See
Python Initialization Configuration
for details on how to configure the interpreter prior to initialization.
Before Python initialization
In an application embedding Python, the
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:
the runtime pre-initialization functions covered in
Python Initialization Configuration
Configuration functions:
PyInitFrozenExtensions()
the configuration functions covered in
Python Initialization Configuration
Informative functions:
Utilities:
the status reporting and utility functions covered in
Python Initialization Configuration
Memory allocators:
Synchronization:
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:
,
, and
.
Global configuration variables
Python has variables for the global configuration to control different features and options. By default, these flags are controlled by
.
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
to 1 and -bb sets Py_BytesWarningFlag to 2.
intPy_BytesWarningFlag
This API is kept for backward compatibility: setting
should be used instead, see
Python Initialization Configuration
.
Issue a warning when comparing
or
with
or bytes with
. Issue an error if greater or equal to 2.
Set by the
option.
Deprecated since version 3.12, will be removed in version 3.15.
intPy_DebugFlag
This API is kept for backward compatibility: setting
should be used instead, see
Python Initialization Configuration
.
Turn on parser debugging output (for expert only, depending on compilation options).
Set by the
option and the
environment variable.
Deprecated since version 3.12, will be removed in version 3.15.
intPy_DontWriteBytecodeFlag
This API is kept for backward compatibility: setting
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
option and the
environment variable.
Deprecated since version 3.12, will be removed in version 3.15.
intPy_FrozenFlag
This API is kept for backward compatibility: setting
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.
intPy_HashRandomizationFlag
This API is kept for backward compatibility: setting
and
should be used instead, see
Python Initialization Configuration
.
Set to 1 if the
environment variable is set to a non-empty string.
If the flag is non-zero, read the
environment variable to initialize the secret hash seed.
Deprecated since version 3.12, will be removed in version 3.15.
intPy_IgnoreEnvironmentFlag
This API is kept for backward compatibility: setting
should be used instead, see
Python Initialization Configuration
.
Ignore all PYTHON* environment variables, e.g.
and
, that might be set.
Set by the
and
options.
Deprecated since version 3.12, will be removed in version 3.15.
intPy_InspectFlag
This API is kept for backward compatibility: setting
should be used instead, see
Python Initialization Configuration
.
When a script is passed as first argument or the
option is used, enter interactive mode after executing the script or the command, even when
does not appear to be a terminal.
Set by the
option and the
environment variable.
Deprecated since version 3.12, will be removed in version 3.15.
intPy_InteractiveFlag
This API is kept for backward compatibility: setting
should be used instead, see
Python Initialization Configuration
.
Set by the
option.
Deprecated since version 3.12, will be removed in version 3.15.
intPy_IsolatedFlag
This API is kept for backward compatibility: setting
should be used instead, see
Python Initialization Configuration
.
Run Python in isolated mode. In isolated mode
contains neither the script’s directory nor the user’s site-packages directory.
Set by the
option.
Added in version 3.4.
Deprecated since version 3.12, will be removed in version 3.15.
intPy_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
environment variable is set to a non-empty string.
See
for more details.
Deprecated since version 3.12, will be removed in version 3.15.
intPy_LegacyWindowsStdioFlag
This API is kept for backward compatibility: setting
should be used instead, see
Python Initialization Configuration
.
If the flag is non-zero, use
instead of io._WindowsConsoleIO for
standard streams.
Set to 1 if the
environment variable is set to a non-empty string.
See
for more details.
Deprecated since version 3.12, will be removed in version 3.15.
intPy_NoSiteFlag
This API is kept for backward compatibility: setting
should be used instead, see
Python Initialization Configuration
.
Disable the import of the module
and the site-dependent manipulations of
that it entails. Also disable these manipulations if site is explicitly imported later (call
if you want them to be triggered).
Set by the
option.
Deprecated since version 3.12, will be removed in version 3.15.
intPy_NoUserSiteDirectory
This API is kept for backward compatibility: setting
should be used instead, see
Python Initialization Configuration
.
Don’t add the
to
.
Set by the
and
options, and the
environment variable.
Deprecated since version 3.12, will be removed in version 3.15.
intPy_OptimizeFlag
This API is kept for backward compatibility: setting
should be used instead, see
Python Initialization Configuration
.
Set by the
option and the
environment variable.
