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

New in version 3.8.

Structures:

* "PyConfig"

* "PyPreConfig"

* "PyStatus"

* "PyWideStringList"

Functions:

* "PyConfig_Clear()"

* "PyConfig_InitIsolatedConfig()"

* "PyConfig_InitPythonConfig()"

* "PyConfig_Read()"

* "PyConfig_SetArgv()"

* "PyConfig_SetBytesArgv()"

* "PyConfig_SetBytesString()"

* "PyConfig_SetString()"

* "PyConfig_SetWideStringList()"

* "PyPreConfig_InitIsolatedConfig()"

* "PyPreConfig_InitPythonConfig()"

* "PyStatus_Error()"

* "PyStatus_Exception()"

* "PyStatus_Exit()"

* "PyStatus_IsError()"

* "PyStatus_IsExit()"

* "PyStatus_NoMemory()"

* "PyStatus_Ok()"

* "PyWideStringList_Append()"

* "PyWideStringList_Insert()"

* "Py_ExitStatusException()"

* "Py_InitializeFromConfig()"

* "Py_PreInitialize()"

* "Py_PreInitializeFromArgs()"

* "Py_PreInitializeFromBytesArgs()"

* "Py_RunMain()"

The preconfiguration ("PyPreConfig" type) is stored in
"_PyRuntime.preconfig" and the configuration ("PyConfig" type) is
stored in "PyInterpreterState.config".

See also Initialization, Finalization, and Threads.

See also: **PEP 587** “Python Initialization Configuration”.


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

PyWideStringList

   List of "wchar_t*" strings.

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

   Methods:

   PyStatus PyWideStringList_Append(PyWideStringList *list, const wchar_t *item)

      Append *item* to *list*.

      Python must be preinitialized to call this 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:

   Py_ssize_t length

      List length.

   wchar_t** items

      List items.


PyStatus
========

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:

   int exitcode

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

   const char *err_msg

      Error message.

   const char *func

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

   Functions to create a status:

   PyStatus PyStatus_Ok(void)

      Success.

   PyStatus PyStatus_Error(const char *err_msg)

      Initialization error with a message.

   PyStatus PyStatus_NoMemory(void)

      Memory allocation failure (out of memory).

   PyStatus PyStatus_Exit(int exitcode)

      Exit Python with the specified exit code.

   Functions to handle a status:

   int PyStatus_Exception(PyStatus status)

      Is the status an error or an exit? If true, the exception must
      be handled; by calling "Py_ExitStatusException()" for example.

   int PyStatus_IsError(PyStatus status)

      Is the result an error?

   int PyStatus_IsExit(PyStatus status)

      Is the result an exit?

   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
===========

PyPreConfig

   Structure used to preinitialize Python:

   * Set the Python memory allocator

   * Configure the LC_CTYPE locale

   * Set the UTF-8 mode

   Function to initialize a preconfiguration:

   void PyPreConfig_InitIsolatedConfig(PyPreConfig *preconfig)

      Initialize the preconfiguration with Python Configuration.

   void PyPreConfig_InitPythonConfig(PyPreConfig *preconfig)

      Initialize the preconfiguration with Isolated Configuration.

   Structure fields:

   int allocator

      Name of the memory allocator:

      * "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"): Python pymalloc memory
        allocator

      * "PYMEM_ALLOCATOR_PYMALLOC_DEBUG" ("6"): Python pymalloc
        memory allocator with debug hooks

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

      See Memory Management.

   int configure_locale

      Set the LC_CTYPE locale to the user preferred locale? If equals
      to 0, set "coerce_c_locale" and "coerce_c_locale_warn" to 0.

   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.

   int coerce_c_locale_warn

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

   int dev_mode

      See "PyConfig.dev_mode".

   int isolated

      See "PyConfig.isolated".

   int legacy_windows_fs_encoding(Windows only)

      If non-zero, disable UTF-8 Mode, set the Python filesystem
      encoding to "mbcs", set the filesystem error handler to
      "replace".

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

   int parse_argv

      If non-zero, "Py_PreInitializeFromArgs()" and
      "Py_PreInitializeFromBytesArgs()" parse their "argv" argument
      the same way the regular Python parses command line arguments:
      see Command Line Arguments.

   int use_environment

      See "PyConfig.use_environment".

   int utf8_mode

      If non-zero, enable the UTF-8 mode.


