  Python.

* The :ref:`Isolated Configuration <init-isolated-conf>` can be used to embed
  Python into an application. It isolates Python from the system. For example,
  environments variables are ignored, the LC_CTYPE locale is left unchanged and
  no signal handler is registred.

The :c:func:`Py_RunMain` function can be used to write a customized Python
program.

See also :ref:`Initialization, Finalization, and Threads <initialization>`.

.. seealso::
   :pep:`587` "Python Initialization Configuration".


Example
=======

Example of customized Python always running in isolated mode::

    int main(int argc, char **argv)
    {
        PyStatus status;

        PyConfig config;
        PyConfig_InitPythonConfig(&config);
        config.isolated = 1;

        /* Decode command line arguments.
           Implicitly preinitialize Python (in isolated mode). */
        status = PyConfig_SetBytesArgv(&config, argc, argv);
        if (PyStatus_Exception(status)) {
            goto exception;
        }

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

        return Py_RunMain();

    exception:
        PyConfig_Clear(&config);
        if (PyStatus_IsExit(status)) {
            return status.exitcode;
        }
        /* Display the error message and exit the process with
           non-zero exit code */
        Py_ExitStatusException(status);
    }


PyWideStringList
================

.. c:type:: PyWideStringList

   List of ``wchar_t*`` strings.

   If *length* is non-zero, *items* must be non-``NULL`` and all strings must be
   non-``NULL``.

   Methods:

   .. c:function:: PyStatus PyWideStringList_Append(PyWideStringList *list, const wchar_t *item)

      Append *item* to *list*.

      Python must be preinitialized to call this function.

   .. c:function:: PyStatus PyWideStringList_Insert(PyWideStringList *list, Py_ssize_t index, const wchar_t *item)

      Insert *item* into *list* at *index*.

      If *index* is greater than or equal to *list* length, append *item* to
      *list*.

      *index* must be greater than or equal to 0.

      Python must be preinitialized to call this function.

   Structure fields:

   .. c:member:: Py_ssize_t length

      List length.

   .. c:member:: wchar_t** items

      List items.

PyStatus
========

.. c:type:: PyStatus

   Structure to store an initialization function status: success, error
   or exit.

   For an error, it can store the C function name which created the error.

   Structure fields:

   .. c:member:: int exitcode

      Exit code. Argument passed to ``exit()``.

   .. c:member:: const char *err_msg

      Error message.

   .. c:member:: const char *func

      Name of the function which created an error, can be ``NULL``.

   Functions to create a status:

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

      Success.

   .. c:function:: PyStatus PyStatus_Error(const char *err_msg)

      Initialization error with a message.

      *err_msg* must not be ``NULL``.

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

      Memory allocation failure (out of memory).

   .. c:function:: PyStatus PyStatus_Exit(int exitcode)

      Exit Python with the specified exit code.

   Functions to handle a status:

   .. c:function:: int PyStatus_Exception(PyStatus status)

      Is the status an error or an exit? If true, the exception must be
      handled; by calling :c:func:`Py_ExitStatusException` for example.

   .. c:function:: int PyStatus_IsError(PyStatus status)

      Is the result an error?

   .. c:function:: int PyStatus_IsExit(PyStatus status)

      Is the result an exit?

   .. c:function:: void Py_ExitStatusException(PyStatus status)

      Call ``exit(exitcode)`` if *status* is an exit. Print the error
      message and exit with a non-zero exit code if *status* is an error.  Must
      only be called if ``PyStatus_Exception(status)`` is non-zero.

.. note::
   Internally, Python uses macros which set ``PyStatus.func``,
   whereas functions to create a status set ``func`` to ``NULL``.

Example::

    PyStatus alloc(void **ptr, size_t size)
    {
        *ptr = PyMem_RawMalloc(size);
        if (*ptr == NULL) {
            return PyStatus_NoMemory();
        }
        return PyStatus_Ok();
    }

    int main(int argc, char **argv)
    {
        void *ptr;
        PyStatus status = alloc(&ptr, 16);
        if (PyStatus_Exception(status)) {
            Py_ExitStatusException(status);
        }
        PyMem_Free(ptr);
        return 0;
    }


PyPreConfig
===========

.. c:type:: PyPreConfig

   Structure used to preinitialize Python.

   Function to initialize a preconfiguration:

   .. c:function:: void PyPreConfig_InitPythonConfig(PyPreConfig *preconfig)

      Initialize the preconfiguration with :ref:`Python Configuration
      <init-python-config>`.

   .. c:function:: void PyPreConfig_InitIsolatedConfig(PyPreConfig *preconfig)

      Initialize the preconfiguration with :ref:`Isolated Configuration
      <init-isolated-conf>`.

   Structure fields:

   .. c:member:: int allocator

      Name of the Python memory allocators:

      * ``PYMEM_ALLOCATOR_NOT_SET`` (``0``): don't change memory allocators
        (use defaults)
      * ``PYMEM_ALLOCATOR_DEFAULT`` (``1``): default memory allocators
      * ``PYMEM_ALLOCATOR_DEBUG`` (``2``): default memory allocators with
        debug hooks
      * ``PYMEM_ALLOCATOR_MALLOC`` (``3``): force usage of ``malloc()``
      * ``PYMEM_ALLOCATOR_MALLOC_DEBUG`` (``4``): force usage of
        ``malloc()`` with debug hooks
      * ``PYMEM_ALLOCATOR_PYMALLOC`` (``5``): :ref:`Python pymalloc memory
        allocator <pymalloc>`
      * ``PYMEM_ALLOCATOR_PYMALLOC_DEBUG`` (``6``): :ref:`Python pymalloc
        memory allocator <pymalloc>` with debug hooks

      ``PYMEM_ALLOCATOR_PYMALLOC`` and ``PYMEM_ALLOCATOR_PYMALLOC_DEBUG``
      are not supported if Python is configured using ``--without-pymalloc``

      See :ref:`Memory Management <memory>`.

