\documentclass{manual}

% XXX PM explain how to add new types to Python

\title{Extending and Embedding the Python Interpreter}

\input{boilerplate}

% Tell \index to actually write the .idx file

\makeindex

\begin{document}

\maketitle

\ifhtml

\chapter*{Front Matter\label{front}}

\fi

\input{copyright}

\begin{abstract}

\noindent

Python is an interpreted, object-oriented programming language. This

document describes how to write modules in C or \Cpp{} to extend the

Python interpreter with new modules. Those modules can define new

functions but also new object types and their methods. The document

also describes how to embed the Python interpreter in another

application, for use as an extension language. Finally, it shows how

to compile and link extension modules so that they can be loaded

dynamically (at run time) into the interpreter, if the underlying

operating system supports this feature.

This document assumes basic knowledge about Python. For an informal

introduction to the language, see the

\citetitle[../tut/tut.html]{Python Tutorial}. The

\citetitle[../ref/ref.html]{Python Reference Manual} gives a more

formal definition of the language. The

\citetitle[../lib/lib.html]{Python Library Reference} documents the

existing object types, functions and modules (both built-in and

written in Python) that give the language its wide application range.

For a detailed description of the whole Python/C API, see the separate

\citetitle[../api/api.html]{Python/C API Reference Manual}.

\end{abstract}

\tableofcontents

\chapter{Extending Python with C or \Cpp{} \label{intro}}

It is quite easy to add new built-in modules to Python, if you know

how to program in C. Such \dfn{extension modules} can do two things

that can't be done directly in Python: they can implement new built-in

object types, and they can call C library functions and system calls.

To support extensions, the Python API (Application Programmers

Interface) defines a set of functions, macros and variables that

provide access to most aspects of the Python run-time system. The

Python API is incorporated in a C source file by including the header

\code{"Python.h"}.

The compilation of an extension module depends on its intended use as

well as on your system setup; details are given in later chapters.

\section{A Simple Example

\label{simpleExample}}

Let's create an extension module called \samp{spam} (the favorite food

of Monty Python fans...) and let's say we want to create a Python

interface to the C library function \cfunction{system()}.\footnote{An

interface for this function already exists in the standard module

\module{os} --- it was chosen as a simple and straightfoward example.}

This function takes a null-terminated character string as argument and

returns an integer. We want this function to be callable from Python

as follows:

\begin{verbatim}

>>> import spam

>>> status = spam.system("ls -l")

\end{verbatim}

Begin by creating a file \file{spammodule.c}. (Historically, if a

module is called \samp{spam}, the C file containing its implementation

is called \file{spammodule.c}; if the module name is very long, like

\samp{spammify}, the module name can be just \file{spammify.c}.)

The first line of our file can be:

\begin{verbatim}

#include <Python.h>

\end{verbatim}

which pulls in the Python API (you can add a comment describing the

purpose of the module and a copyright notice if you like).

All user-visible symbols defined by \code{"Python.h"} have a prefix of

\samp{Py} or \samp{PY}, except those defined in standard header files.

For convenience, and since they are used extensively by the Python

interpreter, \code{"Python.h"} includes a few standard header files:

\code{<stdio.h>}, \code{<string.h>}, \code{<errno.h>}, and

\code{<stdlib.h>}. If the latter header file does not exist on your

system, it declares the functions \cfunction{malloc()},

\cfunction{free()} and \cfunction{realloc()} directly.

The next thing we add to our module file is the C function that will

be called when the Python expression \samp{spam.system(\var{string})}

is evaluated (we'll see shortly how it ends up being called):

\begin{verbatim}

static PyObject *

spam_system(self, args)

PyObject *self;

PyObject *args;

{

char *command;

int sts;

if (!PyArg_ParseTuple(args, "s", &command))

return NULL;

sts = system(command);

return Py_BuildValue("i", sts);

}

\end{verbatim}

There is a straightforward translation from the argument list in

Python (e.g.\ the single expression \code{"ls -l"}) to the arguments

passed to the C function. The C function always has two arguments,

conventionally named \var{self} and \var{args}.

The \var{self} argument is only used when the C function implements a

built-in method, not a function. In the example, \var{self} will

always be a \NULL{} pointer, since we are defining a function, not a

method. (This is done so that the interpreter doesn't have to

understand two different types of C functions.)

