Python Initialization Configuration

PyInitConfig C API

Python can be initialized with Py_InitializeFromInitConfig.

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

See also Initialization, Finalization, and Threads <initialization>.

Example

Example of customized Python always running with the Python Development Mode <devmode> enabled; return -1 on error:

int init_python(void)
{
    PyInitConfig *config = PyInitConfig_Create();
    if (config == NULL) {
        printf("PYTHON INIT ERROR: memory allocation failed\n");
        return -1;
    }

    // Enable the Python Development Mode
    if (PyInitConfig_SetInt(config, "dev_mode", 1) < 0) {
        goto error;
    }

    // Initialize Python with the configuration
    if (Py_InitializeFromInitConfig(config) < 0) {
        goto error;
    }
    PyInitConfig_Free(config);
    return 0;

error:
    {
        // Display the error message.
        //
        // This uncommon braces style is used, because you cannot make
        // goto targets point to variable declarations.
        const char *err_msg;
        (void)PyInitConfig_GetError(config, &err_msg);
        printf("PYTHON INIT ERROR: %s\n", err_msg);
        PyInitConfig_Free(config);
        return -1;
    }
}

Create Config

Opaque structure to configure the Python initialization.

Create a new initialization configuration using Isolated Configuration <init-isolated-conf> default values.

It must be freed by PyInitConfig_Free.

Return NULL on memory allocation failure.

Free memory of the initialization configuration config.

If config is NULL, no operation is performed.

Error Handling

Get the config error message.

• Set *err_msg and return 1 if an error is set.
• Set *err_msg to NULL and return 0 otherwise.

An error message is a UTF-8 encoded string.

If config has an exit code, format the exit code as an error message.

The error message remains valid until another PyInitConfig function is called with config. The caller doesn't have to free the error message.

Get the config exit code.

Return 0 and leave *exitcode unchanged.

In Python 3.15, Py_InitializeFromInitConfig sets an exit code if a command line option wants to exit Python. This is no longer the case in Python 3.16. Instead, the option is now processed in Py_RunMain. This function became useless.

3.16

Get Options

The configuration option name parameter must be a non-NULL null-terminated UTF-8 encoded string. See Configuration Options <pyinitconfig-opts>.

Test if the configuration has an option called name.

Return 1 if the option exists, or return 0 otherwise.

Get an integer configuration option.

• Set *value, and return 0 on success.
• Set an error in config and return -1 on error.

Get a string configuration option as a null-terminated UTF-8 encoded string.

• Set *value, and return 0 on success.
• Set an error in config and return -1 on error.

*value can be set to NULL if the option is an optional string and the option is unset.

On success, the string must be released with free(value) if it's not NULL.

Get a string list configuration option as an array of null-terminated UTF-8 encoded strings.

• Set *length and *value, and return 0 on success.
• Set an error in config and return -1 on error.

On success, the string list must be released with PyInitConfig_FreeStrList(length, items).

Free memory of a string list created by PyInitConfig_GetStrList().

Set Options

The configuration option name parameter must be a non-NULL null-terminated UTF-8 encoded string. See Configuration Options <pyinitconfig-opts>.

Some configuration options have side effects on other options. This logic is only implemented when Py_InitializeFromInitConfig() is called, not by the "Set" functions below. For example, setting dev_mode to 1 does not set faulthandler to 1.

Set an integer configuration option.

• Return 0 on success.
• Set an error in config and return -1 on error.

Set a string configuration option from a null-terminated UTF-8 encoded string. The string is copied.

• Return 0 on success.
• Set an error in config and return -1 on error.

Set a string list configuration option from an array of null-terminated UTF-8 encoded strings. The string list is copied.

• Return 0 on success.
• Set an error in config and return -1 on error.

Module

Add a built-in extension module to the table of built-in modules.

The new module can be imported by the name name, and uses the function initfunc as the initialization function called on the first attempted import.