      Default: ``PYMEM_ALLOCATOR_NOT_SET``.

   .. c:member:: int configure_locale

      Set the LC_CTYPE locale to the user preferred locale?

      If equals to 0, set :c:member:`~PyPreConfig.coerce_c_locale` and
      :c:member:`~PyPreConfig.coerce_c_locale_warn` members to 0.

      See the :term:`locale encoding`.

      Default: ``1`` in Python config, ``0`` in isolated config.

   .. c:member:: int coerce_c_locale

      If equals to 2, coerce the C locale.

      If equals to 1, read the LC_CTYPE locale to decide if it should be
      coerced.

      See the :term:`locale encoding`.

      Default: ``-1`` in Python config, ``0`` in isolated config.

   .. c:member:: int coerce_c_locale_warn

      If non-zero, emit a warning if the C locale is coerced.

      Default: ``-1`` in Python config, ``0`` in isolated config.

   .. c:member:: int dev_mode

      If non-zero, enables the :ref:`Python Development Mode <devmode>`:
      see :c:member:`PyConfig.dev_mode`.

      Default: ``-1`` in Python mode, ``0`` in isolated mode.

   .. c:member:: int isolated

      Isolated mode: see :c:member:`PyConfig.isolated`.

      Default: ``0`` in Python mode, ``1`` in isolated mode.

   .. c:member:: int legacy_windows_fs_encoding

      If non-zero:

      * Set :c:member:`PyPreConfig.utf8_mode` to ``0``,
      * Set :c:member:`PyConfig.filesystem_encoding` to ``"mbcs"``,
      * Set :c:member:`PyConfig.filesystem_errors` to ``"replace"``.

      Initialized the from :envvar:`PYTHONLEGACYWINDOWSFSENCODING` environment
      variable value.

      Only available on Windows. ``#ifdef MS_WINDOWS`` macro can be used for
      Windows specific code.

      Default: ``0``.

   .. c:member:: int parse_argv

      If non-zero, :c:func:`Py_PreInitializeFromArgs` and
      :c:func:`Py_PreInitializeFromBytesArgs` parse their ``argv`` argument the
      same way the regular Python parses command line arguments: see
      :ref:`Command Line Arguments <using-on-cmdline>`.

      Default: ``1`` in Python config, ``0`` in isolated config.

   .. c:member:: int use_environment

      Use :ref:`environment variables <using-on-envvars>`? See
      :c:member:`PyConfig.use_environment`.

      Default: ``1`` in Python config and ``0`` in isolated config.

   .. c:member:: int utf8_mode

      If non-zero, enable the :ref:`Python UTF-8 Mode <utf8-mode>`.

      Set by the :option:`-X utf8 <-X>` command line option and the
      :envvar:`PYTHONUTF8` environment variable.

      Default: ``-1`` in Python config and ``0`` in isolated config.


.. _c-preinit:

Preinitialize Python with PyPreConfig
=====================================

The preinitialization of Python:

* Set the Python memory allocators (:c:member:`PyPreConfig.allocator`)
* Configure the LC_CTYPE locale (:term:`locale encoding`)
* Set the :ref:`Python UTF-8 Mode <utf8-mode>`
  (:c:member:`PyPreConfig.utf8_mode`)

The current preconfiguration (``PyPreConfig`` type) is stored in
``_PyRuntime.preconfig``.

Functions to preinitialize Python:

.. c:function:: PyStatus Py_PreInitialize(const PyPreConfig *preconfig)

   Preinitialize Python from *preconfig* preconfiguration.

   *preconfig* must not be ``NULL``.

.. c:function:: PyStatus Py_PreInitializeFromBytesArgs(const PyPreConfig *preconfig, int argc, char * const *argv)

   Preinitialize Python from *preconfig* preconfiguration.

   Parse *argv* command line arguments (bytes strings) if
   :c:member:`~PyPreConfig.parse_argv` of *preconfig* is non-zero.

   *preconfig* must not be ``NULL``.

.. c:function:: PyStatus Py_PreInitializeFromArgs(const PyPreConfig *preconfig, int argc, wchar_t * const * argv)

   Preinitialize Python from *preconfig* preconfiguration.

   Parse *argv* command line arguments (wide strings) if
   :c:member:`~PyPreConfig.parse_argv` of *preconfig* is non-zero.

   *preconfig* must not be ``NULL``.

The caller is responsible to handle exceptions (error or exit) using
:c:func:`PyStatus_Exception` and :c:func:`Py_ExitStatusException`.

For :ref:`Python Configuration <init-python-config>`
(:c:func:`PyPreConfig_InitPythonConfig`), if Python is initialized with
command line arguments, the command line arguments must also be passed to
preinitialize Python, since they have an effect on the pre-configuration
like encodings. For example, the :option:`-X utf8 <-X>` command line option
enables the :ref:`Python UTF-8 Mode <utf8-mode>`.

``PyMem_SetAllocator()`` can be called after :c:func:`Py_PreInitialize` and
before :c:func:`Py_InitializeFromConfig` to install a custom memory allocator.
It can be called before :c:func:`Py_PreInitialize` if
:c:member:`PyPreConfig.allocator` is set to ``PYMEM_ALLOCATOR_NOT_SET``.

Python memory allocation functions like :c:func:`PyMem_RawMalloc` must not be
used before the Python preinitialization, whereas calling directly ``malloc()``
and ``free()`` is always safe. :c:func:`Py_DecodeLocale` must not be called
before the Python preinitialization.