Preinitialization with PyPreConfig
==================================

Functions to preinitialize Python:

PyStatus Py_PreInitialize(const PyPreConfig *preconfig)

   Preinitialize Python from *preconfig* preconfiguration.

PyStatus Py_PreInitializeFromBytesArgs(const PyPreConfig *preconfig, int argc, char * const *argv)

   Preinitialize Python from *preconfig* preconfiguration and command
   line arguments (bytes strings).

PyStatus Py_PreInitializeFromArgs(const PyPreConfig *preconfig, int argc, wchar_t * const * argv)

   Preinitialize Python from *preconfig* preconfiguration and command
   line arguments (wide strings).

The caller is responsible to handle exceptions (error or exit) using
"PyStatus_Exception()" and "Py_ExitStatusException()".

For Python Configuration ("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 "-X utf8"
command line option enables the UTF-8 Mode.

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

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

Example using the preinitialization to enable the UTF-8 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 will speak UTF-8 */

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


PyConfig
========

PyConfig

   Structure containing most parameters to configure Python.

   Structure methods:

   void PyConfig_InitPythonConfig(PyConfig *config)

      Initialize configuration with Python Configuration.

   void PyConfig_InitIsolatedConfig(PyConfig *config)

      Initialize configuration with Isolated Configuration.

   PyStatus PyConfig_SetString(PyConfig *config, wchar_t * const *config_str, const wchar_t *str)

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

      Preinitialize Python if needed.

   PyStatus PyConfig_SetBytesString(PyConfig *config, wchar_t * const *config_str, const char *str)

      Decode *str* using "Py_DecodeLocale()" and set the result into
      "*config_str".

      Preinitialize Python if needed.

   PyStatus PyConfig_SetArgv(PyConfig *config, int argc, wchar_t * const *argv)

      Set command line arguments from wide character strings.

      Preinitialize Python if needed.

   PyStatus PyConfig_SetBytesArgv(PyConfig *config, int argc, char * const *argv)

      Set command line arguments: decode bytes using
      "Py_DecodeLocale()".

      Preinitialize Python if needed.

   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*.

      Preinitialize Python if needed.

   PyStatus PyConfig_Read(PyConfig *config)

      Read all Python configuration.

      Fields which are already initialized are left unchanged.

      Preinitialize Python if needed.

   void PyConfig_Clear(PyConfig *config)

      Release configuration memory.

   Most "PyConfig" methods preinitialize Python if needed. In that
   case, the Python preinitialization configuration in based on the
   "PyConfig". If configuration fields which are in common with
   "PyPreConfig" are tuned, they must be set before calling a
   "PyConfig" method:

   * "dev_mode"

   * "isolated"

   * "parse_argv"

   * "use_environment"

   Moreover, if "PyConfig_SetArgv()" or "PyConfig_SetBytesArgv()" is
   used, this method must be called first, before other methods, since
   the preinitialization configuration depends on command line
   arguments (if "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:

   PyWideStringList argv

      Command line arguments, "sys.argv". See "parse_argv" to parse
      "argv" the same way the regular Python parses Python command
      line arguments. If "argv" is empty, an empty string is added to
      ensure that "sys.argv" always exists and is never empty.

   wchar_t* base_exec_prefix

      "sys.base_exec_prefix".

   wchar_t* base_executable

      "sys._base_executable": "__PYVENV_LAUNCHER__" environment
      variable value, or copy of "PyConfig.executable".

   wchar_t* base_prefix

      "sys.base_prefix".

   int buffered_stdio

      If equals to 0, enable unbuffered mode, making the stdout and
      stderr streams unbuffered.

      stdin is always opened in buffered mode.

   int bytes_warning

      If equals to 1, issue a warning when comparing "bytes" or
      "bytearray" with "str", or comparing "bytes" with "int". If
      equal or greater to 2, raise a "BytesWarning" exception.

   wchar_t* check_hash_pycs_mode

      Control the validation behavior of hash-based ".pyc" files (see
      **PEP 552**): "--check-hash-based-pycs" command line option
      value.

      Valid values: "always", "never" and "default".

      The default value is: "default".

   int configure_c_stdio

      If non-zero, configure C standard streams ("stdio", "stdout",
      "stdout"). For example, set their mode to "O_BINARY" on Windows.

   int dev_mode

      Development mode: see "-X dev".

   int dump_refs

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

      Require a debug build of Python ("Py_REF_DEBUG" macro must be
      defined).

   wchar_t* exec_prefix

      "sys.exec_prefix".

   wchar_t* executable

      "sys.executable".

   int faulthandler

      If non-zero, call "faulthandler.enable()" at startup.

   wchar_t* filesystem_encoding

      Filesystem encoding, "sys.getfilesystemencoding()".

   wchar_t* filesystem_errors

      Filesystem encoding errors, "sys.getfilesystemencodeerrors()".

   unsigned long hash_seed

   int use_hash_seed

      Randomized hash function seed.

