Interpreter initialization and finalization

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Before Python initialization#
Global configuration variables#
Initializing and finalizing the interpreter#
Cautions regarding runtime finalization#
Process-wide parameters#

Interpreter initialization and finalization#

See Python Initialization Configuration for details on how to configure the interpreter prior to initialization.

Before Python initialization#

In an application embedding Python, the "Py_Initialize()" function must be called before using any other Python/C API functions; with the exception of a few functions and the global configuration variables.

The following functions can be safely called before Python is initialized:

Functions that initialize the interpreter:
"Py_Initialize()"
"Py_InitializeEx()"
"Py_InitializeFromConfig()"
"Py_BytesMain()"
"Py_Main()"
the runtime pre-initialization functions covered in Configuración de inicialización de Python
Configuration functions:
"PyImport_AppendInittab()"
"PyImport_ExtendInittab()"
"PyInitFrozenExtensions()"
"PyMem_SetAllocator()"
"PyMem_SetupDebugHooks()"
"PyObject_SetArenaAllocator()"
"Py_SetProgramName()"
"Py_SetPythonHome()"
the configuration functions covered in Configuración de inicialización de Python
Informative functions:
"Py_IsInitialized()"
"PyMem_GetAllocator()"
"PyObject_GetArenaAllocator()"
"Py_GetBuildInfo()"
"Py_GetCompiler()"
"Py_GetCopyright()"
"Py_GetPlatform()"
"Py_GetVersion()"
"Py_IsInitialized()"
Utilities:
"Py_DecodeLocale()"
the status reporting and utility functions covered in Configuración de inicialización de Python
Memory allocators:
"PyMem_RawMalloc()"
"PyMem_RawRealloc()"
"PyMem_RawCalloc()"
"PyMem_RawFree()"
Synchronization:
"PyMutex_Lock()"
"PyMutex_Unlock()"

Nota:

Despite their apparent similarity to some of the functions listed above, the following functions should not be called before the interpreter has been initialized: "Py_EncodeLocale()", "PyEval_InitThreads()", and "Py_RunMain()".

Global configuration variables#

Python has variables for the global configuration to control different features and options. By default, these flags are controlled by command line options.

When a flag is set by an option, the value of the flag is the number of times that the option was set. For example, "-b" sets "Py_BytesWarningFlag" to 1 and "-bb" sets "Py_BytesWarningFlag" to 2.

int Py_BytesWarningFlag

This API is kept for backward compatibility: setting "PyConfig.bytes_warning" should be used instead, see Python Initialization Configuration.

Issue a warning when comparing "bytes" or "bytearray" with "str" or "bytes" with "int". Issue an error if greater or equal to "2".

Set by the "-b" option.

Deprecated since version 3.12, will be removed in version 3.15.

int Py_DebugFlag

This API is kept for backward compatibility: setting "PyConfig.parser_debug" should be used instead, see Python Initialization Configuration.

Turn on parser debugging output (for expert only, depending on compilation options).

Set by the "-d" option and the "PYTHONDEBUG" environment variable.

Deprecated since version 3.12, will be removed in version 3.15.

int Py_DontWriteBytecodeFlag

This API is kept for backward compatibility: setting "PyConfig.write_bytecode" should be used instead, see Python Initialization Configuration.

If set to non-zero, Python won't try to write ".pyc" files on the import of source modules.

Set by the "-B" option and the "PYTHONDONTWRITEBYTECODE" environment variable.

Deprecated since version 3.12, will be removed in version 3.15.

int Py_FrozenFlag

This API is kept for backward compatibility: setting "PyConfig.pathconfig_warnings" should be used instead, see Python Initialization Configuration.

Private flag used by "_freeze_module" and "frozenmain" programs.

Deprecated since version 3.12, will be removed in version 3.15.

int Py_HashRandomizationFlag

This API is kept for backward compatibility: setting "PyConfig.hash_seed" and "PyConfig.use_hash_seed" should be used instead, see Python Initialization Configuration.

Set to "1" if the "PYTHONHASHSEED" environment variable is set to a non-empty string.

If the flag is non-zero, read the "PYTHONHASHSEED" environment variable to initialize the secret hash seed.

Deprecated since version 3.12, will be removed in version 3.15.

int Py_IgnoreEnvironmentFlag

This API is kept for backward compatibility: setting "PyConfig.use_environment" should be used instead, see Python Initialization Configuration.

Ignore all "PYTHON*" environment variables, e.g. "PYTHONPATH" and "PYTHONHOME", that might be set.

Set by the "-E" and "-I" options.