Example using the preinitialization to enable
the :ref:`Python UTF-8 Mode <utf8-mode>`::

    PyStatus status;
    PyPreConfig preconfig;
    PyPreConfig_InitPythonConfig(&preconfig);

    preconfig.utf8_mode = 1;

    status = Py_PreInitialize(&preconfig);
    if (PyStatus_Exception(status)) {
        Py_ExitStatusException(status);
    }

    /* at this point, Python speaks UTF-8 */

    Py_Initialize();
    /* ... use Python API here ... */
    Py_Finalize();


PyConfig
========

.. c:type:: PyConfig

   Structure containing most parameters to configure Python.

   When done, the :c:func:`PyConfig_Clear` function must be used to release the
   configuration memory.

   Structure methods:

   .. c:function:: void PyConfig_InitPythonConfig(PyConfig *config)

      Initialize configuration with the :ref:`Python Configuration
      <init-python-config>`.

   .. c:function:: void PyConfig_InitIsolatedConfig(PyConfig *config)

      Initialize configuration with the :ref:`Isolated Configuration
      <init-isolated-conf>`.

   .. c:function:: PyStatus PyConfig_SetString(PyConfig *config, wchar_t * const *config_str, const wchar_t *str)

      Copy the wide character string *str* into ``*config_str``.

      :ref:`Preinitialize Python <c-preinit>` if needed.

   .. c:function:: PyStatus PyConfig_SetBytesString(PyConfig *config, wchar_t * const *config_str, const char *str)

      Decode *str* using :c:func:`Py_DecodeLocale` and set the result into
      ``*config_str``.

      :ref:`Preinitialize Python <c-preinit>` if needed.

   .. c:function:: PyStatus PyConfig_SetArgv(PyConfig *config, int argc, wchar_t * const *argv)

      Set command line arguments (:c:member:`~PyConfig.argv` member of
      *config*) from the *argv* list of wide character strings.

      :ref:`Preinitialize Python <c-preinit>` if needed.

   .. c:function:: PyStatus PyConfig_SetBytesArgv(PyConfig *config, int argc, char * const *argv)

      Set command line arguments (:c:member:`~PyConfig.argv` member of
      *config*) from the *argv* list of bytes strings. Decode bytes using
      :c:func:`Py_DecodeLocale`.

      :ref:`Preinitialize Python <c-preinit>` if needed.

   .. c:function:: PyStatus PyConfig_SetWideStringList(PyConfig *config, PyWideStringList *list, Py_ssize_t length, wchar_t **items)

      Set the list of wide strings *list* to *length* and *items*.

      :ref:`Preinitialize Python <c-preinit>` if needed.

   .. c:function:: PyStatus PyConfig_Read(PyConfig *config)

      Read all Python configuration.

      Fields which are already initialized are left unchanged.

      The :c:func:`PyConfig_Read` function only parses
      :c:member:`PyConfig.argv` arguments once: :c:member:`PyConfig.parse_argv`
      is set to ``2`` after arguments are parsed. Since Python arguments are
      strippped from :c:member:`PyConfig.argv`, parsing arguments twice would
      parse the application options as Python options.

      :ref:`Preinitialize Python <c-preinit>` if needed.

      .. versionchanged:: 3.10
         The :c:member:`PyConfig.argv` arguments are now only parsed once,
         :c:member:`PyConfig.parse_argv` is set to ``2`` after arguments are
         parsed, and arguments are only parsed if
         :c:member:`PyConfig.parse_argv` equals ``1``.

   .. c:function:: void PyConfig_Clear(PyConfig *config)

      Release configuration memory.

   Most ``PyConfig`` methods :ref:`preinitialize Python <c-preinit>` if needed.
   In that case, the Python preinitialization configuration
   (:c:type:`PyPreConfig`) in based on the :c:type:`PyConfig`. If configuration
   fields which are in common with :c:type:`PyPreConfig` are tuned, they must
   be set before calling a :c:type:`PyConfig` method:

   * :c:member:`PyConfig.dev_mode`
   * :c:member:`PyConfig.isolated`
   * :c:member:`PyConfig.parse_argv`
   * :c:member:`PyConfig.use_environment`

   Moreover, if :c:func:`PyConfig_SetArgv` or :c:func:`PyConfig_SetBytesArgv`
   is used, this method must be called before other methods, since the
   preinitialization configuration depends on command line arguments (if
   :c:member:`parse_argv` is non-zero).

   The caller of these methods is responsible to handle exceptions (error or
   exit) using ``PyStatus_Exception()`` and ``Py_ExitStatusException()``.

   Structure fields:

   .. c:member:: PyWideStringList argv

      Command line arguments: :data:`sys.argv`.

      Set :c:member:`~PyConfig.parse_argv` to ``1`` to parse
      :c:member:`~PyConfig.argv` the same way the regular Python parses Python
      command line arguments and then to strip Python arguments from
      :c:member:`~PyConfig.argv`.

      If :c:member:`~PyConfig.argv` is empty, an empty string is added to
      ensure that :data:`sys.argv` always exists and is never empty.

      Default: ``NULL``.

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

   .. c:member:: wchar_t* base_exec_prefix

      :data:`sys.base_exec_prefix`.

      Default: ``NULL``.

      Part of the :ref:`Python Path Configuration <init-path-config>` output.

   .. c:member:: wchar_t* base_executable

      Python base executable: :data:`sys._base_executable`.

      Set by the :envvar:`__PYVENV_LAUNCHER__` environment variable.

      Set from :c:member:`PyConfig.executable` if ``NULL``.

      Default: ``NULL``.

      Part of the :ref:`Python Path Configuration <init-path-config>` output.