• Return 0 on success.
• Set an error in config and return -1 on error.

If Python is initialized multiple times, PyInitConfig_AddModule() must be called at each Python initialization.

Similar to the PyImport_AppendInittab function.

Initialize Python

Initialize Python from the initialization configuration.

• Return 0 on success.
• Set an error in config and return -1 on error.

3.16 The function no longer sets an exit code if a command line option wants to exit Python. Instead, the option is now processed in Py_RunMain.

Configuration Options

Visibility:

• Public: Can be retrieved by PyConfig_Get and set by PyConfig_Set.
• Read-only: Can be retrieved by PyConfig_Get, but cannot be set by PyConfig_Set.

Runtime Python configuration API

At runtime, it's possible to get and set configuration options using PyConfig_Get and PyConfig_Set functions.

The configuration option name parameter must be a non-NULL null-terminated UTF-8 encoded string. See Configuration Options <pyinitconfig-opts>.

Some options are read from the sys attributes. For example, the option "argv" is read from sys.argv.

Get the current runtime value of a configuration option as a Python object.

• Return a new reference on success.
• Set an exception and return NULL on error.

The object type depends on the configuration option. It can be:

• bool
• int
• str
• list[str]
• dict[str, str]

The caller must have an attached thread state. The function cannot be called before Python initialization nor after Python finalization.

3.14

Similar to PyConfig_Get, but get the value as a C int.

• Return 0 on success.
• Set an exception and return -1 on error.

3.14

Get all configuration option names as a frozenset.

• Return a new reference on success.
• Set an exception and return NULL on error.

The caller must have an attached thread state. The function cannot be called before Python initialization nor after Python finalization.

3.14

Set the current runtime value of a configuration option.

• Raise a ValueError if there is no option name.
• Raise a ValueError if value is an invalid value.
• Raise a ValueError if the option is read-only (cannot be set).
• Raise a TypeError if value has not the proper type.

The caller must have an attached thread state. The function cannot be called before Python initialization nor after Python finalization.

cpython.PyConfig_Set name,value c.PyConfig_Set

3.14

3.16 The function now replaces sys.flags (create a new object), instead of modifying sys.flags in-place.

PyConfig C API

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 <init-python-config> can be used to build a customized Python which behaves as the regular Python. For example, environment variables and command line arguments are used to configure Python.
• The Isolated Configuration <init-isolated-conf> can be used to embed Python into an application. It isolates Python from the system. For example, environment 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 <initialization>.

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);
    /* Display the error message and exit the process with
       non-zero exit code */
    Py_ExitStatusException(status);
}

PyWideStringList

List of wchar_t* strings.

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

Methods:

Append item to list.

Python must be preinitialized to call this function.

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:

List length.

List items.

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:

Exit code. Argument passed to exit().

3.16

Error message.

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

Functions to create a status:

Success.

Initialization error with a message.

err_msg must not be NULL.

Memory allocation failure (out of memory).

Exit Python with the specified exit code.

3.16 Py_InitializeFromConfig no longer sets an exit code if a command line option wants to exit Python. Instead, the option is now processed in Py_RunMain.

Functions to handle a status:

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

Is the result an error?

Is the result an exit?

3.16 Py_InitializeFromConfig no longer sets an exit code if a command line option wants to exit Python. Instead, the option is now processed in Py_RunMain.

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_RawFree(ptr);
    return 0;
}

PyPreConfig

Structure used to preinitialize Python.

Function to initialize a preconfiguration:

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

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

Structure fields:

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 <default-memory-allocators>.
• PYMEM_ALLOCATOR_DEBUG (2): default memory allocators <default-memory-allocators> with debug hooks <pymem-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-debug-hooks>.
• PYMEM_ALLOCATOR_PYMALLOC (5): Python pymalloc memory allocator <pymalloc>.
• PYMEM_ALLOCATOR_PYMALLOC_DEBUG (6): Python pymalloc memory allocator <pymalloc> with debug hooks <pymem-debug-hooks>.
• PYMEM_ALLOCATOR_MIMALLOC (7): use mimalloc, a fast malloc replacement.
• PYMEM_ALLOCATOR_MIMALLOC_DEBUG (8): use mimalloc, a fast malloc replacement with debug hooks <pymem-debug-hooks>.