Deprecated since version 3.12, will be removed in version 3.15.

int Py_InspectFlag

This API is kept for backward compatibility: setting "PyConfig.inspect" should be used instead, see Python Initialization Configuration.

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.

Set by the "-i" option and the "PYTHONINSPECT" environment variable.

Deprecated since version 3.12, will be removed in version 3.15.

int Py_InteractiveFlag

This API is kept for backward compatibility: setting "PyConfig.interactive" should be used instead, see Python Initialization Configuration.

Set by the "-i" option.

Deprecated since version 3.12, will be removed in version 3.15.

int Py_IsolatedFlag

This API is kept for backward compatibility: setting "PyConfig.isolated" should be used instead, see Python Initialization Configuration.

Run Python in isolated mode. In isolated mode "sys.path" contains neither the script's directory nor the user's site-packages directory.

Set by the "-I" option.

Added in version 3.4.

Deprecated since version 3.12, will be removed in version 3.15.

int Py_LegacyWindowsFSEncodingFlag

This API is kept for backward compatibility: setting "PyPreConfig.legacy_windows_fs_encoding" should be used instead, see Python Initialization Configuration.

If the flag is non-zero, use the "mbcs" encoding with "replace" error handler, instead of the UTF-8 encoding with "surrogatepass" error handler, for the filesystem encoding and error handler.

Set to "1" if the "PYTHONLEGACYWINDOWSFSENCODING" environment variable is set to a non-empty string.

See PEP 529 for more details.

Availability: Windows.

Deprecated since version 3.12, will be removed in version 3.15.

int Py_LegacyWindowsStdioFlag

This API is kept for backward compatibility: setting "PyConfig.legacy_windows_stdio" should be used instead, see Python Initialization Configuration.

If the flag is non-zero, use "io.FileIO" instead of "io._WindowsConsoleIO" for "sys" standard streams.

Set to "1" if the "PYTHONLEGACYWINDOWSSTDIO" environment variable is set to a non-empty string.

See PEP 528 for more details.

Availability: Windows.

Deprecated since version 3.12, will be removed in version 3.15.

int Py_NoSiteFlag

This API is kept for backward compatibility: setting "PyConfig.site_import" should be used instead, see Python Initialization Configuration.

Disable the import of the module "site" and the site-dependent manipulations of "sys.path" that it entails. Also disable these manipulations if "site" is explicitly imported later (call "site.main()" if you want them to be triggered).

Set by the "-S" option.

Deprecated since version 3.12, will be removed in version 3.15.

int Py_NoUserSiteDirectory

This API is kept for backward compatibility: setting "PyConfig.user_site_directory" should be used instead, see Python Initialization Configuration.

Don't add the "user site-packages directory" to "sys.path".

Set by the "-s" and "-I" options, and the "PYTHONNOUSERSITE" environment variable.

Deprecated since version 3.12, will be removed in version 3.15.

int Py_OptimizeFlag

This API is kept for backward compatibility: setting "PyConfig.optimization_level" should be used instead, see Python Initialization Configuration.

Set by the "-O" option and the "PYTHONOPTIMIZE" environment variable.

Deprecated since version 3.12, will be removed in version 3.15.

int Py_QuietFlag

This API is kept for backward compatibility: setting "PyConfig.quiet" should be used instead, see Python Initialization Configuration.

Don't display the copyright and version messages even in interactive mode.

Set by the "-q" option.

Added in version 3.2.

Deprecated since version 3.12, will be removed in version 3.15.

int Py_UnbufferedStdioFlag

This API is kept for backward compatibility: setting "PyConfig.buffered_stdio" should be used instead, see Python Initialization Configuration.

Force the stdout and stderr streams to be unbuffered.

Set by the "-u" option and the "PYTHONUNBUFFERED" environment variable.

Deprecated since version 3.12, will be removed in version 3.15.

int Py_VerboseFlag

This API is kept for backward compatibility: setting "PyConfig.verbose" should be used instead, see Python Initialization Configuration.

Print a message each time a module is initialized, 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.

Set by the "-v" option and the "PYTHONVERBOSE" environment variable.

Deprecated since version 3.12, will be removed in version 3.15.

Initializing and finalizing the interpreter#

void Py_Initialize() * Part of the Stable ABI.*

Initialize the Python interpreter. In an application embedding Python, this should be called before using any other Python/C API functions; see Before Python Initialization for the few exceptions.

This initializes the table of loaded modules ("sys.modules"), and creates the fundamental modules "builtins", "main" and "sys". It also initializes the module search path ("sys.path"). It does not set "sys.argv"; use the Python Initialization Configuration API for that. This is a no-op when called for a second time (without calling "Py_FinalizeEx()" first). There is no return value; it is a fatal error if the initialization fails.