   .. c:member:: wchar_t* base_prefix

      :data:`sys.base_prefix`.

      Default: ``NULL``.

      Part of the :ref:`Python Path Configuration <init-path-config>` output.

   .. c:member:: int buffered_stdio

      If equals to 0 and :c:member:`~PyConfig.configure_c_stdio` is non-zero,
      disable buffering on the C streams stdout and stderr.

      Set to 0 by the :option:`-u` command line option and the
      :envvar:`PYTHONUNBUFFERED` environment variable.

      stdin is always opened in buffered mode.

      Default: ``1``.

   .. c:member:: int bytes_warning

      If equals to 1, issue a warning when comparing :class:`bytes` or
      :class:`bytearray` with :class:`str`, or comparing :class:`bytes` with
      :class:`int`.

      If equal or greater to 2, raise a :exc:`BytesWarning` exception in these
      cases.

      Incremented by the :option:`-b` command line option.

      Default: ``0``.

   .. c:member:: wchar_t* check_hash_pycs_mode

      Control the validation behavior of hash-based ``.pyc`` files:
      value of the :option:`--check-hash-based-pycs` command line option.

      Valid values:

      - ``L"always"``: Hash the source file for invalidation regardless of
        value of the 'check_source' flag.
      - ``L"never"``: Assume that hash-based pycs always are valid.
      - ``L"default"``: The 'check_source' flag in hash-based pycs
        determines invalidation.

      Default: ``L"default"``.

      See also :pep:`552` "Deterministic pycs".

   .. c:member:: int configure_c_stdio

      If non-zero, configure C standard streams:

      * On Windows, set the binary mode (``O_BINARY``) on stdin, stdout and
        stderr.
      * If :c:member:`~PyConfig.buffered_stdio` equals zero, disable buffering
        of stdin, stdout and stderr streams.
      * If :c:member:`~PyConfig.interactive` is non-zero, enable stream
        buffering on stdin and stdout (only stdout on Windows).

      Default: ``1`` in Python config, ``0`` in isolated config.

   .. c:member:: int dev_mode

      If non-zero, enable the :ref:`Python Development Mode <devmode>`.

      Default: ``-1`` in Python mode, ``0`` in isolated mode.

   .. c:member:: int dump_refs

      Dump Python refererences?

      If non-zero, dump all objects which are still alive at exit.

      Set to ``1`` by the :envvar:`PYTHONDUMPREFS` environment variable.

      Need a special build of Python with the ``Py_TRACE_REFS`` macro defined.

      Default: ``0``.

   .. c:member:: wchar_t* exec_prefix

      The site-specific directory prefix where the platform-dependent Python
      files are installed: :data:`sys.exec_prefix`.

      Default: ``NULL``.

      Part of the :ref:`Python Path Configuration <init-path-config>` output.

   .. c:member:: wchar_t* executable

      The absolute path of the executable binary for the Python interpreter:
      :data:`sys.executable`.

      Default: ``NULL``.

      Part of the :ref:`Python Path Configuration <init-path-config>` output.

   .. c:member:: int faulthandler

      Enable faulthandler?

      If non-zero, call :func:`faulthandler.enable` at startup.

      Set to ``1`` by :option:`-X faulthandler <-X>` and the
      :envvar:`PYTHONFAULTHANDLER` environment variable.

      Default: ``-1`` in Python mode, ``0`` in isolated mode.

   .. c:member:: wchar_t* filesystem_encoding

      :term:`Filesystem encoding <filesystem encoding and error handler>`:
      :func:`sys.getfilesystemencoding`.

      On macOS, Android and VxWorks: use ``"utf-8"`` by default.

      On Windows: use ``"utf-8"`` by default, or ``"mbcs"`` if
      :c:member:`~PyPreConfig.legacy_windows_fs_encoding` of
      :c:type:`PyPreConfig` is non-zero.

      Default encoding on other platforms:

      * ``"utf-8"`` if :c:member:`PyPreConfig.utf8_mode` is non-zero.
      * ``"ascii"`` if Python detects that ``nl_langinfo(CODESET)`` announces
        the ASCII encoding (or Roman8 encoding on HP-UX), whereas the
        ``mbstowcs()`` function decodes from a different encoding (usually
        Latin1).
      * ``"utf-8"`` if ``nl_langinfo(CODESET)`` returns an empty string.
      * Otherwise, use the :term:`locale encoding`:
        ``nl_langinfo(CODESET)`` result.

      At Python statup, the encoding name is normalized to the Python codec
      name. For example, ``"ANSI_X3.4-1968"`` is replaced with ``"ascii"``.

      See also the :c:member:`~PyConfig.filesystem_errors` member.

   .. c:member:: wchar_t* filesystem_errors

      :term:`Filesystem error handler <filesystem encoding and error handler>`:
      :func:`sys.getfilesystemencodeerrors`.

      On Windows: use ``"surrogatepass"`` by default, or ``"replace"``  if
      :c:member:`~PyPreConfig.legacy_windows_fs_encoding` of
      :c:type:`PyPreConfig` is non-zero.

      On other platforms: use ``"surrogateescape"`` by default.

      Supported error handlers:

      * ``"strict"``
      * ``"surrogateescape"``
      * ``"surrogatepass"`` (only supported with the UTF-8 encoding)

      See also the :c:member:`~PyConfig.filesystem_encoding` member.

   .. c:member:: unsigned long hash_seed
   .. c:member:: int use_hash_seed

      Randomized hash function seed.

      If :c:member:`~PyConfig.use_hash_seed` is zero, a seed is chosen randomly
      at Python startup, and :c:member:`~PyConfig.hash_seed` is ignored.

      Set by the :envvar:`PYTHONHASHSEED` environment variable.