The \var{args} argument will be a pointer to a Python tuple object

containing the arguments. Each item of the tuple corresponds to an

argument in the call's argument list. The arguments are Python

objects --- in order to do anything with them in our C function we have

to convert them to C values. The function \cfunction{PyArg_ParseTuple()}

in the Python API checks the argument types and converts them to C

values. It uses a template string to determine the required types of

the arguments as well as the types of the C variables into which to

store the converted values. More about this later.

\cfunction{PyArg_ParseTuple()} returns true (nonzero) if all arguments have

the right type and its components have been stored in the variables

whose addresses are passed. It returns false (zero) if an invalid

argument list was passed. In the latter case it also raises an

appropriate exception so the calling function can return

\NULL{} immediately (as we saw in the example).

\section{Intermezzo: Errors and Exceptions

\label{errors}}

An important convention throughout the Python interpreter is the

following: when a function fails, it should set an exception condition

and return an error value (usually a \NULL{} pointer). Exceptions

are stored in a static global variable inside the interpreter; if this

variable is \NULL{} no exception has occurred. A second global

variable stores the ``associated value'' of the exception (the second

argument to \keyword{raise}). A third variable contains the stack

traceback in case the error originated in Python code. These three

variables are the C equivalents of the Python variables

\code{sys.exc_type}, \code{sys.exc_value} and \code{sys.exc_traceback} (see

the section on module \module{sys} in the

\citetitle[../lib/lib.html]{Python Library Reference}). It is

important to know about them to understand how errors are passed

around.

The Python API defines a number of functions to set various types of

exceptions.

The most common one is \cfunction{PyErr_SetString()}. Its arguments

are an exception object and a C string. The exception object is

usually a predefined object like \cdata{PyExc_ZeroDivisionError}. The

C string indicates the cause of the error and is converted to a

Python string object and stored as the ``associated value'' of the

exception.

Another useful function is \cfunction{PyErr_SetFromErrno()}, which only

takes an exception argument and constructs the associated value by

inspection of the global variable \cdata{errno}. The most

general function is \cfunction{PyErr_SetObject()}, which takes two object

arguments, the exception and its associated value. You don't need to

\cfunction{Py_INCREF()} the objects passed to any of these functions.

You can test non-destructively whether an exception has been set with

\cfunction{PyErr_Occurred()}. This returns the current exception object,

or \NULL{} if no exception has occurred. You normally don't need

to call \cfunction{PyErr_Occurred()} to see whether an error occurred in a

function call, since you should be able to tell from the return value.

When a function \var{f} that calls another function \var{g} detects

that the latter fails, \var{f} should itself return an error value

(e.g.\ \NULL{} or \code{-1}). It should \emph{not} call one of the

\cfunction{PyErr_*()} functions --- one has already been called by \var{g}.

\var{f}'s caller is then supposed to also return an error indication

to \emph{its} caller, again \emph{without} calling \cfunction{PyErr_*()},

and so on --- the most detailed cause of the error was already

reported by the function that first detected it. Once the error

reaches the Python interpreter's main loop, this aborts the currently

executing Python code and tries to find an exception handler specified

by the Python programmer.

(There are situations where a module can actually give a more detailed

error message by calling another \cfunction{PyErr_*()} function, and in

such cases it is fine to do so. As a general rule, however, this is

not necessary, and can cause information about the cause of the error

to be lost: most operations can fail for a variety of reasons.)

To ignore an exception set by a function call that failed, the exception

condition must be cleared explicitly by calling \cfunction{PyErr_Clear()}.

The only time C code should call \cfunction{PyErr_Clear()} is if it doesn't

want to pass the error on to the interpreter but wants to handle it

completely by itself (e.g.\ by trying something else or pretending

nothing happened).

Every failing \cfunction{malloc()} call must be turned into an

exception --- the direct caller of \cfunction{malloc()} (or

\cfunction{realloc()}) must call \cfunction{PyErr_NoMemory()} and

return a failure indicator itself. All the object-creating functions

(for example, \cfunction{PyInt_FromLong()}) already do this, so this

note is only relevant to those who call \cfunction{malloc()} directly.

Also note that, with the important exception of

\cfunction{PyArg_ParseTuple()} and friends, functions that return an

integer status usually return a positive value or zero for success and

\code{-1} for failure, like \UNIX{} system calls.