Use "Py_InitializeFromConfig()" to customize the Python Initialization Configuration.

Nota:

 On Windows, changes the console mode from "O_TEXT" to "O_BINARY",
 which will also affect non-Python uses of the console using the C
 Runtime.

void Py_InitializeEx(int initsigs) * Part of the Stable ABI.*

This function works like "Py_Initialize()" if initsigs is "1". If initsigs is "0", it skips initialization registration of signal handlers, which may be useful when CPython is embedded as part of a larger application.

Use "Py_InitializeFromConfig()" to customize the Python Initialization Configuration.

PyStatus Py_InitializeFromConfig(const PyConfig *config)

Initialize Python from config configuration, as described in Inicialización con PyConfig.

See the Configuración de inicialización de Python section for details on pre-initializing the interpreter, populating the runtime configuration structure, and querying the returned status structure.

int Py_IsInitialized() * Part of the Stable ABI.*

Return true (nonzero) when the Python interpreter has been initialized, false (zero) if not. After "Py_FinalizeEx()" is called, this returns false until "Py_Initialize()" is called again.

int Py_IsFinalizing() * Part of the Stable ABI since version 3.13.*

Return true (non-zero) if the main Python interpreter is shutting down. Return false (zero) otherwise.

Added in version 3.13.

int Py_FinalizeEx() * Part of the Stable ABI since version 3.6.*

Undo all initializations made by "Py_Initialize()" and subsequent use of Python/C API functions, and destroy all sub-interpreters (see "Py_NewInterpreter()" below) that were created and not yet destroyed since the last call to "Py_Initialize()". This is a no- op when called for a second time (without calling "Py_Initialize()" again first).

Since this is the reverse of "Py_Initialize()", it should be called in the same thread with the same interpreter active. That means the main thread and the main interpreter. This should never be called while "Py_RunMain()" is running.

Normally the return value is "0". If there were errors during finalization (flushing buffered data), "-1" is returned.

Note that Python will do a best effort at freeing all memory allocated by the Python interpreter. Therefore, any C-Extension should make sure to correctly clean up all of the previously allocated PyObjects before using them in subsequent calls to "Py_Initialize()". Otherwise it could introduce vulnerabilities and incorrect behavior.

This function is provided for a number of reasons. An embedding application might want to restart Python without having to restart the application itself. An application that has loaded the Python interpreter from a dynamically loadable library (or DLL) might want to free all memory allocated by Python before unloading the DLL. During a hunt for memory leaks in an application a developer might want to free all memory allocated by Python before exiting from the application.

Bugs and caveats: The destruction of modules and objects in modules is done in random order; this may cause destructors ("del()" methods) to fail when they depend on other objects (even functions) or modules. Dynamically loaded extension modules loaded by Python are not unloaded. Small amounts of memory allocated by the Python interpreter may not be freed (if you find a leak, please report it). Memory tied up in circular references between objects is not freed. Interned strings will all be deallocated regardless of their reference count. Some memory allocated by extension modules may not be freed. Some extensions may not work properly if their initialization routine is called more than once; this can happen if an application calls "Py_Initialize()" and "Py_FinalizeEx()" more than once. "Py_FinalizeEx()" must not be called recursively from within itself. Therefore, it must not be called by any code that may be run as part of the interpreter shutdown process, such as "atexit" handlers, object finalizers, or any code that may be run while flushing the stdout and stderr files.

Raises an auditing event "cpython._PySys_ClearAuditHooks" with no arguments.

Added in version 3.6.

void Py_Finalize() * Part of the Stable ABI.*

This is a backwards-compatible version of "Py_FinalizeEx()" that disregards the return value.

int Py_BytesMain(int argc, char *argv) * Part of the Stable ABI since version 3.8.

Similar to "Py_Main()" but argv is an array of bytes strings, allowing the calling application to delegate the text decoding step to the CPython runtime.

Added in version 3.8.

int Py_Main(int argc, wchar_t *argv) * Part of the Stable ABI.

The main program for the standard interpreter, encapsulating a full initialization/finalization cycle, as well as additional behaviour to implement reading configurations settings from the environment and command line, and then executing "main" in accordance with Línea de comando.

This is made available for programs which wish to support the full CPython command line interface, rather than just embedding a Python runtime in a larger application.

The argc and argv parameters are similar to those which are passed to a C program's "main()" function, except that the argv entries are first converted to "wchar_t" using "Py_DecodeLocale()". It is also important to note that the argument list entries may be modified to point to strings other than those passed in (however, the contents of the strings pointed to by the argument list are not modified).