      If "use_hash_seed" is zero, a seed is chosen randomly at
      Pythonstartup, and "hash_seed" is ignored.

   wchar_t* home

      Python home directory.

      Initialized from "PYTHONHOME" environment variable value by
      default.

   int import_time

      If non-zero, profile import time.

   int inspect

      Enter interactive mode after executing a script or a command.

   int install_signal_handlers

      Install signal handlers?

   int interactive

      Interactive mode.

   int isolated

      If greater than 0, enable isolated mode:

      * "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 "readline" nor enable default
        readline configuration on interactive prompts.

      * Set "use_environment" and "user_site_directory" to 0.

   int legacy_windows_stdio

      If non-zero, use "io.FileIO" instead of "io.WindowsConsoleIO"
      for "sys.stdin", "sys.stdout" and "sys.stderr".

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

   int malloc_stats

      If non-zero, dump statistics on Python pymalloc memory allocator
      at exit.

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

   wchar_t* pythonpath_env

      Module search paths as a string separated by "DELIM"
      ("os.path.pathsep").

      Initialized from "PYTHONPATH" environment variable value by
      default.

   PyWideStringList module_search_paths

   int module_search_paths_set

      "sys.path". If "module_search_paths_set" is equal to 0, the
      "module_search_paths" is overridden by the function calculating
      the Path Configuration.

   int optimization_level

      Compilation optimization level:

      * 0: Peephole optimizer (and "__debug__" is set to "True")

      * 1: Remove assertions, set "__debug__" to "False"

      * 2: Strip docstrings

   int parse_argv

      If non-zero, parse "argv" the same way the regular Python
      command line arguments, and strip Python arguments from "argv":
      see Command Line Arguments.

   int parser_debug

      If non-zero, turn on parser debugging output (for expert only,
      depending on compilation options).

   int pathconfig_warnings

      If equal to 0, suppress warnings when calculating the Path
      Configuration (Unix only, Windows does not log any warning).
      Otherwise, warnings are written into "stderr".

   wchar_t* prefix

      "sys.prefix".

   wchar_t* program_name

      Program name. Used to initialize "executable", and in early
      error messages.

   wchar_t* pycache_prefix

      "sys.pycache_prefix": ".pyc" cache prefix.

      If NULL, "sys.pycache_prefix" is set to "None".

   int quiet

      Quiet mode. For example, don’t display the copyright and version
      messages in interactive mode.

   wchar_t* run_command

      "python3 -c COMMAND" argument. Used by "Py_RunMain()".

   wchar_t* run_filename

      "python3 FILENAME" argument. Used by "Py_RunMain()".

   wchar_t* run_module

      "python3 -m MODULE" argument. Used by "Py_RunMain()".

   int show_alloc_count

      Show allocation counts at exit?

      Set to 1 by "-X showalloccount" command line option.

      Need a special Python build with "COUNT_ALLOCS" macro defined.

   int show_ref_count

      Show total reference count at exit?

      Set to 1 by "-X showrefcount" command line option.

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

   int site_import

      Import the "site" module at startup?

   int skip_source_first_line

      Skip the first line of the source?

   wchar_t* stdio_encoding

   wchar_t* stdio_errors

      Encoding and encoding errors of "sys.stdin", "sys.stdout" and
      "sys.stderr".

   int tracemalloc

      If non-zero, call "tracemalloc.start()" at startup.

   int use_environment

      If greater than 0, use environment variables.

   int user_site_directory

      If non-zero, add user site directory to "sys.path".

   int verbose

      If non-zero, enable verbose mode.

   PyWideStringList warnoptions

      "sys.warnoptions": options of the "warnings" module to build
      warnings filters: lowest to highest priority.

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

   int write_bytecode

      If non-zero, write ".pyc" files.

      "sys.dont_write_bytecode" is initialized to the inverted value
      of "write_bytecode".

   PyWideStringList xoptions

      "sys._xoptions".

If "parse_argv" is non-zero, "argv" arguments are parsed the same way
the regular Python parses command line arguments, and Python arguments
are stripped from "argv": see Command Line Arguments.

