\chapter{Initialization, Finalization, and Threads

\label{initialization}}

\begin{cfuncdesc}{void}{Py_Initialize}{}

Initialize the Python interpreter. In an application embedding

Python, this should be called before using any other Python/C API

functions; with the exception of

\cfunction{Py_SetProgramName()}\ttindex{Py_SetProgramName()},

\cfunction{PyEval_InitThreads()}\ttindex{PyEval_InitThreads()},

\cfunction{PyEval_ReleaseLock()}\ttindex{PyEval_ReleaseLock()},

and \cfunction{PyEval_AcquireLock()}\ttindex{PyEval_AcquireLock()}.

This initializes the table of loaded modules (\code{sys.modules}),

and\withsubitem{(in module sys)}{\ttindex{modules}\ttindex{path}}

creates the fundamental modules

\module{__builtin__}\refbimodindex{__builtin__},

\module{__main__}\refbimodindex{__main__} and

\module{sys}\refbimodindex{sys}. It also initializes the module

search\indexiii{module}{search}{path} path (\code{sys.path}).

It does not set \code{sys.argv}; use

\cfunction{PySys_SetArgv()}\ttindex{PySys_SetArgv()} for that. This

is a no-op when called for a second time (without calling

\cfunction{Py_Finalize()}\ttindex{Py_Finalize()} first). There is

no return value; it is a fatal error if the initialization fails.

\end{cfuncdesc}

\begin{cfuncdesc}{int}{Py_IsInitialized}{}

Return true (nonzero) when the Python interpreter has been

initialized, false (zero) if not. After \cfunction{Py_Finalize()}

is called, this returns false until \cfunction{Py_Initialize()} is

called again.

\end{cfuncdesc}

\begin{cfuncdesc}{void}{Py_Finalize}{}

Undo all initializations made by \cfunction{Py_Initialize()} and

subsequent use of Python/C API functions, and destroy all

sub-interpreters (see \cfunction{Py_NewInterpreter()} below) that

were created and not yet destroyed since the last call to

\cfunction{Py_Initialize()}. Ideally, this frees all memory

allocated by the Python interpreter. This is a no-op when called

for a second time (without calling \cfunction{Py_Initialize()} again

first). There is no return value; errors during finalization are

ignored.

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.

\strong{Bugs and caveats:} The destruction of modules and objects in

modules is done in random order; this may cause destructors

(\method{__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. 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 \cfunction{Py_Initialize()} and

\cfunction{Py_Finalize()} more than once.

\end{cfuncdesc}

\begin{cfuncdesc}{PyThreadState*}{Py_NewInterpreter}{}

Create a new sub-interpreter. This is an (almost) totally separate

environment for the execution of Python code. In particular, the

new interpreter has separate, independent versions of all imported

modules, including the fundamental modules

\module{__builtin__}\refbimodindex{__builtin__},

\module{__main__}\refbimodindex{__main__} and

\module{sys}\refbimodindex{sys}. The table of loaded modules

(\code{sys.modules}) and the module search path (\code{sys.path})

are also separate. The new environment has no \code{sys.argv}

variable. It has new standard I/O stream file objects

\code{sys.stdin}, \code{sys.stdout} and \code{sys.stderr} (however

these refer to the same underlying \ctype{FILE} structures in the C

library).

\withsubitem{(in module sys)}{

\ttindex{stdout}\ttindex{stderr}\ttindex{stdin}}

The return value points to the first thread state created in the new

sub-interpreter. This thread state is made the current thread

state. Note that no actual thread is created; see the discussion of

thread states below. If creation of the new interpreter is

unsuccessful, \NULL{} is returned; no exception is set since the

exception state is stored in the current thread state and there may

not be a current thread state. (Like all other Python/C API

functions, the global interpreter lock must be held before calling

this function and is still held when it returns; however, unlike

most other Python/C API functions, there needn't be a current thread

state on entry.)