PYMEM_ALLOCATOR_PYMALLOC and PYMEM_ALLOCATOR_PYMALLOC_DEBUG are not supported if Python is configured using --without-pymalloc <--without-pymalloc>.

PYMEM_ALLOCATOR_MIMALLOC and PYMEM_ALLOCATOR_MIMALLOC_DEBUG are not supported if Python is configured using --without-mimalloc <--without-mimalloc> or if the underlying atomic support isn't available.

See Memory Management <memory>.

Default: PYMEM_ALLOCATOR_NOT_SET.

Set the LC_CTYPE locale to the user preferred locale.

If equals to 0, set ~PyPreConfig.coerce_c_locale and ~PyPreConfig.coerce_c_locale_warn members to 0.

See the locale encoding.

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

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.

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

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

Python Development Mode <devmode>: see PyConfig.dev_mode.

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

Isolated mode: see PyConfig.isolated.

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

If non-zero:

• Set PyPreConfig.utf8_mode to 0,
• Set PyConfig.filesystem_encoding to "mbcs",
• Set PyConfig.filesystem_errors to "replace".

Initialized from the PYTHONLEGACYWINDOWSFSENCODING environment variable value.

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

Default: 0.

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 <using-on-cmdline>.

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

Use environment variables <using-on-envvars>? See PyConfig.use_environment.

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

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

Set to 0 or 1 by the -X utf8 <-X> command line option and the PYTHONUTF8 environment variable.

Default: 1.

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 <utf8-mode> (PyPreConfig.utf8_mode)

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

Functions to preinitialize Python:

Preinitialize Python from preconfig preconfiguration.

preconfig must not be NULL.

Preinitialize Python from preconfig preconfiguration.

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

preconfig must not be NULL.

Preinitialize Python from preconfig preconfiguration.

Parse argv command line arguments (wide strings) if ~PyPreConfig.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 <init-python-config> (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 <-X> command line option enables the Python UTF-8 Mode <utf8-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 <utf8-mode>:

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

preconfig.utf8_mode = 1;

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

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

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

PyConfig

Structure containing most parameters to configure Python.

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

Structure methods:

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

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

Copy the wide character string str into *config_str.

Preinitialize Python <c-preinit> if needed.

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

Preinitialize Python <c-preinit> if needed.

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

Preinitialize Python <c-preinit> if needed.

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

Preinitialize Python <c-preinit> if needed.

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

Preinitialize Python <c-preinit> if needed.

Read all Python configuration.

Fields which are already initialized are left unchanged.

Fields for path configuration <init-path-config> are no longer calculated or modified when calling this function, as of Python 3.11.

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 stripped from PyConfig.argv, parsing arguments twice would parse the application options as Python options.

Preinitialize Python <c-preinit> if needed.

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.

3.11 PyConfig_Read no longer calculates all paths, and so fields listed under Python Path Configuration <init-path-config> may no longer be updated until Py_InitializeFromConfig is called.

Release configuration memory.

Most PyConfig methods preinitialize Python <c-preinit> if needed. In that case, the Python preinitialization configuration (PyPreConfig) is 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 ~PyConfig.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:

single: main() single: argv (in module sys)

Set sys.argv command line arguments based on ~PyConfig.argv. These parameters are similar to those passed to the program's main function with the difference that the first entry should refer to the script file to be executed rather than the executable hosting the Python interpreter. If there isn't a script that will be run, the first entry in ~PyConfig.argv can be an empty string.

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

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

Default: NULL.

See also the ~PyConfig.orig_argv member.

