Python Initialization Configuration — Python 3.10.0 documentation

# Python Initialization Configuration

New in version 3.8.

Python can be initialized with `Py_InitializeFromConfig()` and the `PyConfig` structure. It can be preinitialized with `Py_PreInitialize()` and the `PyPreConfig` structure.

There are two kinds of configuration:

- The Python Configuration 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 Isolated Configuration 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 registered.

The `Py_RunMain()` function can be used to write a customized Python program.

See also Initialization, Finalization, and Threads.

See also

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

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:

  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

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:

  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.

    err_msg must not be `NULL`.

  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

type `PyPreConfig`

  Structure used to preinitialize Python.

  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 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`): use `malloc()` of the C library.

    - `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.

    Default: `PYMEM_ALLOCATOR_NOT_SET`.

  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` members to 0.

    See the locale encoding.

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

  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 locale encoding.

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

  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.

  int `dev_mode`

    If non-zero, enables the Python Development Mode: see `PyConfig.dev_mode`.

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

  int `isolated`

    Isolated mode: see `PyConfig.isolated`.

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

  int `legacy_windows_fs_encoding`

    If non-zero:

    - Set `PyPreConfig.utf8_mode` to `0`,

    - Set `PyConfig.filesystem_encoding` to `"mbcs"`,

    - Set `PyConfig.filesystem_errors` to `"replace"`.

    Initialized the from `PYTHONLEGACYWINDOWSFSENCODING` environment variable value.

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

    Default: `0`.

  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.

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

  int `use_environment`

    Use environment variables? See `PyConfig.use_environment`.

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

  int `utf8_mode`

    If non-zero, enable the Python UTF-8 Mode.

    Set by the `-X utf8` command line option and the `PYTHONUTF8` environment variable.

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

## Preinitialize Python with PyPreConfig

The preinitialization of Python:

- Set the Python memory allocators (`PyPreConfig.allocator`)

- Configure the LC_CTYPE locale (locale encoding)

- Set the Python UTF-8 Mode (`PyPreConfig.utf8_mode`)

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

Functions to preinitialize Python:

PyStatus `Py_PreInitialize`(const PyPreConfig *preconfig)

  Preinitialize Python from preconfig preconfiguration.

  preconfig must not be `NULL`.

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

  Preinitialize Python from preconfig preconfiguration.

  Parse argv command line arguments (bytes strings) if `parse_argv` of preconfig is non-zero.

  preconfig must not be `NULL`.

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 `parse_argv` of preconfig is non-zero.

  preconfig must not be `NULL`.

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 Python 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 the Python preinitialization, whereas calling directly `malloc()` and `free()` is always safe. `Py_DecodeLocale()` must not be called before the Python preinitialization.

Example using the preinitialization to enable the Python 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 speaks UTF-8 */

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

## PyConfig

type `PyConfig`

  Structure containing most parameters to configure Python.

  When done, the `PyConfig_Clear()` function must be used to release the configuration memory.

  Structure methods:

  void `PyConfig_InitPythonConfig`(PyConfig *config)

    Initialize configuration with the Python Configuration.

  void `PyConfig_InitIsolatedConfig`(PyConfig *config)

    Initialize configuration with the 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 (`argv` member of config) from the argv list of wide character strings.

    Preinitialize Python if needed.

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

    Set command line arguments (`argv` member of config) from the argv list of bytes strings. 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.

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

    Preinitialize Python if needed.

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

  void `PyConfig_Clear`(PyConfig *config)

    Release configuration memory.

  Most `PyConfig` methods preinitialize Python if needed. In that case, the Python preinitialization configuration (`PyPreConfig`) 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:

  - `PyConfig.dev_mode`

  - `PyConfig.isolated`

  - `PyConfig.parse_argv`

  - `PyConfig.use_environment`

  Moreover, if `PyConfig_SetArgv()` or `PyConfig_SetBytesArgv()` is used, this method must be called 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`.

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

    If `argv` is empty, an empty string is added to ensure that `sys.argv` always exists and is never empty.

    Default: `NULL`.

    See also the `orig_argv` member.

  wchar_t *`base_exec_prefix`

    `sys.base_exec_prefix`.

    Default: `NULL`.

    Part of the Python Path Configuration output.

  wchar_t *`base_executable`

    Python base executable: `sys._base_executable`.