Extension modules are shared between (sub-)interpreters as follows:

the first time a particular extension is imported, it is initialized

normally, and a (shallow) copy of its module's dictionary is

squirreled away. When the same extension is imported by another

(sub-)interpreter, a new module is initialized and filled with the

contents of this copy; the extension's \code{init} function is not

called. Note that this is different from what happens when an

extension is imported after the interpreter has been completely

re-initialized by calling

\cfunction{Py_Finalize()}\ttindex{Py_Finalize()} and

\cfunction{Py_Initialize()}\ttindex{Py_Initialize()}; in that case,

the extension's \code{init\var{module}} function \emph{is} called

again.

\strong{Bugs and caveats:} Because sub-interpreters (and the main

interpreter) are part of the same process, the insulation between

them isn't perfect --- for example, using low-level file operations

like \withsubitem{(in module os)}{\ttindex{close()}}

\function{os.close()} they can (accidentally or maliciously) affect

each other's open files. Because of the way extensions are shared

between (sub-)interpreters, some extensions may not work properly;

this is especially likely when the extension makes use of (static)

global variables, or when the extension manipulates its module's

dictionary after its initialization. It is possible to insert

objects created in one sub-interpreter into a namespace of another

sub-interpreter; this should be done with great care to avoid

sharing user-defined functions, methods, instances or classes

between sub-interpreters, since import operations executed by such

objects may affect the wrong (sub-)interpreter's dictionary of

loaded modules. (XXX This is a hard-to-fix bug that will be

addressed in a future release.)

\end{cfuncdesc}

\begin{cfuncdesc}{void}{Py_EndInterpreter}{PyThreadState *tstate}

Destroy the (sub-)interpreter represented by the given thread state.

The given thread state must be the current thread state. See the

discussion of thread states below. When the call returns, the

current thread state is \NULL. All thread states associated with

this interpreted are destroyed. (The global interpreter lock must

be held before calling this function and is still held when it

returns.) \cfunction{Py_Finalize()}\ttindex{Py_Finalize()} will

destroy all sub-interpreters that haven't been explicitly destroyed

at that point.

\end{cfuncdesc}

\begin{cfuncdesc}{void}{Py_SetProgramName}{char *name}

This function should be called before

\cfunction{Py_Initialize()}\ttindex{Py_Initialize()} is called

for the first time, if it is called at all. It tells the

interpreter the value of the \code{argv[0]} argument to the

\cfunction{main()}\ttindex{main()} function of the program. This is

used by \cfunction{Py_GetPath()}\ttindex{Py_GetPath()} and some

other functions below to find the Python run-time libraries relative

to the interpreter executable. The default value is

\code{'python'}. 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.

\end{cfuncdesc}

\begin{cfuncdesc}{char*}{Py_GetProgramName}{}

Return the program name set with

\cfunction{Py_SetProgramName()}\ttindex{Py_SetProgramName()}, or the

default. The returned string points into static storage; the caller

should not modify its value.

\end{cfuncdesc}

\begin{cfuncdesc}{char*}{Py_GetPrefix}{}

Return the \emph{prefix} for installed platform-independent files.

This is derived through a number of complicated rules from the

program name set with \cfunction{Py_SetProgramName()} and some

environment variables; for example, if the program name is

\code{'/usr/local/bin/python'}, the prefix is \code{'/usr/local'}.

The returned string points into static storage; the caller should

not modify its value. This corresponds to the \makevar{prefix}

variable in the top-level \file{Makefile} and the

\longprogramopt{prefix} argument to the \program{configure} script

at build time. The value is available to Python code as

\code{sys.prefix}. It is only useful on \UNIX. See also the next

function.

