# 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_InitPythonConfig`(PyPreConfig *preconfig)

    Initialize the preconfiguration with Python Configuration.

  void `PyPreConfig_InitIsolatedConfig`(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 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;
    }

## 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);
        }
    }