Deprecated since version 3.12, will be removed in version 3.15.
intPy_QuietFlag
This API is kept for backward compatibility: setting
should be used instead, see
Python Initialization Configuration
.
Don’t display the copyright and version messages even in interactive mode.
Set by the
option.
Added in version 3.2.
Deprecated since version 3.12, will be removed in version 3.15.
intPy_UnbufferedStdioFlag
This API is kept for backward compatibility: setting
should be used instead, see
Python Initialization Configuration
.
Force the stdout and stderr streams to be unbuffered.
Set by the
option and the
environment variable.
Deprecated since version 3.12, will be removed in version 3.15.
intPy_VerboseFlag
This API is kept for backward compatibility: setting
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
option and the
environment variable.
Deprecated since version 3.12, will be removed in version 3.15.
Initializing and finalizing the interpreter
voidPy_Initialize()
Part of the
.Initialize the Python interpreter. In an application embedding Python, this should be called before using any other Python/C API functions; see
for the few exceptions.
This initializes the table of loaded modules (sys.modules), and creates the fundamental modules
,
and
. 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
first). There is no return value; it is a fatal error if the initialization fails.
Use
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.
voidPy_InitializeEx(intinitsigs)
Part of the
.This function works like
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
to customize the
Python Initialization Configuration
.
Py_InitializeFromConfig(const
*config)
Initialize Python from config configuration, as described in
.
See the
Python Initialization Configuration
section for details on pre-initializing the interpreter, populating the runtime configuration structure, and querying the returned status structure.
intPy_IsInitialized()
Part of the
.Return true (nonzero) when the Python interpreter has been initialized, false (zero) if not. After
is called, this returns false until
is called again.
intPy_IsFinalizing()
Part of the
since version 3.13.Return true (non-zero) if the main Python interpreter is
. Return false (zero) otherwise.
Added in version 3.13.
intPy_FinalizeEx()
Part of the
since version 3.6.Undo all initializations made by
and subsequent use of Python/C API functions, and destroy all sub-interpreters (see
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
, 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
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
. 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 (
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
and
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
handlers, object finalizers, or any code that may be run while flushing the stdout and stderr files.
Raises an
cpython._PySys_ClearAuditHooks with no arguments.
Added in version 3.6.
voidPy_Finalize()
Part of the
.This is a backwards-compatible version of
that disregards the return value.
intPy_BytesMain(intargc, char**argv)
Part of the
since version 3.8.Similar to
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.
intPy_Main(intargc, wchar_t**argv)
Part of the
.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
.
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
. 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
.
In terms of the CPython runtime configuration APIs documented in the
section (and without accounting for error handling), Py_Main is approximately equivalent to:
PyConfigconfig;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
,
or
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).
intPy_RunMain(void)
Executes the main module in a fully configured CPython runtime.
Executes the command (
), the script (
) or the 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
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
, or 1 for any other unhandled exception.
If
is set (such as when the
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
, as specified above.
This function always finalizes the Python interpreter before it returns.
See
for an example of a customized Python that always runs in isolated mode using
.
intPyUnstable_AtExit(
*interp, void(*func)(void*), void*data)
This is
. It may change without warning in minor releases.
Register an
callback for the target interpreter interp. This is similar to
, but takes an explicit interpreter and data pointer for the callback.
There must be an
for interp.
Added in version 3.13.
Cautions regarding runtime finalization
In the late stage of
, after attempting to wait for non-daemon threads to exit (though this can be interrupted by
) and running the
functions, the runtime is marked as finalizing:
and
return true. At this point, only the finalization thread that initiated finalization (typically the main thread) is allowed to acquire the
.
If any thread, other than the finalization thread, attempts to attach a
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
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
voidPy_SetProgramName(constwchar_t*name)
Part of the
.This API is kept for backward compatibility: setting
should be used instead, see
Python Initialization Configuration
.
This function should be called before
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
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
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
.Return the program name set with
, or the default. The returned string points into static storage; the caller should not modify its value.
This function should not be called before
, otherwise it returns NULL.
Changed in version 3.10: It now returns NULL if called before
.
wchar_t*Py_GetPrefix()
Part of the
.Return the prefix for installed platform-independent files. This is derived through a number of complicated rules from the program name set with
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
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
, otherwise it returns NULL.