\end{cfuncdesc}

\begin{cfuncdesc}{char*}{Py_GetExecPrefix}{}

Return the \emph{exec-prefix} for installed

platform-\emph{de}pendent files. This is derived through a number

of complicated rules from the program name set with

\cfunction{Py_SetProgramName()} and some environment variables; for

example, if the program name is \code{'/usr/local/bin/python'}, the

exec-prefix is \code{'/usr/local'}. The returned string points into

static storage; the caller should not modify its value. This

corresponds to the \makevar{exec_prefix} variable in the top-level

\file{Makefile} and the \longprogramopt{exec-prefix} argument to the

\program{configure} script at build time. The value is available

to Python code as \code{sys.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

\file{/usr/local/plat} subtree while platform independent may be

installed in \file{/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 \program{mount}

or \program{automount} programs to share \file{/usr/local} between

platforms while having \file{/usr/local/plat} be a different

filesystem for each platform.

\end{cfuncdesc}

\begin{cfuncdesc}{char*}{Py_GetProgramFullPath}{}

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

\cfunction{Py_SetProgramName()}\ttindex{Py_SetProgramName()} above).

The returned string points into static storage; the caller should

not modify its value. The value is available to Python code as

\code{sys.executable}.

\withsubitem{(in module sys)}{\ttindex{executable}}

\end{cfuncdesc}

\begin{cfuncdesc}{char*}{Py_GetPath}{}

\indexiii{module}{search}{path}

Return the default module search path; this is computed from the

program name (set by \cfunction{Py_SetProgramName()} above) 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 \character{:} on \UNIX,

\character{;} on Windows, and \character{\e n} (the \ASCII{}

newline character) on Macintosh. The returned string points into

static storage; the caller should not modify its value. The value

is available to Python code as the list

\code{sys.path}\withsubitem{(in module sys)}{\ttindex{path}}, which

may be modified to change the future search path for loaded

modules.

% XXX should give the exact rules

\end{cfuncdesc}

\begin{cfuncdesc}{const char*}{Py_GetVersion}{}

Return the version of this Python interpreter. This is a string

that looks something like

\begin{verbatim}

"1.5 (#67, Dec 31 1997, 22:34:28) [GCC 2.7.2.2]"

\end{verbatim}

The first word (up to the first space character) is the current

Python version; the first three 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 \code{sys.version}.

\withsubitem{(in module sys)}{\ttindex{version}}

\end{cfuncdesc}

\begin{cfuncdesc}{const char*}{Py_GetPlatform}{}

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 \code{'sunos5'}. On Macintosh, it is \code{'mac'}. On Windows,

it is \code{'win'}. The returned string points into static storage;

the caller should not modify its value. The value is available to

Python code as \code{sys.platform}.

\withsubitem{(in module sys)}{\ttindex{platform}}

\end{cfuncdesc}

\begin{cfuncdesc}{const char*}{Py_GetCopyright}{}

Return the official copyright string for the current Python version,

for example

\code{'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

\code{sys.copyright}.

\withsubitem{(in module sys)}{\ttindex{copyright}}

\end{cfuncdesc}

\begin{cfuncdesc}{const char*}{Py_GetCompiler}{}

Return an indication of the compiler used to build the current

Python version, in square brackets, for example:

\begin{verbatim}

"[GCC 2.7.2.2]"

\end{verbatim}

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 \code{sys.version}.

\withsubitem{(in module sys)}{\ttindex{version}}

\end{cfuncdesc}

\begin{cfuncdesc}{const char*}{Py_GetBuildInfo}{}

Return information about the sequence number and build date and time

of the current Python interpreter instance, for example

\begin{verbatim}

"#67, Aug 1 1997, 22:34:28"

\end{verbatim}

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 \code{sys.version}.

\withsubitem{(in module sys)}{\ttindex{version}}

\end{cfuncdesc}

\begin{cfuncdesc}{int}{PySys_SetArgv}{int argc, char **argv}

Set \code{sys.argv} based on \var{argc} and \var{argv}. These

parameters are similar to those passed to the program's

\cfunction{main()}\ttindex{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 \var{argv} can

be an empty string. If this function fails to initialize

\code{sys.argv}, a fatal condition is signalled using

\cfunction{Py_FatalError()}\ttindex{Py_FatalError()}.