      Default *use_hash_seed* value: ``-1`` in Python mode, ``0`` in isolated
      mode.

   .. c:member:: wchar_t* home

      Python home directory.

      If :c:func:`Py_SetPythonHome` has been called, use its argument if it is
      not ``NULL``.

      Set by the :envvar:`PYTHONHOME` environment variable.

      Default: ``NULL``.

      Part of the :ref:`Python Path Configuration <init-path-config>` input.

   .. c:member:: int import_time

      If non-zero, profile import time.

      Set the ``1`` by the :option:`-X importtime <-X>` option and the
      :envvar:`PYTHONPROFILEIMPORTTIME` environment variable.

      Default: ``0``.

   .. c:member:: int inspect

      Enter interactive mode after executing a script or a command.

      If greater than 0, enable inspect: when a script is passed as first
      argument or the -c option is used, enter interactive mode after executing
      the script or the command, even when :data:`sys.stdin` does not appear to
      be a terminal.

      Incremented by the :option:`-i` command line option. Set to ``1`` if the
      :envvar:`PYTHONINSPECT` environment variable is non-empty.

      Default: ``0``.

   .. c:member:: int install_signal_handlers

      Install Python signal handlers?

      Default: ``1`` in Python mode, ``0`` in isolated mode.

   .. c:member:: int interactive

      If greater than 0, enable the interactive mode (REPL).

      Incremented by the :option:`-i` command line option.

      Default: ``0``.

   .. c:member:: int isolated

      If greater than 0, enable isolated mode:

      * :data:`sys.path` contains neither the script's directory (computed from
        ``argv[0]`` or the current directory) nor the user's site-packages
        directory.
      * Python REPL doesn't import :mod:`readline` nor enable default readline
        configuration on interactive prompts.
      * Set :c:member:`~PyConfig.use_environment` and
        :c:member:`~PyConfig.user_site_directory` to 0.

      Default: ``0`` in Python mode, ``1`` in isolated mode.

      See also :c:member:`PyPreConfig.isolated`.

   .. c:member:: int legacy_windows_stdio

      If non-zero, use :class:`io.FileIO` instead of
      :class:`io.WindowsConsoleIO` for :data:`sys.stdin`, :data:`sys.stdout`
      and :data:`sys.stderr`.

      Set to ``1`` if the :envvar:`PYTHONLEGACYWINDOWSSTDIO` environment
      variable is set to a non-empty string.

      Only available on Windows. ``#ifdef MS_WINDOWS`` macro can be used for
      Windows specific code.

      Default: ``0``.

      See also the :pep:`528` (Change Windows console encoding to UTF-8).

   .. c:member:: int malloc_stats

      If non-zero, dump statistics on :ref:`Python pymalloc memory allocator
      <pymalloc>` at exit.

      Set to ``1`` by the :envvar:`PYTHONMALLOCSTATS` environment variable.

      The option is ignored if Python is built using ``--without-pymalloc``.

      Default: ``0``.

   .. c:member:: wchar_t* platlibdir

      Platform library directory name: :data:`sys.platlibdir`.

      Set by the :envvar:`PYTHONPLATLIBDIR` environment variable.

      Default: value of the ``PLATLIBDIR`` macro which is set at configure time
      by ``--with-platlibdir`` (default: ``"lib"``).

      Part of the :ref:`Python Path Configuration <init-path-config>` input.

      .. versionadded:: 3.9

   .. c:member:: wchar_t* pythonpath_env

      Module search paths (:data:`sys.path`) as a string separated by ``DELIM``
      (:data:`os.path.pathsep`).

      Set by the :envvar:`PYTHONPATH` environment variable.

      Default: ``NULL``.

      Part of the :ref:`Python Path Configuration <init-path-config>` input.

   .. c:member:: PyWideStringList module_search_paths
   .. c:member:: int module_search_paths_set

      Module search paths: :data:`sys.path`.

      If :c:member:`~PyConfig.module_search_paths_set` is equal to 0, the
      function calculating the :ref:`Python Path Configuration <init-path-config>`
      overrides the :c:member:`~PyConfig.module_search_paths` and sets
      :c:member:`~PyConfig.module_search_paths_set` to ``1``.

      Default: empty list (``module_search_paths``) and ``0``
      (``module_search_paths_set``).

      Part of the :ref:`Python Path Configuration <init-path-config>` output.

   .. c:member:: int optimization_level

      Compilation optimization level:

      * ``0``: Peephole optimizer, set ``__debug__`` to ``True``.
      * ``1``: Level 0, remove assertions, set ``__debug__`` to ``False``.
      * ``2``: Level 1, strip docstrings.

      Incremented by the :option:`-O` command line option. Set to the
      :envvar:`PYTHONOPTIMIZE` environment variable value.

      Default: ``0``.

   .. c:member:: PyWideStringList orig_argv

      The list of the original command line arguments passed to the Python
      executable: :data:`sys.orig_argv`.

      If :c:member:`~PyConfig.orig_argv` list is empty and
      :c:member:`~PyConfig.argv` is not a list only containing an empty
      string, :c:func:`PyConfig_Read` copies :c:member:`~PyConfig.argv` into
      :c:member:`~PyConfig.orig_argv` before modifying
      :c:member:`~PyConfig.argv` (if :c:member:`~PyConfig.parse_argv` is
      non-zero).

      See also the :c:member:`~PyConfig.argv` member and the
      :c:func:`Py_GetArgcArgv` function.

      Default: empty list.

      .. versionadded:: 3.10

   .. c:member:: int parse_argv

      Parse command line arguments?

      If equals to ``1``, parse :c:member:`~PyConfig.argv` the same way the regular
      Python parses :ref:`command line arguments <using-on-cmdline>`, and strip
      Python arguments from :c:member:`~PyConfig.argv`.