    Set by the `__PYVENV_LAUNCHER__` environment variable.

    Set from `PyConfig.executable` if `NULL`.

    Default: `NULL`.

    Part of the Python Path Configuration output.

  wchar_t *`base_prefix`

    `sys.base_prefix`.

    Default: `NULL`.

    Part of the Python Path Configuration output.

  int `buffered_stdio`

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

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

    stdin is always opened in buffered mode.

    Default: `1`.

  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 in these cases.

    Incremented by the `-b` command line option.

    Default: `0`.

  int `warn_default_encoding`

    If non-zero, emit a `EncodingWarning` warning when `io.TextIOWrapper` uses its default encoding. See Opt-in EncodingWarning for details.

    Default: `0`.

    New in version 3.10.

  wchar_t *`check_hash_pycs_mode`

    Control the validation behavior of hash-based `.pyc` files: value of the `--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”.

  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 `buffered_stdio` equals zero, disable buffering of stdin, stdout and stderr streams.

    - If `interactive` is non-zero, enable stream buffering on stdin and stdout (only stdout on Windows).

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

  int `dev_mode`

    If non-zero, enable the Python Development Mode.

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

  int `dump_refs`

    Dump Python refererences?

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

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

    Need a special build of Python with the `Py_TRACE_REFS` macro defined: see the `configure --with-trace-refs option`.

    Default: `0`.

  wchar_t *`exec_prefix`

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

    Default: `NULL`.

    Part of the Python Path Configuration output.

  wchar_t *`executable`

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

    Default: `NULL`.

    Part of the Python Path Configuration output.

  int `faulthandler`

    Enable faulthandler?

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

    Set to `1` by `-X faulthandler` and the `PYTHONFAULTHANDLER` environment variable.

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

  wchar_t *`filesystem_encoding`

    Filesystem encoding: `sys.getfilesystemencoding()`.

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

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

    Default encoding on other platforms:

    - `"utf-8"` if `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 locale encoding: `nl_langinfo(CODESET)` result.

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

    See also the `filesystem_errors` member.

  wchar_t *`filesystem_errors`

    Filesystem error handler: `sys.getfilesystemencodeerrors()`.

    On Windows: use `"surrogatepass"` by default, or `"replace"` if `legacy_windows_fs_encoding` of `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 `filesystem_encoding` member.

  unsigned long `hash_seed`
  int `use_hash_seed`

    Randomized hash function seed.

    If `use_hash_seed` is zero, a seed is chosen randomly at Python startup, and `hash_seed` is ignored.

    Set by the `PYTHONHASHSEED` environment variable.

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

  wchar_t *`home`

    Python home directory.

    If `Py_SetPythonHome()` has been called, use its argument if it is not `NULL`.

    Set by the `PYTHONHOME` environment variable.

    Default: `NULL`.

    Part of the Python Path Configuration input.

  int `import_time`

    If non-zero, profile import time.

    Set the `1` by the `-X importtime` option and the `PYTHONPROFILEIMPORTTIME` environment variable.

    Default: `0`.

  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 `sys.stdin` does not appear to be a terminal.

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

    Default: `0`.

  int `install_signal_handlers`

    Install Python signal handlers?

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

  int `interactive`

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

    Incremented by the `-i` command line option.

    Default: `0`.

  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.

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

    See also `PyPreConfig.isolated`.

  int `legacy_windows_stdio`

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

    Set to `1` if the `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).

  int `malloc_stats`

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

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

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

    Default: `0`.

  wchar_t *`platlibdir`

    Platform library directory name: `sys.platlibdir`.

    Set by the `PYTHONPLATLIBDIR` environment variable.

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

    Part of the Python Path Configuration input.

    New in version 3.9.

  wchar_t *`pythonpath_env`

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

    Set by the `PYTHONPATH` environment variable.

    Default: `NULL`.

    Part of the Python Path Configuration input.

  PyWideStringList `module_search_paths`
  int `module_search_paths_set`

    Module search paths: `sys.path`.

    If `module_search_paths_set` is equal to 0, the function calculating the Python Path Configuration overrides the `module_search_paths` and sets `module_search_paths_set` to `1`.

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

    Part of the Python Path Configuration output.

  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 `-O` command line option. Set to the `PYTHONOPTIMIZE` environment variable value.

    Default: `0`.

  PyWideStringList `orig_argv`

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

    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.

    Default: empty list.