\withsubitem{(in module sys)}{\ttindex{argv}}

% XXX impl. doesn't seem consistent in allowing 0/NULL for the params;

% check w/ Guido.

\end{cfuncdesc}

% XXX Other PySys thingies (doesn't really belong in this chapter)

\section{Thread State and the Global Interpreter Lock

\label{threads}}

\index{global interpreter lock}

\index{interpreter lock}

\index{lock, interpreter}

The Python interpreter is not fully thread safe. In order to support

multi-threaded Python programs, there's a global lock that must be

held by the current thread before it can safely access Python objects.

Without the lock, even the simplest operations could cause problems in

a multi-threaded program: for example, when two threads simultaneously

increment the reference count of the same object, the reference count

could end up being incremented only once instead of twice.

Therefore, the rule exists that only the thread that has acquired the

global interpreter lock may operate on Python objects or call Python/C

API functions. In order to support multi-threaded Python programs,

the interpreter regularly releases and reacquires the lock --- by

default, every ten bytecode instructions (this can be changed with

\withsubitem{(in module sys)}{\ttindex{setcheckinterval()}}

\function{sys.setcheckinterval()}). The lock is also released and

reacquired around potentially blocking I/O operations like reading or

writing a file, so that other threads can run while the thread that

requests the I/O is waiting for the I/O operation to complete.

The Python interpreter needs to keep some bookkeeping information

separate per thread --- for this it uses a data structure called

\ctype{PyThreadState}\ttindex{PyThreadState}. This is new in Python

1.5; in earlier versions, such state was stored in global variables,

and switching threads could cause problems. In particular, exception

handling is now thread safe, when the application uses

\withsubitem{(in module sys)}{\ttindex{exc_info()}}

\function{sys.exc_info()} to access the exception last raised in the

current thread.

There's one global variable left, however: the pointer to the current

\ctype{PyThreadState}\ttindex{PyThreadState} structure. While most

thread packages have a way to store ``per-thread global data,''

Python's internal platform independent thread abstraction doesn't

support this yet. Therefore, the current thread state must be

manipulated explicitly.

This is easy enough in most cases. Most code manipulating the global

interpreter lock has the following simple structure:

\begin{verbatim}

Save the thread state in a local variable.

Release the interpreter lock.

...Do some blocking I/O operation...

Reacquire the interpreter lock.

Restore the thread state from the local variable.

\end{verbatim}

This is so common that a pair of macros exists to simplify it:

\begin{verbatim}

Py_BEGIN_ALLOW_THREADS

...Do some blocking I/O operation...

Py_END_ALLOW_THREADS

\end{verbatim}

The

\csimplemacro{Py_BEGIN_ALLOW_THREADS}\ttindex{Py_BEGIN_ALLOW_THREADS}

macro opens a new block and declares a hidden local variable; the

\csimplemacro{Py_END_ALLOW_THREADS}\ttindex{Py_END_ALLOW_THREADS}

macro closes the block. Another advantage of using these two macros

is that when Python is compiled without thread support, they are

defined empty, thus saving the thread state and lock manipulations.

When thread support is enabled, the block above expands to the

following code:

\begin{verbatim}

PyThreadState *_save;

_save = PyEval_SaveThread();

...Do some blocking I/O operation...

PyEval_RestoreThread(_save);

\end{verbatim}

Using even lower level primitives, we can get roughly the same effect

as follows:

\begin{verbatim}

PyThreadState *_save;

_save = PyThreadState_Swap(NULL);

PyEval_ReleaseLock();

...Do some blocking I/O operation...

