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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 Python Initialization Configuration
Configuration functions:
- PyImport_AppendInittab()
- PyImport_ExtendInittab()
- PyInitFrozenExtensions()
- PyMem_SetAllocator()
- PyMem_SetupDebugHooks()
- PyObject_SetArenaAllocator()
- Py_SetProgramName()
- Py_SetPythonHome()
- the configuration functions covered in Python Initialization Configuration
Informative functions:
Utilities:
- Py_DecodeLocale()
- the status reporting and utility functions covered in Python Initialization Configuration
Memory allocators:
Synchronization:
- PyMutex_Lock()
- PyMutex_Unlock()

Note

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, 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, 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, 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, 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, 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, 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, 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, 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, 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, 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, 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, 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, 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, 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, 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, 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, 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.

Note

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 Initialization with PyConfig.

See the Python Initialization Configuration 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.

Changed in version 3.15: This function no longer returns true until initialization has fully completed, including import of the site module. Previously it could return true while Py_Initialize() was still running.

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 Command line.

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_FrozenMain(int argc, char **argv)¶

The main program for a “frozen” Python program (an executable that embeds the interpreter together with the bytecode of the program’s modules, as produced by the freeze utility; see Tools/freeze).

This function initializes the interpreter, imports the frozen module named __main__, and finalizes the interpreter. The frozen modules must have been registered in PyImport_FrozenModules before calling this function; it is a fatal error if __main__ is not among them.

argc and argv are byte strings like the arguments of a C program’s main function, and are passed to the interpreter with PyConfig_SetBytesArgv(). Unlike Py_Main(), the arguments are not parsed as Python command line options (PyConfig.parse_argv is 0), so all of them are made available to the program in sys.argv. Apart from that, the interpreter uses the Python Configuration, so environment variables such as PYTHONPATH are honored. Warnings about the path configuration (PyConfig.pathconfig_warnings) are suppressed. If PYTHONINSPECT is set and standard input is a terminal, an interactive prompt is run after __main__ has finished, as with the -i option.

Return 0 if __main__ ran without an unhandled exception, 1 otherwise, and 120 if finalizing the interpreter failed (see Py_FinalizeEx()). An unhandled SystemExit terminates the process. If the interpreter cannot be initialized, the process exits via Py_ExitStatusException().

On Windows, this function is not exported by the Python DLL. The freeze utility compiles Python/frozenmain.c into the frozen program instead, together with the Windows-specific hooks that the function requires.

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 interpreter 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, meaning that Py_IsFinalizing() and sys.is_finalizing() return true. At this point, only the finalization thread (the thread that initiated finalization; this is typically the main thread) is allowed to attach a thread state.

Other threads that attempt to attach during finalization, either explicitly (such as via PyThreadState_Ensure() or Py_END_ALLOW_THREADS) or implicitly (such as in-between bytecode instructions), will enter a permanently blocked state. Generally, this is harmless, but this can result in deadlocks. For example, a thread may be permanently blocked while holding a lock, meaning that the finalization thread can never acquire that lock.

Prior to CPython 3.13, the thread would exit instead of hanging, which led to other issues (see the warning note at PyThread_exit_thread()).

Gross? Yes. Starting in Python 3.15, there are a number of C APIs that make it possible to avoid these issues by temporarily preventing finalization:

Interpreter views¶

In some cases, it may be necessary to access an interpreter that may have been deleted. This can be done using interpreter views.

type PyInterpreterView¶

Part of the Stable ABI (as an opaque struct) since version 3.15.

An opaque view of an interpreter.

This is a thread-safe way to access an interpreter that may be finalizing or already destroyed.

Added in version 3.15.

PyInterpreterView *PyInterpreterView_FromCurrent(void)¶

Part of the Stable ABI since version 3.15.

Create a view to the current interpreter.

This function is generally meant to be used alongside PyInterpreterGuard_FromView() or PyThreadState_EnsureFromView().

On success, this function returns a view to the current interpreter; on failure, it returns NULL with an exception set.

The caller must hold an attached thread state.

Added in version 3.15.

void PyInterpreterView_Close(PyInterpreterView *view)¶

Part of the Stable ABI since version 3.15.

Close an interpreter view.

If an interpreter view is never closed, the view’s memory will never be freed, but there are no other consequences. (In contrast, forgetting to close a guard will infinitely hang the main thread during finalization.)

This function cannot fail, and the caller doesn’t need to hold an attached thread state.

Added in version 3.15.

PyInterpreterView *PyInterpreterView_FromMain(void)¶

Part of the Stable ABI since version 3.15.

Create a view for the main interpreter (the first and default interpreter in a Python process; see PyInterpreterState_Main()).

On success, this function returns a view to the main interpreter; on failure, it returns NULL without an exception set. Failure indicates that the process is out of memory.

Use this function when an interpreter pointer or view cannot be supplied by the caller, such as when a native threading library does not provide a void *arg parameter that could carry a PyInterpreterGuard or PyInterpreterView. In code that supports subinterpreters, prefer PyInterpreterView_FromCurrent() so the guard tracks the calling interpreter rather than the main one.

The caller does not need to hold an attached thread state.

Added in version 3.15.

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 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, removed in version 3.15.

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.