Finally, be careful to clean up garbage (by making

\cfunction{Py_XDECREF()} or \cfunction{Py_DECREF()} calls for objects

you have already created) when you return an error indicator!

The choice of which exception to raise is entirely yours. There are

predeclared C objects corresponding to all built-in Python exceptions,

e.g.\ \cdata{PyExc_ZeroDivisionError}, which you can use directly. Of

course, you should choose exceptions wisely --- don't use

\cdata{PyExc_TypeError} to mean that a file couldn't be opened (that

should probably be \cdata{PyExc_IOError}). If something's wrong with

the argument list, the \cfunction{PyArg_ParseTuple()} function usually

raises \cdata{PyExc_TypeError}. If you have an argument whose value

must be in a particular range or must satisfy other conditions,

\cdata{PyExc_ValueError} is appropriate.

You can also define a new exception that is unique to your module.

For this, you usually declare a static object variable at the

beginning of your file, e.g.

\begin{verbatim}

static PyObject *SpamError;

\end{verbatim}

and initialize it in your module's initialization function

(\cfunction{initspam()}) with an exception object, e.g.\ (leaving out

the error checking for now):

\begin{verbatim}

void

initspam()

{

PyObject *m, *d;

m = Py_InitModule("spam", SpamMethods);

d = PyModule_GetDict(m);

SpamError = PyErr_NewException("spam.error", NULL, NULL);

PyDict_SetItemString(d, "error", SpamError);

}

\end{verbatim}

Note that the Python name for the exception object is

\exception{spam.error}. The \cfunction{PyErr_NewException()} function

may create a class with the base class being \exception{Exception}

(unless another class is passed in instead of \NULL), described in the

\citetitle[../lib/lib.html]{Python Library Reference} under ``Built-in

Exceptions.''

Note also that the \cdata{SpamError} variable retains a reference to

the newly created exception class; this is intentional! Since the

exception could be removed from the module by external code, an owned

reference to the class is needed to ensure that it will not be

discarded, causing \cdata{SpamError} to become a dangling pointer.

Should it become a dangling pointer, C code which raises the exception

could cause a core dump or other unintended side effects.

\section{Back to the Example

\label{backToExample}}

Going back to our example function, you should now be able to

understand this statement:

\begin{verbatim}

if (!PyArg_ParseTuple(args, "s", &command))

return NULL;

\end{verbatim}

It returns \NULL{} (the error indicator for functions returning

object pointers) if an error is detected in the argument list, relying

on the exception set by \cfunction{PyArg_ParseTuple()}. Otherwise the

string value of the argument has been copied to the local variable

\cdata{command}. This is a pointer assignment and you are not supposed

to modify the string to which it points (so in Standard C, the variable

\cdata{command} should properly be declared as \samp{const char

*command}).

The next statement is a call to the \UNIX{} function

\cfunction{system()}, passing it the string we just got from

\cfunction{PyArg_ParseTuple()}:

\begin{verbatim}

sts = system(command);

\end{verbatim}

Our \function{spam.system()} function must return the value of

\cdata{sts} as a Python object. This is done using the function

\cfunction{Py_BuildValue()}, which is something like the inverse of

\cfunction{PyArg_ParseTuple()}: it takes a format string and an

arbitrary number of C values, and returns a new Python object.

More info on \cfunction{Py_BuildValue()} is given later.

\begin{verbatim}

return Py_BuildValue("i", sts);

\end{verbatim}

In this case, it will return an integer object. (Yes, even integers

are objects on the heap in Python!)

If you have a C function that returns no useful argument (a function

returning \ctype{void}), the corresponding Python function must return

\code{None}. You need this idiom to do so:

\begin{verbatim}

Py_INCREF(Py_None);

return Py_None;

\end{verbatim}

\cdata{Py_None} is the C name for the special Python object

\code{None}. It is a genuine Python object rather than a \NULL{}

pointer, which means ``error'' in most contexts, as we have seen.

\section{The Module's Method Table and Initialization Function

\label{methodTable}}

I promised to show how \cfunction{spam_system()} is called from Python

programs. First, we need to list its name and address in a ``method

table'':

\begin{verbatim}

static PyMethodDef SpamMethods[] = {

...