PyEval_AcquireLock();

PyThreadState_Swap(_save);

\end{verbatim}

There are some subtle differences; in particular,

\cfunction{PyEval_RestoreThread()}\ttindex{PyEval_RestoreThread()} saves

and restores the value of the global variable

\cdata{errno}\ttindex{errno}, since the lock manipulation does not

guarantee that \cdata{errno} is left alone. Also, when thread support

is disabled,

\cfunction{PyEval_SaveThread()}\ttindex{PyEval_SaveThread()} and

\cfunction{PyEval_RestoreThread()} don't manipulate the lock; in this

case, \cfunction{PyEval_ReleaseLock()}\ttindex{PyEval_ReleaseLock()} and

\cfunction{PyEval_AcquireLock()}\ttindex{PyEval_AcquireLock()} are not

available. This is done so that dynamically loaded extensions

compiled with thread support enabled can be loaded by an interpreter

that was compiled with disabled thread support.

The global interpreter lock is used to protect the pointer to the

current thread state. When releasing the lock and saving the thread

state, the current thread state pointer must be retrieved before the

lock is released (since another thread could immediately acquire the

lock and store its own thread state in the global variable).

Conversely, when acquiring the lock and restoring the thread state,

the lock must be acquired before storing the thread state pointer.

Why am I going on with so much detail about this? Because when

threads are created from C, they don't have the global interpreter

lock, nor is there a thread state data structure for them. Such

threads must bootstrap themselves into existence, by first creating a

thread state data structure, then acquiring the lock, and finally

storing their thread state pointer, before they can start using the

Python/C API. When they are done, they should reset the thread state

pointer, release the lock, and finally free their thread state data

structure.

When creating a thread data structure, you need to provide an

interpreter state data structure. The interpreter state data

structure hold global data that is shared by all threads in an

interpreter, for example the module administration

(\code{sys.modules}). Depending on your needs, you can either create

a new interpreter state data structure, or share the interpreter state

data structure used by the Python main thread (to access the latter,

you must obtain the thread state and access its \member{interp} member;

this must be done by a thread that is created by Python or by the main

thread after Python is initialized).

Assuming you have access to an interpreter object, the typical idiom

for calling into Python from a C thread is

\begin{verbatim}

PyThreadState *tstate;

PyObject *result;

/* interp is your reference to an interpreter object. */

tstate = PyThreadState_New(interp);

PyEval_AcquireThread(tstate);

/* Perform Python actions here. */

result = CallSomeFunction();

/* evaluate result */

/* Release the thread. No Python API allowed beyond this point. */

PyEval_ReleaseThread(tstate);

/* You can either delete the thread state, or save it

until you need it the next time. */

PyThreadState_Delete(tstate);

\end{verbatim}

\begin{ctypedesc}{PyInterpreterState}

This data structure represents the state shared by a number of

cooperating threads. Threads belonging to the same interpreter

share their module administration and a few other internal items.

There are no public members in this structure.

Threads belonging to different interpreters initially share nothing,

except process state like available memory, open file descriptors

and such. The global interpreter lock is also shared by all

threads, regardless of to which interpreter they belong.

\end{ctypedesc}

\begin{ctypedesc}{PyThreadState}

This data structure represents the state of a single thread. The

only public data member is \ctype{PyInterpreterState

*}\member{interp}, which points to this thread's interpreter state.

\end{ctypedesc}

\begin{cfuncdesc}{void}{PyEval_InitThreads}{}

Initialize and acquire the global interpreter lock. It should be

called in the main thread before creating a second thread or

engaging in any other thread operations such as

\cfunction{PyEval_ReleaseLock()}\ttindex{PyEval_ReleaseLock()} or

\code{PyEval_ReleaseThread(\var{tstate})}\ttindex{PyEval_ReleaseThread()}.

It is not needed before calling

\cfunction{PyEval_SaveThread()}\ttindex{PyEval_SaveThread()} or

\cfunction{PyEval_RestoreThread()}\ttindex{PyEval_RestoreThread()}.

This is a no-op when called for a second time. It is safe to call

this function before calling

\cfunction{Py_Initialize()}\ttindex{Py_Initialize()}.

When only the main thread exists, no lock operations are needed.

This is a common situation (most Python programs do not use

threads), and the lock operations slow the interpreter down a bit.