      The :c:func:`PyConfig_Read` function only parses
      :c:member:`PyConfig.argv` arguments once: :c:member:`PyConfig.parse_argv`
      is set to ``2`` after arguments are parsed. Since Python arguments are
      strippped from :c:member:`PyConfig.argv`, parsing arguments twice would
      parse the application options as Python options.

      Default: ``1`` in Python mode, ``0`` in isolated mode.

      .. versionchanged:: 3.10
         The :c:member:`PyConfig.argv` arguments are now only parsed if
         :c:member:`PyConfig.parse_argv` equals to ``1``.

   .. c:member:: int parser_debug

      Parser debug mode. If greater than 0, turn on parser debugging output (for expert only, depending
      on compilation options).

      Incremented by the :option:`-d` command line option. Set to the
      :envvar:`PYTHONDEBUG` environment variable value.

      Default: ``0``.

   .. c:member:: int pathconfig_warnings

      On Unix, if non-zero, calculating the :ref:`Python Path Configuration
      <init-path-config>` can log warnings into ``stderr``. If equals to 0,
      suppress these warnings.

      It has no effect on Windows.

      Default: ``1`` in Python mode, ``0`` in isolated mode.

      Part of the :ref:`Python Path Configuration <init-path-config>` input.

   .. c:member:: wchar_t* prefix

      The site-specific directory prefix where the platform independent Python
      files are installed: :data:`sys.prefix`.

      Default: ``NULL``.

      Part of the :ref:`Python Path Configuration <init-path-config>` output.

   .. c:member:: wchar_t* program_name

      Program name used to initialize :c:member:`~PyConfig.executable` and in
      early error messages during Python initialization.

      * If :func:`Py_SetProgramName` has been called, use its argument.
      * On macOS, use :envvar:`PYTHONEXECUTABLE` environment variable if set.
      * If the ``WITH_NEXT_FRAMEWORK`` macro is defined, use
        :envvar:`__PYVENV_LAUNCHER__` environment variable if set.
      * Use ``argv[0]`` of :c:member:`~PyConfig.argv` if available and
        non-empty.
      * Otherwise, use ``L"python"`` on Windows, or ``L"python3"`` on other
        platforms.

      Default: ``NULL``.

      Part of the :ref:`Python Path Configuration <init-path-config>` input.

   .. c:member:: wchar_t* pycache_prefix

      Directory where cached ``.pyc`` files are written:
      :data:`sys.pycache_prefix`.

      Set by the :option:`-X pycache_prefix=PATH <-X>` command line option and
      the :envvar:`PYTHONPYCACHEPREFIX` environment variable.

      If ``NULL``, :data:`sys.pycache_prefix` is set to ``None``.

      Default: ``NULL``.

   .. c:member:: int quiet

      Quiet mode. If greater than 0, don't display the copyright and version at
      Python startup in interactive mode.

      Incremented by the :option:`-q` command line option.

      Default: ``0``.

   .. c:member:: wchar_t* run_command

      Value of the :option:`-c` command line option.

      Used by :c:func:`Py_RunMain`.

      Default: ``NULL``.

   .. c:member:: wchar_t* run_filename

      Filename passed on the command line: trailing command line argument
      without :option:`-c` or :option:`-m`.

      For example, it is set to ``script.py`` by the ``python3 script.py arg``
      command.

      Used by :c:func:`Py_RunMain`.

      Default: ``NULL``.

   .. c:member:: wchar_t* run_module

      Value of the :option:`-m` command line option.

      Used by :c:func:`Py_RunMain`.

      Default: ``NULL``.

   .. c:member:: int show_ref_count

      Show total reference count at exit?

      Set to 1 by :option:`-X showrefcount <-X>` command line option.

      Need a debug build of Python (``Py_REF_DEBUG`` macro must be defined).

      Default: ``0``.

   .. c:member:: int site_import

      Import the :mod:`site` module at startup?

      If equal to zero, disable the import of the module site and the
      site-dependent manipulations of :data:`sys.path` that it entails.

      Also disable these manipulations if the :mod:`site` module is explicitly
      imported later (call :func:`site.main` if you want them to be triggered).

      Set to ``0`` by the :option:`-S` command line option.

      :data:`sys.flags.no_site` is set to the inverted value of
      :c:member:`~PyConfig.site_import`.

      Default: ``1``.

   .. c:member:: int skip_source_first_line

      If non-zero, skip the first line of the :c:member:`PyConfig.run_filename`
      source.

      It allows the usage of non-Unix forms of ``#!cmd``. This is intended for
      a DOS specific hack only.

      Set to ``1`` by the :option:`-x` command line option.

      Default: ``0``.

   .. c:member:: wchar_t* stdio_encoding
   .. c:member:: wchar_t* stdio_errors

      Encoding and encoding errors of :data:`sys.stdin`, :data:`sys.stdout` and
      :data:`sys.stderr` (but :data:`sys.stderr` always uses
      ``"backslashreplace"`` error handler).

      If :c:func:`Py_SetStandardStreamEncoding` has been called, use its
      *error* and *errors* arguments if they are not ``NULL``.

      Use the :envvar:`PYTHONIOENCODING` environment variable if it is
      non-empty.

      Default encoding:

      * ``"UTF-8"`` if :c:member:`PyPreConfig.utf8_mode` is non-zero.
      * Otherwise, use the :term:`locale encoding`.

      Default error handler:

      * On Windows: use ``"surrogateescape"``.
      * ``"surrogateescape"`` if :c:member:`PyPreConfig.utf8_mode` is non-zero,
        or if the LC_CTYPE locale is "C" or "POSIX".
      * ``"strict"`` otherwise.