If equals to zero, Py_RunMain() prepends a potentially unsafe path to sys.path at startup:

• If argv[0] <PyConfig.argv> is equal to L"-m" (python -m module), prepend the current working directory.
• If running a script (python script.py), prepend the script's directory. If it's a symbolic link, resolve symbolic links.
• Otherwise (python -c code and python), prepend an empty string, which means the current working directory.

Set to 1 by the -P command line option and the PYTHONSAFEPATH environment variable.

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

3.11

sys.base_exec_prefix.

Default: NULL.

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

See also PyConfig.exec_prefix.

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 <init-path-config> output.

See also PyConfig.executable.

sys.base_prefix.

Default: NULL.

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

See also PyConfig.prefix.

If equals to 0 and ~PyConfig.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.

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.

3.15 3.17

The -b and !-bb options will become no-op in 3.17. ~PyConfig.bytes_warning member will be removed in 3.17.

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

Default: 0.

3.10

If equals to 0, disables the inclusion of the end line and column mappings in code objects. Also disables traceback printing carets to specific error locations.

Set to 0 by the PYTHONNODEBUGRANGES environment variable and by the -X no_debug_ranges <-X> command line option.

Default: 1.

3.11

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 552 "Deterministic pycs".

If non-zero, configure C standard streams:

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

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

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

Set to 1 by the -X dev <-X> option and the PYTHONDEVMODE environment variable.

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

Dump Python references?

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

Set to 1 by the PYTHONDUMPREFS environment variable.

Needs a special build of Python with the Py_TRACE_REFS macro defined: see the configure --with-trace-refs option <--with-trace-refs>.

Default: 0.

Filename where to dump Python references.

Set by the PYTHONDUMPREFSFILE environment variable.

Default: NULL.

3.11

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

Default: NULL.

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

See also PyConfig.base_exec_prefix.

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

Default: NULL.

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

See also PyConfig.base_executable.

Enable faulthandler?

If non-zero, call faulthandler.enable at startup.

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

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

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

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

On Windows: use "utf-8" by default, or "mbcs" if ~PyPreConfig.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, 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 ~PyConfig.filesystem_errors member.

See also PyUnicode_EncodeFSDefault and related functions <capi-filesystem-encoding> for encoding paths at runtime.

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

On Windows: use "surrogatepass" by default, or "replace" if ~PyPreConfig.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 ~PyConfig.filesystem_encoding member.

If non-zero, use frozen modules.

Set by the PYTHON_FROZEN_MODULES environment variable.

Default: 1 in a release build, or 0 in a debug build <debug-build>.

Randomized hash function seed.

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

Set by the PYTHONHASHSEED environment variable.

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

Set the default Python "home" directory, that is, the location of the standard Python libraries (see PYTHONHOME).

Set by the PYTHONHOME environment variable.

Default: NULL.

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

If 1, profile import time. If 2, include additional output that indicates when an imported module has already been loaded.

Set by the -X importtime <-X> option and the PYTHONPROFILEIMPORTTIME environment variable.

Default: 0.

3.14

Added support for import_time = 2

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.

Install Python signal handlers?

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

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

Incremented by the -i command line option.

Default: 0.

Configures the integer string conversion length limitation <int_max_str_digits>. An initial value of -1 means the value will be taken from the command line or environment or otherwise default to 4300 (sys.int_info.default_max_str_digits). A value of 0 disables the limitation. Values greater than zero but less than 640 (sys.int_info.str_digits_check_threshold) are unsupported and will produce an error.

Configured by the -X int_max_str_digits <-X> command line flag or the PYTHONINTMAXSTRDIGITS environment variable.

Default: -1 in Python mode. 4300 (sys.int_info.default_max_str_digits) in isolated mode.

3.12

If the value of ~PyConfig.cpu_count is not -1 then it will override the return values of os.cpu_count, os.process_cpu_count, and multiprocessing.cpu_count.