Therefore, the lock is not created initially. This situation is

equivalent to having acquired the lock: when there is only a single

thread, all object accesses are safe. Therefore, when this function

initializes the lock, it also acquires it. Before the Python

\module{thread}\refbimodindex{thread} module creates a new thread,

knowing that either it has the lock or the lock hasn't been created

yet, it calls \cfunction{PyEval_InitThreads()}. When this call

returns, it is guaranteed that the lock has been created and that it

has acquired it.

It is \strong{not} safe to call this function when it is unknown

which thread (if any) currently has the global interpreter lock.

This function is not available when thread support is disabled at

compile time.

\end{cfuncdesc}

\begin{cfuncdesc}{void}{PyEval_AcquireLock}{}

Acquire the global interpreter lock. The lock must have been

created earlier. If this thread already has the lock, a deadlock

ensues. This function is not available when thread support is

disabled at compile time.

\end{cfuncdesc}

\begin{cfuncdesc}{void}{PyEval_ReleaseLock}{}

Release the global interpreter lock. The lock must have been

created earlier. This function is not available when thread support

is disabled at compile time.

\end{cfuncdesc}

\begin{cfuncdesc}{void}{PyEval_AcquireThread}{PyThreadState *tstate}

Acquire the global interpreter lock and then set the current thread

state to \var{tstate}, which should not be \NULL. The lock must

have been created earlier. If this thread already has the lock,

deadlock ensues. This function is not available when thread support

is disabled at compile time.

\end{cfuncdesc}

\begin{cfuncdesc}{void}{PyEval_ReleaseThread}{PyThreadState *tstate}

Reset the current thread state to \NULL{} and release the global

interpreter lock. The lock must have been created earlier and must

be held by the current thread. The \var{tstate} argument, which

must not be \NULL, is only used to check that it represents the

current thread state --- if it isn't, a fatal error is reported.

This function is not available when thread support is disabled at

compile time.

\end{cfuncdesc}

\begin{cfuncdesc}{PyThreadState*}{PyEval_SaveThread}{}

Release the interpreter lock (if it has been created and thread

support is enabled) and reset the thread state to \NULL, returning

the previous thread state (which is not \NULL). If the lock has

been created, the current thread must have acquired it. (This

function is available even when thread support is disabled at

compile time.)

\end{cfuncdesc}

\begin{cfuncdesc}{void}{PyEval_RestoreThread}{PyThreadState *tstate}

Acquire the interpreter lock (if it has been created and thread

support is enabled) and set the thread state to \var{tstate}, which

must not be \NULL. If the lock has been created, the current thread

must not have acquired it, otherwise deadlock ensues. (This

function is available even when thread support is disabled at

compile time.)

\end{cfuncdesc}

The following macros are normally used without a trailing semicolon;

look for example usage in the Python source distribution.

\begin{csimplemacrodesc}{Py_BEGIN_ALLOW_THREADS}

This macro expands to

\samp{\{ PyThreadState *_save; _save = PyEval_SaveThread();}.

Note that it contains an opening brace; it must be matched with a

following \csimplemacro{Py_END_ALLOW_THREADS} macro. See above for

further discussion of this macro. It is a no-op when thread support

is disabled at compile time.

\end{csimplemacrodesc}

\begin{csimplemacrodesc}{Py_END_ALLOW_THREADS}

This macro expands to \samp{PyEval_RestoreThread(_save); \}}.

Note that it contains a closing brace; it must be matched with an

earlier \csimplemacro{Py_BEGIN_ALLOW_THREADS} macro. See above for

further discussion of this macro. It is a no-op when thread support

is disabled at compile time.

\end{csimplemacrodesc}

\begin{csimplemacrodesc}{Py_BLOCK_THREADS}

This macro expands to \samp{PyEval_RestoreThread(_save);}: it is

equivalent to \csimplemacro{Py_END_ALLOW_THREADS} without the

closing brace. It is a no-op when thread support is disabled at

compile time.

