See Python Initialization Configuration for details on how to configure the interpreter prior to 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:
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: Py_EncodeLocale(), PyEval_InitThreads(), and
Py_RunMain().
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.
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.
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.
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.
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.
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.
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.
Deprecated since version 3.12, removed in version 3.15.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
Return true (non-zero) if the main Python interpreter is shutting down. Return false (zero) otherwise.
Added in version 3.13.
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.
This is a backwards-compatible version of Py_FinalizeEx() that
disregards the return value.
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.
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).
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().
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.
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:
See also
PEP 788 explains the design, motivation and rationale for these APIs.
An opaque interpreter guard structure.
By holding an interpreter guard, the caller can ensure that the interpreter
will not finalize until the guard is closed (through
PyInterpreterGuard_Close()).
When a guard is held, a thread attempting to finalize the interpreter will block until the guard is closed before starting finalization. After finalization has started, threads are forever unable to acquire guards for that interpreter. This means that if you forget to close an interpreter guard, the process will permanently hang during finalization!
Holding a guard for an interpreter is similar to holding a
strong reference to a Python object, except finalization does not happen
automatically after all guards are released: it requires an explicit
Py_EndInterpreter() call.
Added in version 3.15.
Create a finalization guard for the current interpreter. This will prevent finalization until the guard is closed.
For example:
// Temporarily prevent finalization.
PyInterpreterGuard *guard = PyInterpreterGuard_FromCurrent();
if (guard == NULL) {
// Finalization has already started or we're out of memory.
return NULL;
}
Py_BEGIN_ALLOW_THREADS;
// Do some critical processing here. For example, we can safely acquire
// locks that might be acquired by the finalization thread.
Py_END_ALLOW_THREADS;
// Now that we're done with our critical processing, the interpreter is
// allowed to finalize again.
PyInterpreterGuard_Close(guard);
On success, this function returns a guard for the current interpreter;
on failure, it returns NULL with an exception set.
This function will fail only if the current interpreter has already started finalizing, or if the process is out of memory.
The guard pointer returned by this function must be eventually closed
with PyInterpreterGuard_Close(); failing to do so will result in
the Python process infinitely hanging.
The caller must hold an attached thread state.
Added in version 3.15.
Create a finalization guard for an interpreter through a view.
On success, this function returns a guard to the interpreter
represented by view. The view is still valid after calling this
function. The guard must eventually be closed with
PyInterpreterGuard_Close().
If the interpreter no longer exists, is already finalizing, or out of memory,
then this function returns NULL without setting an exception.
The caller does not need to hold an attached thread state.
Added in version 3.15.
Close an interpreter guard, allowing the interpreter to start finalization if no other guards remain. If an interpreter guard is never closed, the interpreter will infinitely wait when trying to enter finalization!
After an interpreter guard is closed, it may not be used in
PyThreadState_Ensure(). Doing so will result in undefined
behavior.
This function cannot fail, and the caller doesn’t need to hold an attached thread state.
Added in version 3.15.
In some cases, it may be necessary to access an interpreter that may have been deleted. This can be done using interpreter views.
An opaque view of an interpreter.
This is a thread-safe way to access an interpreter that may have be finalizing or already destroyed.
Added in 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.
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.
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.
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.
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.
See also the Py_Version constant.
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.
Return the official copyright string for the current Python version, for example
'Copyright 1991-1995 Stichting Mathematisch Centrum, 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.
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.
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.
This API is kept for backward compatibility: setting
PyConfig.argv, PyConfig.parse_argv and
PyConfig.safe_path should be used instead, see Python
Initialization Configuration.
Set sys.argv 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 Py_FatalError().
If updatepath is zero, this is all the function does. If updatepath
is non-zero, the function also modifies sys.path 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
sys.path.
Otherwise (that is, if argc is 0 or argv[0] doesn’t point
to an existing file name), an empty string is prepended to
sys.path, which is the same as prepending the current working
directory (".").
Use Py_DecodeLocale() to decode a bytes string to get a
wchar_t* string.
See also PyConfig.orig_argv and PyConfig.argv
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 sys.path themselves if desired.
See CVE 2008-5983.
On versions before 3.1.3, you can achieve the same effect by manually
popping the first sys.path element after having called
PySys_SetArgv(), for example using:
PyRun_SimpleString("import sys; sys.path.pop(0)\n");
Added in version 3.1.3.
Deprecated since version 3.11, removed in version 3.15.
This API is kept for backward compatibility: setting
PyConfig.argv and PyConfig.parse_argv should be used
instead, see Python Initialization Configuration.
This function works like PySys_SetArgvEx() with updatepath set
to 1 unless the python interpreter was started with the
-I.
Use Py_DecodeLocale() to decode a bytes string to get a
wchar_t* string.
See also PyConfig.orig_argv and PyConfig.argv
members of the Python Initialization Configuration.
Changed in version 3.4: The updatepath value depends on -I.
Deprecated since version 3.11, removed in version 3.15.
This API is kept for backward compatibility: setting
PyConfig.home should be used instead, see Python
Initialization Configuration.
Set the default “home” directory, that is, the location of the standard
Python libraries. See PYTHONHOME 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 Py_DecodeLocale() to decode a bytes string to get a
wchar_t* string.
Deprecated since version 3.11, removed in version 3.15.