Configured by the -X cpu_count={n|default} command line flag or the PYTHON_CPU_COUNT environment variable.

Default: -1.

3.13

If greater than 0, enable isolated mode:

• Set ~PyConfig.safe_path to 1: don't prepend a potentially unsafe path to sys.path at Python startup, such as the current directory, the script's directory or an empty string.
• Set ~PyConfig.use_environment to 0: ignore PYTHON environment variables.
• Set ~PyConfig.user_site_directory to 0: don't add the user site directory to sys.path.
• Python REPL doesn't import readline nor enable default readline configuration on interactive prompts.

Set to 1 by the -I command line option.

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

See also the Isolated Configuration <init-isolated-conf> and PyPreConfig.isolated.

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 528 (Change Windows console encoding to UTF-8).

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

Set to 1 by the PYTHONMALLOCSTATS environment variable.

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

Default: 0.

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 <--with-platlibdir> (default: "lib", or "DLLs" on Windows).

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

3.9

3.11 This macro is now used on Windows to locate the standard library extension modules, typically under DLLs. However, for compatibility, note that this value is ignored for any non-standard layouts, including in-tree builds and virtual environments.

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

Set by the PYTHONPATH environment variable.

Default: NULL.

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

Module search paths: sys.path.

If ~PyConfig.module_search_paths_set is equal to 0, Py_InitializeFromConfig will replace ~PyConfig.module_search_paths and sets ~PyConfig.module_search_paths_set to 1.

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

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

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.

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

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

See also the ~PyConfig.argv member and the Py_GetArgcArgv function.

Default: empty list.

3.10

Parse command line arguments?

If equals to 1, parse ~PyConfig.argv the same way the regular Python parses command line arguments <using-on-cmdline>, and strip Python arguments from ~PyConfig.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 stripped from PyConfig.argv, parsing arguments twice would parse the application options as Python options.

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

3.10 The PyConfig.argv arguments are now only parsed if PyConfig.parse_argv equals to 1.

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.

Needs a debug build of Python <debug-build> (the Py_DEBUG macro must be defined).

Default: 0.

If non-zero, calculation of path configuration is allowed to log warnings into stderr. If equals to 0, suppress these warnings.

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

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

3.11 Now also applies on Windows.

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

Default: NULL.

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

See also PyConfig.base_prefix.

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

• 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 ~PyConfig.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 <init-path-config> input.

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

Set by the -X pycache_prefix=PATH <-X> command line option and the PYTHONPYCACHEPREFIX environment variable. The command-line option takes precedence.

If NULL, sys.pycache_prefix is set to None.

Default: NULL.

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.

Value of the -c command line option.

Used by Py_RunMain.

Default: NULL.

Filename passed on the command line: trailing command line argument without -c or -m. It is used by the Py_RunMain function.

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

See also the PyConfig.skip_source_first_line option.

Default: NULL.

Value of the -m command line option.

Used by Py_RunMain.

Default: NULL.

module or module:func entry point that should be executed before the site module is imported.

Set by the -X presite=module:func <-X> command-line option and the PYTHON_PRESITE environment variable. The command-line option takes precedence.

Needs a debug build of Python <debug-build> (the Py_DEBUG macro must be defined).

Default: NULL.

Show total reference count at exit (excluding immortal objects)?

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

Needs a debug build of Python <debug-build> (the Py_REF_DEBUG macro must be defined).

Default: 0.

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 <sys.flags> is set to the inverted value of ~PyConfig.site_import.

Default: 1.

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.

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

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.

See also PyConfig.legacy_windows_stdio.

Enable tracemalloc?

If non-zero, call tracemalloc.start at startup.

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

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

Enable the Linux perf profiler support?

If equals to 1, enable support for the Linux perf profiler.

If equals to 2, enable support for the Linux perf profiler with DWARF JIT support.

Set to 1 by -X perf <-X> command-line option and the PYTHONPERFSUPPORT environment variable.