\end{csimplemacrodesc}

\begin{csimplemacrodesc}{Py_UNBLOCK_THREADS}

This macro expands to \samp{_save = PyEval_SaveThread();}: it is

equivalent to \csimplemacro{Py_BEGIN_ALLOW_THREADS} without the

opening brace and variable declaration. It is a no-op when thread

support is disabled at compile time.

\end{csimplemacrodesc}

All of the following functions are only available when thread support

is enabled at compile time, and must be called only when the

interpreter lock has been created.

\begin{cfuncdesc}{PyInterpreterState*}{PyInterpreterState_New}{}

Create a new interpreter state object. The interpreter lock need

not be held, but may be held if it is necessary to serialize calls

to this function.

\end{cfuncdesc}

\begin{cfuncdesc}{void}{PyInterpreterState_Clear}{PyInterpreterState *interp}

Reset all information in an interpreter state object. The

interpreter lock must be held.

\end{cfuncdesc}

\begin{cfuncdesc}{void}{PyInterpreterState_Delete}{PyInterpreterState *interp}

Destroy an interpreter state object. The interpreter lock need not

be held. The interpreter state must have been reset with a previous

call to \cfunction{PyInterpreterState_Clear()}.

\end{cfuncdesc}

\begin{cfuncdesc}{PyThreadState*}{PyThreadState_New}{PyInterpreterState *interp}

Create a new thread state object belonging to the given interpreter

object. The interpreter lock need not be held, but may be held if

it is necessary to serialize calls to this function.

\end{cfuncdesc}

\begin{cfuncdesc}{void}{PyThreadState_Clear}{PyThreadState *tstate}

Reset all information in a thread state object. The interpreter lock

must be held.

\end{cfuncdesc}

\begin{cfuncdesc}{void}{PyThreadState_Delete}{PyThreadState *tstate}

Destroy a thread state object. The interpreter lock need not be

held. The thread state must have been reset with a previous call to

\cfunction{PyThreadState_Clear()}.

\end{cfuncdesc}

\begin{cfuncdesc}{PyThreadState*}{PyThreadState_Get}{}

Return the current thread state. The interpreter lock must be

held. When the current thread state is \NULL, this issues a fatal

error (so that the caller needn't check for \NULL).

\end{cfuncdesc}

\begin{cfuncdesc}{PyThreadState*}{PyThreadState_Swap}{PyThreadState *tstate}

Swap the current thread state with the thread state given by the

argument \var{tstate}, which may be \NULL. The interpreter lock

must be held.

\end{cfuncdesc}

\begin{cfuncdesc}{PyObject*}{PyThreadState_GetDict}{}

Return a dictionary in which extensions can store thread-specific

state information. Each extension should use a unique key to use to

store state in the dictionary. If this function returns \NULL, an

exception has been raised and the caller should allow it to

propagate.

\end{cfuncdesc}

\section{Profiling and Tracing \label{profiling}}

\sectionauthor{Fred L. Drake, Jr.}{[email protected]}

The Python interpreter provides some low-level support for attaching

profiling and execution tracing facilities. These are used for

profiling, debugging, and coverage analysis tools.

Starting with Python 2.2, the implementation of this facility was

substantially revised, and an interface from C was added. This C

interface allows the profiling or tracing code to avoid the overhead

of calling through Python-level callable objects, making a direct C

function call instead. The essential attributes of the facility have

not changed; the interface allows trace functions to be installed

per-thread, and the basic events reported to the trace function are

the same as had been reported to the Python-level trace functions in

previous versions.

\begin{ctypedesc}[Py_tracefunc]{int (*Py_tracefunc)(PyObject *obj,

PyFrameObject *frame, int what,

PyObject *arg)}

The type of the trace function registered using

\cfunction{PyEval_SetProfile()} and \cfunction{PyEval_SetTrace()}.