Changed in version 3.10: It now returns NULL if called before
.
wchar_t*Py_GetExecPrefix()
Part of the
.Return the exec-prefix for installed platform-dependent files. This is derived through a number of complicated rules from the program name set with
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
, otherwise it returns NULL.
Changed in version 3.10: It now returns NULL if called before
.
wchar_t*Py_GetProgramFullPath()
Part of the
.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
). 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
, otherwise it returns NULL.
Changed in version 3.10: It now returns NULL if called before
.
wchar_t*Py_GetPath()
Part of the
.Return the default module search path; this is computed from the program name (set by
) 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
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
, otherwise it returns NULL.
Changed in version 3.10: It now returns NULL if called before
.
constchar*Py_GetVersion()
Part of the
.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
.
See also the
constant.
constchar*Py_GetPlatform()
Part of the
.Return the platform identifier for the current platform. On Unix, this is formed from the “official” name of the operating system, converted to lower case, followed by the major revision number; e.g., for Solaris 2.x, which is also known as SunOS 5.x, the value is 'sunos5'. On macOS, it is 'darwin'. On Windows, it is 'win'. The returned string points into static storage; the caller should not modify its value. The value is available to Python code as sys.platform.
constchar*Py_GetCopyright()
Part of the
.Return the official copyright string for the current Python version, for example
'Copyright1991-1995StichtingMathematischCentrum,Amsterdam'
The returned string points into static storage; the caller should not modify its value. The value is available to Python code as sys.copyright.
constchar*Py_GetCompiler()
Part of the
.Return an indication of the compiler used to build the current Python version, in square brackets, for example:
"[GCC 2.7.2.2]"The returned string points into static storage; the caller should not modify its value. The value is available to Python code as part of the variable sys.version.
constchar*Py_GetBuildInfo()
Part of the
.Return information about the sequence number and build date and time of the current Python interpreter instance, for example
"#67, Aug 1 1997, 22:34:28"The returned string points into static storage; the caller should not modify its value. The value is available to Python code as part of the variable sys.version.
voidPySys_SetArgvEx(intargc, wchar_t**argv, intupdatepath)
Part of the
.This API is kept for backward compatibility: setting
,
and
should be used instead, see
Python Initialization Configuration
.
Set
based on argc and 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 argv can be an empty string. If this function fails to initialize sys.argv, a fatal condition is signalled using
.
If updatepath is zero, this is all the function does. If updatepath is non-zero, the function also modifies
according to the following algorithm:
If the name of an existing script is passed in argv[0], the absolute path of the directory where the script is located is prepended to
.
Otherwise (that is, if argc is 0 or argv[0] doesn’t point to an existing file name), an empty string is prepended to
, which is the same as prepending the current working directory (".").
Use
to decode a bytes string to get a wchar_t* string.
See also
and
members of the
Python Initialization Configuration
.
Note
It is recommended that applications embedding the Python interpreter for purposes other than executing a single script pass 0 as updatepath, and update
themselves if desired. See
.
On versions before 3.1.3, you can achieve the same effect by manually popping the first
element after having called
, for example using:
PyRun_SimpleString("import sys; sys.path.pop(0)\n");Added in version 3.1.3.
Deprecated since version 3.11, will be removed in version 3.15.
voidPySys_SetArgv(intargc, wchar_t**argv)
Part of the
.This API is kept for backward compatibility: setting
and
should be used instead, see
Python Initialization Configuration
.
This function works like
with updatepath set to 1 unless the python interpreter was started with the
.
Use
to decode a bytes string to get a wchar_t* string.
See also
and
members of the
Python Initialization Configuration
.
Changed in version 3.4: The updatepath value depends on
.
Deprecated since version 3.11, will be removed in version 3.15.
voidPy_SetPythonHome(constwchar_t*home)
Part of the
.This API is kept for backward compatibility: setting
should be used instead, see
Python Initialization Configuration
.
Set the default “home” directory, that is, the location of the standard Python libraries. See
for the meaning of the argument string.
The argument should point to a zero-terminated 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
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_GetPythonHome()
Part of the
.Return the default “home”, that is, the value set by
, or the value of the
environment variable if it is set.
This function should not be called before
, otherwise it returns NULL.
Changed in version 3.10: It now returns NULL if called before
.
Deprecated since version 3.13, will be removed in version 3.15: Use
or the
environment variable instead.