   .. c:member:: int tracemalloc

      Enable tracemalloc?

      If non-zero, call :func:`tracemalloc.start` at startup.

      Set by :option:`-X tracemalloc=N <-X>` command line option and by the
      :envvar:`PYTHONTRACEMALLOC` environment variable.

      Default: ``-1`` in Python mode, ``0`` in isolated mode.

   .. c:member:: int use_environment

      Use :ref:`environment variables <using-on-envvars>`?

      If equals to zero, ignore the :ref:`environment variables
      <using-on-envvars>`.

      Default: ``1`` in Python config and ``0`` in isolated config.

   .. c:member:: int user_site_directory

      If non-zero, add the user site directory to :data:`sys.path`.

      Set to ``0`` by the :option:`-s` and :option:`-I` command line options.

      Set to ``0`` by the :envvar:`PYTHONNOUSERSITE` environment variable.

      Default: ``1`` in Python mode, ``0`` in isolated mode.

   .. c:member:: int verbose

      Verbose mode. If greater than 0, print a message each time a module is
      imported, showing the place (filename or built-in module) from which
      it is loaded.

      If greater or equal to 2, print a message for each file that is checked
      for when searching for a module. Also provides information on module
      cleanup at exit.

      Incremented by the :option:`-v` command line option.

      Set to the :envvar:`PYTHONVERBOSE` environment variable value.

      Default: ``0``.

   .. c:member:: PyWideStringList warnoptions

      Options of the :mod:`warnings` module to build warnings filters, lowest
      to highest priority: :data:`sys.warnoptions`.

      The :mod:`warnings` module adds :data:`sys.warnoptions` in the reverse
      order: the last :c:member:`PyConfig.warnoptions` item becomes the first
      item of :data:`warnings.filters` which is checked first (highest
      priority).

      Default: empty list.

   .. c:member:: int write_bytecode

      If equal to 0, Python won't try to write ``.pyc`` files on the import of
      source modules.

      Set to ``0`` by the :option:`-B` command line option and the
      :envvar:`PYTHONDONTWRITEBYTECODE` environment variable.

      :data:`sys.dont_write_bytecode` is initialized to the inverted value of
      :c:member:`~PyConfig.write_bytecode`.

      Default: ``1``.

   .. c:member:: PyWideStringList xoptions

      Values of the :option:`-X` command line options: :data:`sys._xoptions`.

      Default: empty list.

If :c:member:`~PyConfig.parse_argv` is non-zero, :c:member:`~PyConfig.argv`
arguments are parsed the same way the regular Python parses :ref:`command line
arguments <using-on-cmdline>`, and Python arguments are stripped from
:c:member:`~PyConfig.argv`.

The :c:member:`~PyConfig.xoptions` options are parsed to set other options: see
the :option:`-X` command line option.

.. versionchanged:: 3.9

   The ``show_alloc_count`` field has been removed.


Initialization with PyConfig
============================

Function to initialize Python:

.. c:function:: PyStatus Py_InitializeFromConfig(const PyConfig *config)

   Initialize Python from *config* configuration.

The caller is responsible to handle exceptions (error or exit) using
:c:func:`PyStatus_Exception` and :c:func:`Py_ExitStatusException`.

If :c:func:`PyImport_FrozenModules`, :c:func:`PyImport_AppendInittab` or
:c:func:`PyImport_ExtendInittab` are used, they must be set or called after
Python preinitialization and before the Python initialization.

The current configuration (``PyConfig`` type) is stored in
``PyInterpreterState.config``.

Example setting the program name::

    void init_python(void)
    {
        PyStatus status;

        PyConfig config;
        PyConfig_InitPythonConfig(&config);

        /* Set the program name. Implicitly preinitialize Python. */
        status = PyConfig_SetString(&config, &config.program_name,
                                    L"/path/to/my_program");
        if (PyStatus_Exception(status)) {
            goto exception;
        }

        status = Py_InitializeFromConfig(&config);
        if (PyStatus_Exception(status)) {
            goto exception;
        }
        PyConfig_Clear(&config);
        return;

    exception:
        PyConfig_Clear(&config);
        Py_ExitStatusException(status);
    }

More complete example modifying the default configuration, read the
configuration, and then override some parameters::

    PyStatus init_python(const char *program_name)
    {
        PyStatus status;

        PyConfig config;
        PyConfig_InitPythonConfig(&config);

        /* Set the program name before reading the configuration
           (decode byte string from the locale encoding).

           Implicitly preinitialize Python. */
        status = PyConfig_SetBytesString(&config, &config.program_name,
                                         program_name);
        if (PyStatus_Exception(status)) {
            goto done;
        }

        /* Read all configuration at once */
        status = PyConfig_Read(&config);
        if (PyStatus_Exception(status)) {
            goto done;
        }

        /* Append our custom search path to sys.path */
        status = PyWideStringList_Append(&config.module_search_paths,
                                         L"/path/to/more/modules");
        if (PyStatus_Exception(status)) {
            goto done;
        }

        /* Override executable computed by PyConfig_Read() */
        status = PyConfig_SetString(&config, &config.executable,
                                    L"/path/to/my_executable");
        if (PyStatus_Exception(status)) {
            goto done;
        }

        status = Py_InitializeFromConfig(&config);

    done:
        PyConfig_Clear(&config);
        return status;
    }


.. _init-isolated-conf:

Isolated Configuration
======================

:c:func:`PyPreConfig_InitIsolatedConfig` and
:c:func:`PyConfig_InitIsolatedConfig` functions create a configuration to
isolate Python from the system. For example, to embed Python into an
application.