{"system", spam_system, METH_VARARGS},

...

{NULL, NULL} /* Sentinel */

};

\end{verbatim}

Note the third entry (\samp{METH_VARARGS}). This is a flag telling

the interpreter the calling convention to be used for the C

function. It should normally always be \samp{METH_VARARGS} or

\samp{METH_VARARGS | METH_KEYWORDS}; a value of \code{0} means that an

obsolete variant of \cfunction{PyArg_ParseTuple()} is used.

When using only \samp{METH_VARARGS}, the function should expect

the Python-level parameters to be passed in as a tuple acceptable for

parsing via \cfunction{PyArg_ParseTuple()}; more information on this

function is provided below.

The \constant{METH_KEYWORDS} bit may be set in the third field if

keyword arguments should be passed to the function. In this case, the

C function should accept a third \samp{PyObject *} parameter which

will be a dictionary of keywords. Use

\cfunction{PyArg_ParseTupleAndKeywords()} to parse the arguments to

such a function.

The method table must be passed to the interpreter in the module's

initialization function. The initialization function must be named

\cfunction{init\var{name}()}, where \var{name} is the name of the

module, and should be the only non-\keyword{static} item defined in

the module file:

\begin{verbatim}

void

initspam()

{

(void) Py_InitModule("spam", SpamMethods);

}

\end{verbatim}

Note that for \Cpp, this method must be declared \code{extern "C"}.

When the Python program imports module \module{spam} for the first

time, \cfunction{initspam()} is called. (See below for comments about

embedding Python.) It calls

\cfunction{Py_InitModule()}, which creates a ``module object'' (which

is inserted in the dictionary \code{sys.modules} under the key

\code{"spam"}), and inserts built-in function objects into the newly

created module based upon the table (an array of \ctype{PyMethodDef}

structures) that was passed as its second argument.

\cfunction{Py_InitModule()} returns a pointer to the module object

that it creates (which is unused here). It aborts with a fatal error

if the module could not be initialized satisfactorily, so the caller

doesn't need to check for errors.

When embedding Python, the \cfunction{initspam()} function is not

called automatically unless there's an entry in the

\cdata{_PyImport_Inittab} table. The easiest way to handle this is to

statically initialize your statically-linked modules by directly

calling \cfunction{initspam()} after the call to

\cfunction{Py_Initialize()} or \cfunction{PyMac_Initialize()}:

\begin{verbatim}

int main(int argc, char **argv)

{

/* Pass argv[0] to the Python interpreter */

Py_SetProgramName(argv[0]);

/* Initialize the Python interpreter. Required. */

Py_Initialize();

/* Add a static module */

initspam();

\end{verbatim}

An example may be found in the file \file{Demo/embed/demo.c} in the

Python source distribution.

\strong{Note:} Removing entries from \code{sys.modules} or importing

compiled modules into multiple interpreters within a process (or

following a \cfunction{fork()} without an intervening

\cfunction{exec()}) can create problems for some extension modules.

Extension module authors should exercise caution when initializing

internal data structures.

Note also that the \function{reload()} function can be used with

extension modules, and will call the module initialization function

(\cfunction{initspam()} in the example), but will not load the module

again if it was loaded from a dynamically loadable object file

(\file{.so} on \UNIX, \file{.dll} on Windows).

A more substantial example module is included in the Python source

distribution as \file{Modules/xxmodule.c}. This file may be used as a

template or simply read as an example. The \program{modulator.py}

script included in the source distribution or Windows install provides

a simple graphical user interface for declaring the functions and

objects which a module should implement, and can generate a template

which can be filled in. The script lives in the

\file{Tools/modulator/} directory; see the \file{README} file there

for more information.

\section{Compilation and Linkage

\label{compilation}}

There are two more things to do before you can use your new extension:

compiling and linking it with the Python system. If you use dynamic

loading, the details depend on the style of dynamic loading your

system uses; see the chapters about building extension modules on

\UNIX{} (chapter \ref{building-on-unix}) and Windows (chapter

\ref{building-on-windows}) for more information about this.