The return value is "2" if the argument list does not represent a valid Python command line, and otherwise the same as "Py_RunMain()".

In terms of the CPython runtime configuration APIs documented in the runtime configuration section (and without accounting for error handling), "Py_Main" is approximately equivalent to:

  PyConfig config;
  PyConfig_InitPythonConfig(&config);
  PyConfig_SetArgv(&config, argc, argv);
  Py_InitializeFromConfig(&config);
  PyConfig_Clear(&config);

  Py_RunMain();

In normal usage, an embedding application will call this function instead of calling "Py_Initialize()", "Py_InitializeEx()" or "Py_InitializeFromConfig()" directly, and all settings will be applied as described elsewhere in this documentation. If this function is instead called after a preceding runtime initialization API call, then exactly which environmental and command line configuration settings will be updated is version dependent (as it depends on which settings correctly support being modified after they have already been set once when the runtime was first initialized).

int Py_RunMain(void)

Executes the main module in a fully configured CPython runtime.

Executes 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. If none of these values are set, runs the interactive Python prompt (REPL) using the "main" module's global namespace.

If "PyConfig.inspect" is not set (the default), the return value will be "0" if the interpreter exits normally (that is, without raising an exception), the exit status of an unhandled "SystemExit", or "1" for any other unhandled exception.

If "PyConfig.inspect" is set (such as when the "-i" option is used), rather than returning when the interpreter exits, execution will instead resume in an interactive Python prompt (REPL) using the "main" module's global namespace. If the interpreter exited with an exception, it is immediately raised in the REPL session. The function return value is then determined by the way the REPL session terminates: "0", "1", or the status of a "SystemExit", as specified above.

This function always finalizes the Python interpreter before it returns.

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

int PyUnstable_AtExit(PyInterpreterState interp, void (func)(void), void data)

This is Unstable API. It may change without warning in minor releases.

Register an "atexit" callback for the target interpreter interp. This is similar to "Py_AtExit()", but takes an explicit interpreter and data pointer for the callback.

There must be an attached thread state for interp.

Added in version 3.13.

Cautions regarding runtime finalization#

In the late stage of interpreter shutdown, after attempting to wait for non-daemon threads to exit (though this can be interrupted by "KeyboardInterrupt") and running the "atexit" functions, the runtime is marked as finalizing: "Py_IsFinalizing()" and "sys.is_finalizing()" return true. At this point, only the finalization thread that initiated finalization (typically the main thread) is allowed to acquire the GIL.

If any thread, other than the finalization thread, attempts to attach a thread state during finalization, either explicitly or implicitly, the thread enters a permanently blocked state where it remains until the program exits. In most cases this is harmless, but this can result in deadlock if a later stage of finalization attempts to acquire a lock owned by the blocked thread, or otherwise waits on the blocked thread.

Gross? Yes. This prevents random crashes and/or unexpectedly skipped C++ finalizations further up the call stack when such threads were forcibly exited here in CPython 3.13 and earlier. The CPython runtime thread state C APIs have never had any error reporting or handling expectations at thread state attachment time that would've allowed for graceful exit from this situation. Changing that would require new stable C APIs and rewriting the majority of C code in the CPython ecosystem to use those with error handling.

Process-wide parameters#

void Py_SetProgramName(const wchar_t name) * Part of the Stable ABI.

This API is kept for backward compatibility: setting "PyConfig.program_name" should be used instead, see Python Initialization Configuration.

This function should be called before "Py_Initialize()" is called for the first time, if it is called at all. It tells the interpreter the value of the "argv[0]" argument to the "main()" function of the program (converted to wide characters). This is used by "Py_GetPath()" and some other functions below to find the Python run-time libraries relative to the interpreter executable. The default value is "'python'". The argument should point to a zero-terminated wide character string in static storage whose contents will not change for the duration of the program's execution. No code in the Python interpreter will change the contents of this storage.

Use "Py_DecodeLocale()" to decode a bytes string to get a wchar_t* string.

Deprecated since version 3.11, will be removed in version 3.15.

wchar_t Py_GetProgramName() * Part of the Stable ABI.

Return the program name set with "PyConfig.program_name", or the default. The returned string points into static storage; the caller should not modify its value.

This function should not be called before "Py_Initialize()", otherwise it returns "NULL".

Distinto en la versión 3.10: It now returns "NULL" if called before "Py_Initialize()".

Deprecated since version 3.13, will be removed in version 3.15: Use "PyConfig_Get("executable")" ("sys.executable") instead.

wchar_t Py_GetPrefix() * Part of the Stable ABI.