Set to 2 by the -X perf_jit <-X> command-line option and the PYTHON_PERF_JIT_SUPPORT environment variable.

Default: -1.

See perf_profiling for more information.

3.12

Directory of the Python standard library.

Default: NULL.

3.11

Use environment variables <using-on-envvars>?

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

Set to 0 by the -E environment variable.

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

If non-zero, stdout and stderr will be redirected to the system log.

Only available on macOS 10.12 and later, and on iOS.

Default: 0 (don't use the system log) on macOS; 1 on iOS (use the system log).

3.14

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.

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 than 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 by the PYTHONVERBOSE environment variable value.

Default: 0.

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 ~PyConfig.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.

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 ~PyConfig.write_bytecode.

Default: 1.

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

Default: empty list.

If non-zero, write performance statistics at Python exit.

Need a special build with the Py_STATS macro: see --enable-pystats.

Default: 0.

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

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

Initialization with PyConfig

Initializing the interpreter from a populated configuration struct is handled by calling Py_InitializeFromConfig.

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. Note that since 3.11, many parameters are not calculated until initialization, and so values cannot be read from the configuration structure. Any values set before initialize is called will be left unchanged by initialization:

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;
    }

    /* Specify sys.path explicitly */
    /* If you want to modify the default set of paths, finish
       initialization first and then use PySys_GetAttrString("path") */
    config.module_search_paths_set = 1;
    status = PyWideStringList_Append(&config.module_search_paths,
                                     L"/path/to/stdlib");
    if (PyStatus_Exception(status)) {
        goto done;
    }
    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, environment 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 to determine paths that are unspecified. Ensure PyConfig.home is specified to avoid 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 (538) and Python UTF-8 Mode <utf8-mode> (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 ~PyConfig.module_search_paths_set is equal to 0, ~PyConfig.module_search_paths is overridden and ~PyConfig.module_search_paths_set is set to 1.

It is possible to completely ignore the function calculating the default path configuration by setting explicitly all path configuration output fields listed above. A string is considered as set even if it is non-empty. module_search_paths is considered as set if module_search_paths_set is set to 1. In this case, module_search_paths will be used without modification.

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

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

Py_RunMain and Py_Main modify sys.path:

• If ~PyConfig.run_filename is set and is a directory which contains a __main__.py script, prepend ~PyConfig.run_filename to sys.path.
• If ~PyConfig.isolated is zero:
  • If ~PyConfig.run_module is set, prepend the current directory to sys.path. Do nothing if the current directory cannot be read.
  • If ~PyConfig.run_filename is set, prepend the directory of the filename to sys.path.
  • Otherwise, prepend an empty string to sys.path.

If ~PyConfig.site_import is non-zero, sys.path can be modified by the site module. If ~PyConfig.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
• ._pth file (ex: python._pth)
• pybuilddir.txt (Unix only)

If a ._pth file is present:

• Set ~PyConfig.isolated to 1.
• Set ~PyConfig.use_environment to 0.
• Set ~PyConfig.site_import to 0.
• Set ~PyConfig.user_site_directory to 0 (since 3.15).
• Set ~PyConfig.safe_path to 1.

If ~PyConfig.home is not set and a pyvenv.cfg file is present in the same directory as ~PyConfig.executable, or its parent, ~PyConfig.prefix and ~PyConfig.exec_prefix are set that location. When this happens, ~PyConfig.base_prefix and ~PyConfig.base_exec_prefix still keep their value, pointing to the base installation. See sys-path-init-virtual-environments for more information.

The __PYVENV_LAUNCHER__ environment variable is used to set PyConfig.base_executable.

Py_GetArgcArgv()

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 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 <init-path-config>;
  • 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:

If set to 0, Py_InitializeFromConfig stops at the "Core" initialization phase.

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

It may become possible to calculate the Path Configuration <init-path-config> in Python, after the Core phase and before the Main phase, which is one of the 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);
    }
}