% XXX Add information about MacOS

If you can't use dynamic loading, or if you want to make your module a

permanent part of the Python interpreter, you will have to change the

configuration setup and rebuild the interpreter. Luckily, this is

very simple: just place your file (\file{spammodule.c} for example) in

the \file{Modules/} directory of an unpacked source distribution, add

a line to the file \file{Modules/Setup.local} describing your file:

\begin{verbatim}

spam spammodule.o

\end{verbatim}

and rebuild the interpreter by running \program{make} in the toplevel

directory. You can also run \program{make} in the \file{Modules/}

subdirectory, but then you must first rebuild \file{Makefile}

there by running `\program{make} Makefile'. (This is necessary each

time you change the \file{Setup} file.)

If your module requires additional libraries to link with, these can

be listed on the line in the configuration file as well, for instance:

\begin{verbatim}

spam spammodule.o -lX11

\end{verbatim}

\section{Calling Python Functions from C

\label{callingPython}}

So far we have concentrated on making C functions callable from

Python. The reverse is also useful: calling Python functions from C.

This is especially the case for libraries that support so-called

``callback'' functions. If a C interface makes use of callbacks, the

equivalent Python often needs to provide a callback mechanism to the

Python programmer; the implementation will require calling the Python

callback functions from a C callback. Other uses are also imaginable.

Fortunately, the Python interpreter is easily called recursively, and

there is a standard interface to call a Python function. (I won't

dwell on how to call the Python parser with a particular string as

input --- if you're interested, have a look at the implementation of

the \programopt{-c} command line option in \file{Python/pythonmain.c}

from the Python source code.)

Calling a Python function is easy. First, the Python program must

somehow pass you the Python function object. You should provide a

function (or some other interface) to do this. When this function is

called, save a pointer to the Python function object (be careful to

\cfunction{Py_INCREF()} it!) in a global variable --- or wherever you

see fit. For example, the following function might be part of a module

definition:

\begin{verbatim}

static PyObject *my_callback = NULL;

static PyObject *

my_set_callback(dummy, args)

PyObject *dummy, *args;

{

PyObject *result = NULL;

PyObject *temp;

if (PyArg_ParseTuple(args, "O:set_callback", &temp)) {

if (!PyCallable_Check(temp)) {

PyErr_SetString(PyExc_TypeError, "parameter must be callable");

return NULL;

}

Py_XINCREF(temp); /* Add a reference to new callback */

Py_XDECREF(my_callback); /* Dispose of previous callback */

my_callback = temp; /* Remember new callback */

/* Boilerplate to return "None" */

Py_INCREF(Py_None);

result = Py_None;

}

return result;

}

\end{verbatim}

This function must be registered with the interpreter using the

\constant{METH_VARARGS} flag; this is described in section

\ref{methodTable}, ``The Module's Method Table and Initialization

Function.'' The \cfunction{PyArg_ParseTuple()} function and its

arguments are documented in section \ref{parseTuple}, ``Format Strings

for \cfunction{PyArg_ParseTuple()}.''

The macros \cfunction{Py_XINCREF()} and \cfunction{Py_XDECREF()}

increment/decrement the reference count of an object and are safe in

the presence of \NULL{} pointers (but note that \var{temp} will not be

\NULL{} in this context). More info on them in section

\ref{refcounts}, ``Reference Counts.''

Later, when it is time to call the function, you call the C function

\cfunction{PyEval_CallObject()}. This function has two arguments, both

pointers to arbitrary Python objects: the Python function, and the

argument list. The argument list must always be a tuple object, whose

length is the number of arguments. To call the Python function with

no arguments, pass an empty tuple; to call it with one argument, pass

a singleton tuple. \cfunction{Py_BuildValue()} returns a tuple when its

format string consists of zero or more format codes between

parentheses. For example:

\begin{verbatim}

int arg;

PyObject *arglist;

PyObject *result;

...

arg = 123;

...

/* Time to call the callback */

arglist = Py_BuildValue("(i)", arg);

result = PyEval_CallObject(my_callback, arglist);

Py_DECREF(arglist);

\end{verbatim}

\cfunction{PyEval_CallObject()} returns a Python object pointer: this is

the return value of the Python function. \cfunction{PyEval_CallObject()} is

``reference-count-neutral'' with respect to its arguments. In the

example a new tuple was created to serve as the argument list, which

is \cfunction{Py_DECREF()}-ed immediately after the call.

