Python Initialization Configuration

New in version 3.8.



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”.



List of wchar_t* strings.

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


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.



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)


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.


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


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)) {
    return 0;



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;

preconfig.utf8_mode = 1;

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

/* at this point, Python will speak UTF-8 */

/* ... use Python API here ... */



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:

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.

See also the orig_argv member.

wchar_t* base_exec_prefix


wchar_t* base_executable

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

wchar_t* base_prefix


wchar_t* platlibdir

sys.platlibdir: platform library directory name, set at configure time by --with-platlibdir, overrideable by the PYTHONPLATLIBDIR environment variable.

New in version 3.9.

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

If non-zero, enable the Python Development Mode.

int dump_refs

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

Py_TRACE_REFS macro must be defined in build.

wchar_t* exec_prefix


wchar_t* 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

PyWideStringList orig_argv

The list of the original command line arguments passed to the Python executable.

If orig_argv list is empty and argv is not a list only containing an empty string, PyConfig_Read() copies argv into orig_argv before modifying argv (if parse_argv is non-zero).

See also the argv member and the Py_GetArgcArgv() function.

New in version 3.10.

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


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_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


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.

Changed in version 3.9: The show_alloc_count field has been removed.

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;

    /* Set the program name. Implicitly preinitialize Python. */
    status = PyConfig_SetString(&config, &config.program_name,
    if (PyStatus_Exception(status)) {
        goto fail;

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


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;

    /* 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,
    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,
    if (PyStatus_Exception(status)) {
        goto done;

    /* Override executable computed by PyConfig_Read() */
    status = PyConfig_SetString(&config, &config.executable,
    if (PyStatus_Exception(status)) {
        goto done;

    status = Py_InitializeFromConfig(&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;
    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;

    return Py_RunMain();

    if (PyStatus_IsExit(status)) {
        return status.exitcode;
    /* Display the error message and exit the process with
       non-zero exit code */

Path Configuration

PyConfig contains multiple fields for the path configuration:

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 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


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().


void Py_GetArgcArgv(int *argc, wchar_t ***argv)

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

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

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

Private provisional API:

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

  • PyConfig._isolated_interpreter: if non-zero, disallow threads, subprocesses and fork.

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;
    config._init_main = 0;

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

    status = Py_InitializeFromConfig(&config);
    if (PyStatus_Exception(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', "
    if (res < 0) {

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

    status = _Py_InitializeMain();
    if (PyStatus_Exception(status)) {