The "xoptions" options are parsed to set other options: see "-X"
option.


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

Function to initialize Python:

PyStatus Py_InitializeFromConfig(const PyConfig *config)

   Initialize Python from *config* configuration.

The caller is responsible to handle exceptions (error or exit) using
"PyStatus_Exception()" and "Py_ExitStatusException()".

If "PyImport_FrozenModules", "PyImport_AppendInittab()" or
"PyImport_ExtendInittab()" are used, they must be set or called after
Python preinitialization and before the Python initialization.

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 fail;
       }

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

   fail:
       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 configuraton
          (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;
   }


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

"PyPreConfig_InitIsolatedConfig()" and "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 ("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
Path Configuration (“output fields”) to ignore these configuration
files and avoid the function computing the default path configuration.


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

"PyPreConfig_InitPythonConfig()" and "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 UTF-8 Mode
(**PEP 540**) depending on the LC_CTYPE locale, "PYTHONUTF8" and
"PYTHONCOERCECLOCALE" environment variables.

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 fail;
       }

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

       return Py_RunMain();

   fail:
       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);
   }


Path Configuration
==================

"PyConfig" contains multiple fields for the path configuration:

* Path configuration inputs:

  * "PyConfig.home"

  * "PyConfig.pathconfig_warnings"

  * "PyConfig.program_name"

  * "PyConfig.pythonpath_env"

  * current working directory: to get absolute paths

  * "PATH" environment variable to get the program full path (from
    "PyConfig.program_name")

  * "__PYVENV_LAUNCHER__" environment variable

  * (Windows only) Application paths in the registry under
    “SoftwarePythonPythonCoreX.YPythonPath” of HKEY_CURRENT_USER and
    HKEY_LOCAL_MACHINE (where X.Y is the Python version).

* Path configuration output fields:

  * "PyConfig.base_exec_prefix"

  * "PyConfig.base_executable"

  * "PyConfig.base_prefix"

  * "PyConfig.exec_prefix"

  * "PyConfig.executable"

  * "PyConfig.module_search_paths_set",
    "PyConfig.module_search_paths"

  * "PyConfig.prefix"

If at least one “output field” is not set, Python calculates the path
configuration to fill unset fields. If "module_search_paths_set" is
equal to 0, "module_search_paths" is overridden and
"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 "pathconfig_warnings" to 0 to suppress warnings when calculating
the path configuration (Unix only, Windows does not log any warning).

If "base_prefix" or "base_exec_prefix" fields are not set, they
inherit their value from "prefix" and "exec_prefix" respectively.

"Py_RunMain()" and "Py_Main()" modify "sys.path":

* If "run_filename" is set and is a directory which contains a
  "__main__.py" script, prepend "run_filename" to "sys.path".

* If "isolated" is zero:

  * If "run_module" is set, prepend the current directory to
    "sys.path". Do nothing if the current directory cannot be read.

  * If "run_filename" is set, prepend the directory of the filename
    to "sys.path".

  * Otherwise, prepend an empty string to "sys.path".

If "site_import" is non-zero, "sys.path" can be modified by the "site"
module. If "user_site_directory" is non-zero and the user’s site-
package directory exists, the "site" module appends the user’s site-
package directory to "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
"PyConfig.base_executable"


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

int Py_RunMain(void)

   Execute the command ("PyConfig.run_command"), the script
   ("PyConfig.run_filename") or the module ("PyConfig.run_module")
   specified on the command line or in the configuration.

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

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

See Python Configuration for an example of customized Python always
running in isolated mode using "Py_RunMain()".


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 "sys" module is only partially initialized (ex: "sys.path"
    doesn’t exist yet).

* “Main” initialization phase, Python is fully initialized:

  * Install and configure "importlib";

  * Apply the Path Configuration;

  * Install signal handlers;

  * Finish "sys" module initialization (ex: create "sys.stdout" and
    "sys.path");

  * Enable optional features like "faulthandler" and "tracemalloc";

  * Import the "site" module;

  * etc.

Private provisional API:

* "PyConfig._init_main": if set to 0, "Py_InitializeFromConfig()"
  stops at the “Core” initialization phase.

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 Path Configuration is only applied
during the “Main” phase. It may allow to customize Python in Python to
override or tune the Path Configuration, maybe install a custom
"sys.meta_path" importer or an import hook, etc.

It may become possible to calculatin the Path Configuration 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);
       }
   }