The return value of \cfunction{PyEval_CallObject()} is ``new'': either it

is a brand new object, or it is an existing object whose reference

count has been incremented. So, unless you want to save it in a

global variable, you should somehow \cfunction{Py_DECREF()} the result,

even (especially!) if you are not interested in its value.

Before you do this, however, it is important to check that the return

value isn't \NULL{}. If it is, the Python function terminated by

raising an exception. If the C code that called

\cfunction{PyEval_CallObject()} is called from Python, it should now

return an error indication to its Python caller, so the interpreter

can print a stack trace, or the calling Python code can handle the

exception. If this is not possible or desirable, the exception should

be cleared by calling \cfunction{PyErr_Clear()}. For example:

\begin{verbatim}

if (result == NULL)

return NULL; /* Pass error back */

...use result...

Py_DECREF(result);

\end{verbatim}

Depending on the desired interface to the Python callback function,

you may also have to provide an argument list to

\cfunction{PyEval_CallObject()}. In some cases the argument list is

also provided by the Python program, through the same interface that

specified the callback function. It can then be saved and used in the

same manner as the function object. In other cases, you may have to

construct a new tuple to pass as the argument list. The simplest way

to do this is to call \cfunction{Py_BuildValue()}. For example, if

you want to pass an integral event code, you might use the following

code:

\begin{verbatim}

PyObject *arglist;

...

arglist = Py_BuildValue("(l)", eventcode);

result = PyEval_CallObject(my_callback, arglist);

Py_DECREF(arglist);

if (result == NULL)

return NULL; /* Pass error back */

/* Here maybe use the result */

Py_DECREF(result);

\end{verbatim}

Note the placement of \samp{Py_DECREF(arglist)} immediately after the

call, before the error check! Also note that strictly spoken this

code is not complete: \cfunction{Py_BuildValue()} may run out of

memory, and this should be checked.

\section{Extracting Parameters in Extension Functions

\label{parseTuple}}

The \cfunction{PyArg_ParseTuple()} function is declared as follows:

\begin{verbatim}

int PyArg_ParseTuple(PyObject *arg, char *format, ...);

\end{verbatim}

The \var{arg} argument must be a tuple object containing an argument

list passed from Python to a C function. The \var{format} argument

must be a format string, whose syntax is explained below. The

remaining arguments must be addresses of variables whose type is

determined by the format string. For the conversion to succeed, the

\var{arg} object must match the format and the format must be

exhausted.

Note that while \cfunction{PyArg_ParseTuple()} checks that the Python

arguments have the required types, it cannot check the validity of the

addresses of C variables passed to the call: if you make mistakes

there, your code will probably crash or at least overwrite random bits

in memory. So be careful!

A format string consists of zero or more ``format units''. A format

unit describes one Python object; it is usually a single character or

a parenthesized sequence of format units. With a few exceptions, a

format unit that is not a parenthesized sequence normally corresponds

to a single address argument to \cfunction{PyArg_ParseTuple()}. In the

following description, the quoted form is the format unit; the entry

in (round) parentheses is the Python object type that matches the

format unit; and the entry in [square] brackets is the type of the C

variable(s) whose address should be passed. (Use the \samp{\&}

operator to pass a variable's address.)

Note that any Python object references which are provided to the

caller are \emph{borrowed} references; do not decrement their

reference count!

\begin{description}

\item[\samp{s} (string or Unicode object) {[char *]}]

Convert a Python string or Unicode object to a C pointer to a

character string. You must not provide storage for the string

itself; a pointer to an existing string is stored into the character

pointer variable whose address you pass. The C string is

null-terminated. The Python string must not contain embedded null

bytes; if it does, a \exception{TypeError} exception is raised.

Unicode objects are converted to C strings using the default

encoding. If this conversion fails, an \exception{UnicodeError} is

raised.

\item[\samp{s\#} (string, Unicode or any read buffer compatible object)

{[char *, int]}]

This variant on \samp{s} stores into two C variables, the first one a

pointer to a character string, the second one its length. In this

case the Python string may contain embedded null bytes. Unicode

objects pass back a pointer to the default encoded string version of the

object if such a conversion is possible. All other read buffer

compatible objects pass back a reference to the raw internal data

representation.

\item[\samp{z} (string or \code{None}) {[char *]}]

Like \samp{s}, but the Python object may also be \code{None}, in which

case the C pointer is set to \NULL{}.