This configuration ignores global configuration variables, environments
variables, command line arguments (:c:member:`PyConfig.argv` is not parsed)
and user site directory. The C standard streams (ex: ``stdout``) and the
LC_CTYPE locale are left unchanged. Signal handlers are not installed.

Configuration files are still used with this configuration. Set the
:ref:`Python Path Configuration <init-path-config>` ("output fields") to ignore these
configuration files and avoid the function computing the default path
configuration.


.. _init-python-config:

Python Configuration
====================

:c:func:`PyPreConfig_InitPythonConfig` and :c:func:`PyConfig_InitPythonConfig`
functions create a configuration to build a customized Python which behaves as
the regular Python.

Environments variables and command line arguments are used to configure
Python, whereas global configuration variables are ignored.

This function enables C locale coercion (:pep:`538`)
and :ref:`Python UTF-8 Mode <utf8-mode>`
(:pep:`540`) depending on the LC_CTYPE locale, :envvar:`PYTHONUTF8` and
:envvar:`PYTHONCOERCECLOCALE` environment variables.


.. _init-path-config:

Python Path Configuration
=========================

:c:type:`PyConfig` contains multiple fields for the path configuration:

* Path configuration inputs:

  * :c:member:`PyConfig.home`
  * :c:member:`PyConfig.platlibdir`
  * :c:member:`PyConfig.pathconfig_warnings`
  * :c:member:`PyConfig.program_name`
  * :c:member:`PyConfig.pythonpath_env`
  * current working directory: to get absolute paths
  * ``PATH`` environment variable to get the program full path
    (from :c:member:`PyConfig.program_name`)
  * ``__PYVENV_LAUNCHER__`` environment variable
  * (Windows only) Application paths in the registry under
    "Software\Python\PythonCore\X.Y\PythonPath" of HKEY_CURRENT_USER and
    HKEY_LOCAL_MACHINE (where X.Y is the Python version).

* Path configuration output fields:

  * :c:member:`PyConfig.base_exec_prefix`
  * :c:member:`PyConfig.base_executable`
  * :c:member:`PyConfig.base_prefix`
  * :c:member:`PyConfig.exec_prefix`
  * :c:member:`PyConfig.executable`
  * :c:member:`PyConfig.module_search_paths_set`,
    :c:member:`PyConfig.module_search_paths`
  * :c:member:`PyConfig.prefix`

If at least one "output field" is not set, Python calculates the path
configuration to fill unset fields. If
:c:member:`~PyConfig.module_search_paths_set` is equal to 0,
:c:member:`~PyConfig.module_search_paths` is overridden and
:c:member:`~PyConfig.module_search_paths_set` is set to 1.

It is possible to completely ignore the function calculating the default
path configuration by setting explicitly all path configuration output
fields listed above. A string is considered as set even if it is non-empty.
``module_search_paths`` is considered as set if
``module_search_paths_set`` is set to 1. In this case, path
configuration input fields are ignored as well.

Set :c:member:`~PyConfig.pathconfig_warnings` to 0 to suppress warnings when
calculating the path configuration (Unix only, Windows does not log any warning).

If :c:member:`~PyConfig.base_prefix` or :c:member:`~PyConfig.base_exec_prefix`
fields are not set, they inherit their value from :c:member:`~PyConfig.prefix`
and :c:member:`~PyConfig.exec_prefix` respectively.

:c:func:`Py_RunMain` and :c:func:`Py_Main` modify :data:`sys.path`:

* If :c:member:`~PyConfig.run_filename` is set and is a directory which contains a
  ``__main__.py`` script, prepend :c:member:`~PyConfig.run_filename` to
  :data:`sys.path`.
* If :c:member:`~PyConfig.isolated` is zero:

  * If :c:member:`~PyConfig.run_module` is set, prepend the current directory
    to :data:`sys.path`. Do nothing if the current directory cannot be read.
  * If :c:member:`~PyConfig.run_filename` is set, prepend the directory of the
    filename to :data:`sys.path`.
  * Otherwise, prepend an empty string to :data:`sys.path`.

If :c:member:`~PyConfig.site_import` is non-zero, :data:`sys.path` can be
modified by the :mod:`site` module. If
:c:member:`~PyConfig.user_site_directory` is non-zero and the user's
site-package directory exists, the :mod:`site` module appends the user's
site-package directory to :data:`sys.path`.

The following configuration files are used by the path configuration:

* ``pyvenv.cfg``
* ``python._pth`` (Windows only)
* ``pybuilddir.txt`` (Unix only)

The ``__PYVENV_LAUNCHER__`` environment variable is used to set
:c:member:`PyConfig.base_executable`


Py_RunMain()
============

.. c:function:: int Py_RunMain(void)

   Execute the command (:c:member:`PyConfig.run_command`), the script
   (:c:member:`PyConfig.run_filename`) or the module
   (:c:member:`PyConfig.run_module`) specified on the command line or in the
   configuration.

   By default and when if :option:`-i` option is used, run the REPL.

   Finally, finalizes Python and returns an exit status that can be passed to
   the ``exit()`` function.

See :ref:`Python Configuration <init-python-config>` for an example of
customized Python always running in isolated mode using
:c:func:`Py_RunMain`.


Py_GetArgcArgv()
================

.. c:function:: void Py_GetArgcArgv(int *argc, wchar_t ***argv)

   Get the original command line arguments, before Python modified them.

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


Multi-Phase Initialization Private Provisional API
==================================================

This section is a private provisional API introducing multi-phase
initialization, the core feature of the :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:`PyConfig._init_main`: if set to 0,
  :c:func:`Py_InitializeFromConfig` stops at the "Core" initialization phase.
* :c:member:`PyConfig._isolated_interpreter`: if non-zero,
  disallow threads, subprocesses and fork.

.. 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 calculatin 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);
        }
    }