    New in version 3.10.

  int `parse_argv`

    Parse command line arguments?

    If equals to `1`, parse `argv` the same way the regular Python parses command line arguments, and strip Python arguments from `argv`.

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

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

    Changed in version 3.10: The `PyConfig.argv` arguments are now only parsed if `PyConfig.parse_argv` equals to `1`.

  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 `-d` command line option. Set to the `PYTHONDEBUG` environment variable value.

    Default: `0`.

  int `pathconfig_warnings`

    On Unix, if non-zero, calculating the Python Path Configuration 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 Python Path Configuration input.

  wchar_t *`prefix`

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

    Default: `NULL`.

    Part of the Python Path Configuration output.

  wchar_t *`program_name`

    Program name used to initialize `executable` and in early error messages during Python initialization.

    - If `Py_SetProgramName()` has been called, use its argument.

    - On macOS, use `PYTHONEXECUTABLE` environment variable if set.

    - If the `WITH_NEXT_FRAMEWORK` macro is defined, use `__PYVENV_LAUNCHER__` environment variable if set.

    - Use `argv[0]` of `argv` if available and non-empty.

    - Otherwise, use `L"python"` on Windows, or `L"python3"` on other platforms.

    Default: `NULL`.

    Part of the Python Path Configuration input.

  wchar_t *`pycache_prefix`

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

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

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

    Default: `NULL`.

  int `quiet`

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

    Incremented by the `-q` command line option.

    Default: `0`.

  wchar_t *`run_command`

    Value of the `-c` command line option.

    Used by `Py_RunMain()`.

    Default: `NULL`.

  wchar_t *`run_filename`

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

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

    Used by `Py_RunMain()`.

    Default: `NULL`.

  wchar_t *`run_module`

    Value of the `-m` command line option.

    Used by `Py_RunMain()`.

    Default: `NULL`.

  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 (the `Py_REF_DEBUG` macro must be defined).

    Default: `0`.

  int `site_import`

    Import the `site` module at startup?

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

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

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

    `sys.flags.no_site` is set to the inverted value of `site_import`.

    Default: `1`.

  int `skip_source_first_line`

    If non-zero, skip the first line of the `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 `-x` command line option.

    Default: `0`.

  wchar_t *`stdio_encoding`
  wchar_t *`stdio_errors`

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

    If `Py_SetStandardStreamEncoding()` has been called, use its error and errors arguments if they are not `NULL`.

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

    Default encoding:

    - `"UTF-8"` if `PyPreConfig.utf8_mode` is non-zero.

    - Otherwise, use the locale encoding.

    Default error handler:

    - On Windows: use `"surrogateescape"`.

    - `"surrogateescape"` if `PyPreConfig.utf8_mode` is non-zero, or if the LC_CTYPE locale is “C” or “POSIX”.

    - `"strict"` otherwise.

  int `tracemalloc`

    Enable tracemalloc?

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

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

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

  int `use_environment`

    Use environment variables?

    If equals to zero, ignore the environment variables.

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

  int `user_site_directory`

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

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

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

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

  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 `-v` command line option.

    Set to the `PYTHONVERBOSE` environment variable value.

    Default: `0`.

  PyWideStringList `warnoptions`

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

    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).

    The `-W` command line options adds its value to `warnoptions`, it can be used multiple times.

    The `PYTHONWARNINGS` environment variable can also be used to add warning options. Multiple options can be specified, separated by commas (`,`).

    Default: empty list.

  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 `-B` command line option and the `PYTHONDONTWRITEBYTECODE` environment variable.

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

    Default: `1`.

  PyWideStringList `xoptions`

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

    Default: empty list.

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`.

The `xoptions` options are parsed to set other options: see the `-X` command line 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. If Python is initialized multiple times, `PyImport_AppendInittab()` or `PyImport_ExtendInittab()` must be called before each 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;
    }

## 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 Python 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 Python UTF-8 Mode (PEP 540) depending on the LC_CTYPE locale, `PYTHONUTF8` and `PYTHONCOERCECLOCALE` environment variables.

## Python Path Configuration

`PyConfig` contains multiple fields for the path configuration:

- Path configuration inputs:

  - `PyConfig.home`

  - `PyConfig.platlibdir`

  - `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()`.

## Py_GetArgcArgv()

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:

  - 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.

- `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;
        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);
        }
    }