Return the prefix for installed platform-independent files. This is derived through a number of complicated rules from the program name set with "PyConfig.program_name" and some environment variables; for example, if the program name is "'/usr/local/bin/python'", the prefix is "'/usr/local'". The returned string points into static storage; the caller should not modify its value. This corresponds to the prefix variable in the top-level "Makefile" and the "--prefix" argument to the configure script at build time. The value is available to Python code as "sys.base_prefix". It is only useful on Unix. See also the next function.

This function should not be called before "Py_Initialize()", otherwise it returns "NULL".

Distinto en la versión 3.10: It now returns "NULL" if called before "Py_Initialize()".

Deprecated since version 3.13, will be removed in version 3.15: Use "PyConfig_Get("base_prefix")" ("sys.base_prefix") instead. Use "PyConfig_Get("prefix")" ("sys.prefix") if virtual environments need to be handled.

wchar_t Py_GetExecPrefix() * Part of the Stable ABI.

Return the exec-prefix for installed platform-dependent files. This is derived through a number of complicated rules from the program name set with "PyConfig.program_name" and some environment variables; for example, if the program name is "'/usr/local/bin/python'", the exec-prefix is "'/usr/local'". The returned string points into static storage; the caller should not modify its value. This corresponds to the exec_prefix variable in the top-level "Makefile" and the "--exec-prefix" argument to the configure script at build time. The value is available to Python code as "sys.base_exec_prefix". It is only useful on Unix.

Background: The exec-prefix differs from the prefix when platform dependent files (such as executables and shared libraries) are installed in a different directory tree. In a typical installation, platform dependent files may be installed in the "/usr/local/plat" subtree while platform independent may be installed in "/usr/local".

Generally speaking, a platform is a combination of hardware and software families, e.g. Sparc machines running the Solaris 2.x operating system are considered the same platform, but Intel machines running Solaris 2.x are another platform, and Intel machines running Linux are yet another platform. Different major revisions of the same operating system generally also form different platforms. Non-Unix operating systems are a different story; the installation strategies on those systems are so different that the prefix and exec-prefix are meaningless, and set to the empty string. Note that compiled Python bytecode files are platform independent (but not independent from the Python version by which they were compiled!).

System administrators will know how to configure the mount or automount programs to share "/usr/local" between platforms while having "/usr/local/plat" be a different filesystem for each platform.

This function should not be called before "Py_Initialize()", otherwise it returns "NULL".

Distinto en la versión 3.10: It now returns "NULL" if called before "Py_Initialize()".

Deprecated since version 3.13, will be removed in version 3.15: Use "PyConfig_Get("base_exec_prefix")" ("sys.base_exec_prefix") instead. Use "PyConfig_Get("exec_prefix")" ("sys.exec_prefix") if virtual environments need to be handled.

wchar_t Py_GetProgramFullPath() * Part of the Stable ABI.

Return the full program name of the Python executable; this is computed as a side-effect of deriving the default module search path from the program name (set by "PyConfig.program_name"). The returned string points into static storage; the caller should not modify its value. The value is available to Python code as "sys.executable".

This function should not be called before "Py_Initialize()", otherwise it returns "NULL".

Distinto en la versión 3.10: It now returns "NULL" if called before "Py_Initialize()".

Deprecated since version 3.13, will be removed in version 3.15: Use "PyConfig_Get("executable")" ("sys.executable") instead.

wchar_t Py_GetPath() * Part of the Stable ABI.

Return the default module search path; this is computed from the program name (set by "PyConfig.program_name") and some environment variables. The returned string consists of a series of directory names separated by a platform dependent delimiter character. The delimiter character is "':'" on Unix and macOS, "';'" on Windows. The returned string points into static storage; the caller should not modify its value. The list "sys.path" is initialized with this value on interpreter startup; it can be (and usually is) modified later to change the search path for loading modules.

This function should not be called before "Py_Initialize()", otherwise it returns "NULL".

Distinto en la versión 3.10: It now returns "NULL" if called before "Py_Initialize()".

Deprecated since version 3.13, will be removed in version 3.15: Use "PyConfig_Get("module_search_paths")" ("sys.path") instead.

const char Py_GetVersion() * Part of the Stable ABI.

Return the version of this Python interpreter. This is a string that looks something like

  "3.0a5+ (py3k:63103M, May 12 2008, 00:53:55) \n[GCC 4.2.3]"

The first word (up to the first space character) is the current Python version; the first characters are the major and minor version separated by a period. The returned string points into static storage; the caller should not modify its value. The value is available to Python code as "sys.version".