The first parameter is the object passed to the registration

function as \var{obj}, \var{frame} is the frame object to which the

event pertains, \var{what} is one of the constants

\constant{PyTrace_CALL}, \constant{PyTrace_EXCEPT},

\constant{PyTrace_LINE} or \constant{PyTrace_RETURN}, and \var{arg}

depends on the value of \var{what}:

\begin{tableii}{l|l}{constant}{Value of \var{what}}{Meaning of \var{arg}}

\lineii{PyTrace_CALL}{Always \NULL.}

\lineii{PyTrace_EXCEPT}{Exception information as returned by

\function{sys.exc_info()}.}

\lineii{PyTrace_LINE}{Always \NULL.}

\lineii{PyTrace_RETURN}{Value being returned to the caller.}

\end{tableii}

\end{ctypedesc}

\begin{cvardesc}{int}{PyTrace_CALL}

The value of the \var{what} parameter to a \ctype{Py_tracefunc}

function when a new call to a function or method is being reported,

or a new entry into a generator. Note that the creation of the

iterator for a generator function is not reported as there is no

control transfer to the Python bytecode in the corresponding frame.

\end{cvardesc}

\begin{cvardesc}{int}{PyTrace_EXCEPT}

The value of the \var{what} parameter to a \ctype{Py_tracefunc}

function when an exception has been raised. The callback function

is called with this value for \var{what} when after any bytecode is

processed after which the exception becomes set within the frame

being executed. The effect of this is that as exception propogation

causes the Python stack to unwind, the callback is called upon

return to each frame as the exception propagates. Only trace

functions receives these events; they are not needed by the

profiler.

\end{cvardesc}

\begin{cvardesc}{int}{PyTrace_LINE}

The value passed as the \var{what} parameter to a trace function

(but not a profiling function) when a line-number event is being

reported.

\end{cvardesc}

\begin{cvardesc}{int}{PyTrace_RETURN}

The value for the \var{what} parameter to \ctype{Py_tracefunc}

functions when a call is returning without propogating an exception.

\end{cvardesc}

\begin{cfuncdesc}{void}{PyEval_SetProfile}{Py_tracefunc func, PyObject *obj}

Set the profiler function to \var{func}. The \var{obj} parameter is

passed to the function as its first parameter, and may be any Python

object, or \NULL. If the profile function needs to maintain state,

using a different value for \var{obj} for each thread provides a

convenient and thread-safe place to store it. The profile function

is called for all monitored events except the line-number events.

\end{cfuncdesc}

\begin{cfuncdesc}{void}{PyEval_SetTrace}{Py_tracefunc func, PyObject *obj}

Set the the tracing function to \var{func}. This is similar to

\cfunction{PyEval_SetProfile()}, except the tracing function does

receive line-number events.

\end{cfuncdesc}

\section{Advanced Debugger Support \label{advanced-debugging}}

\sectionauthor{Fred L. Drake, Jr.}{[email protected]}

These functions are only intended to be used by advanced debugging

tools.

\begin{cfuncdesc}{PyInterpreterState*}{PyInterpreterState_Head}{}

Return the interpreter state object at the head of the list of all

such objects.

\versionadded{2.2}

\end{cfuncdesc}

\begin{cfuncdesc}{PyInterpreterState*}{PyInterpreterState_Next}{PyInterpreterState *interp}

Return the next interpreter state object after \var{interp} from the

list of all such objects.

\versionadded{2.2}

\end{cfuncdesc}

\begin{cfuncdesc}{PyThreadState *}{PyInterpreterState_ThreadHead}{PyInterpreterState *interp}

Return the a pointer to the first \ctype{PyThreadState} object in

the list of threads associated with the interpreter \var{interp}.

\versionadded{2.2}

\end{cfuncdesc}

\begin{cfuncdesc}{PyThreadState*}{PyThreadState_Next}{PyThreadState *tstate}

Return the next thread state object after \var{tstate} from the list

of all such objects belonging to the same \ctype{PyInterpreterState}

object.

\versionadded{2.2}

\end{cfuncdesc}