\item[\samp{z\#} (string or \code{None} or any read buffer compatible object)

{[char *, int]}]

This is to \samp{s\#} as \samp{z} is to \samp{s}.

\item[\samp{u} (Unicode object) {[Py_UNICODE *]}]

Convert a Python Unicode object to a C pointer to a null-terminated

buffer of 16-bit Unicode (UTF-16) data. As with \samp{s}, there is no need

to provide storage for the Unicode data buffer; a pointer to the

existing Unicode data is stored into the Py_UNICODE pointer variable whose

address you pass.

\item[\samp{u\#} (Unicode object) {[Py_UNICODE *, int]}]

This variant on \samp{u} stores into two C variables, the first one

a pointer to a Unicode data buffer, the second one its length.

\item[\samp{es} (string, Unicode object or character buffer compatible

object) {[const char *encoding, char **buffer]}]

This variant on \samp{s} is used for encoding Unicode and objects

convertible to Unicode into a character buffer. It only works for

encoded data without embedded \NULL{} bytes.

The variant reads one C variable and stores into two C variables, the

first one a pointer to an encoding name string (\var{encoding}), and the

second a pointer to a pointer to a character buffer (\var{**buffer},

the buffer used for storing the encoded data).

The encoding name must map to a registered codec. If set to \NULL{},

the default encoding is used.

\cfunction{PyArg_ParseTuple()} will allocate a buffer of the needed

size using \cfunction{PyMem_NEW()}, copy the encoded data into this

buffer and adjust \var{*buffer} to reference the newly allocated

storage. The caller is responsible for calling

\cfunction{PyMem_Free()} to free the allocated buffer after usage.

\item[\samp{et} (string, Unicode object or character buffer compatible

object) {[const char *encoding, char **buffer]}]

Same as \samp{es} except that string objects are passed through without

recoding them. Instead, the implementation assumes that the string

object uses the encoding passed in as parameter.

\item[\samp{es\#} (string, Unicode object or character buffer compatible

object) {[const char *encoding, char **buffer, int *buffer_length]}]

This variant on \samp{s\#} is used for encoding Unicode and objects

convertible to Unicode into a character buffer. It reads one C

variable and stores into three C variables, the first one a pointer to

an encoding name string (\var{encoding}), the second a pointer to a

pointer to a character buffer (\var{**buffer}, the buffer used for

storing the encoded data) and the third one a pointer to an integer

(\var{*buffer_length}, the buffer length).

The encoding name must map to a registered codec. If set to \NULL{},

the default encoding is used.

There are two modes of operation:

If \var{*buffer} points a \NULL{} pointer,

\cfunction{PyArg_ParseTuple()} will allocate a buffer of the needed

size using \cfunction{PyMem_NEW()}, copy the encoded data into this

buffer and adjust \var{*buffer} to reference the newly allocated

storage. The caller is responsible for calling

\cfunction{PyMem_Free()} to free the allocated buffer after usage.

If \var{*buffer} points to a non-\NULL{} pointer (an already allocated

buffer), \cfunction{PyArg_ParseTuple()} will use this location as

buffer and interpret \var{*buffer_length} as buffer size. It will then

copy the encoded data into the buffer and 0-terminate it. Buffer

overflow is signalled with an exception.

In both cases, \var{*buffer_length} is set to the length of the

encoded data without the trailing 0-byte.

\item[\samp{et\#} (string, Unicode object or character buffer compatible

object) {[const char *encoding, char **buffer]}]

Same as \samp{es\#} except that string objects are passed through without

recoding them. Instead, the implementation assumes that the string

object uses the encoding passed in as parameter.

\item[\samp{b} (integer) {[char]}]

Convert a Python integer to a tiny int, stored in a C \ctype{char}.

\item[\samp{h} (integer) {[short int]}]

Convert a Python integer to a C \ctype{short int}.

\item[\samp{i} (integer) {[int]}]

Convert a Python integer to a plain C \ctype{int}.

\item[\samp{l} (integer) {[long int]}]

Convert a Python integer to a C \ctype{long int}.

\item[\samp{c} (string of length 1) {[char]}]

Convert a Python character, represented as a string of length 1, to a

C \ctype{char}.

\item[\samp{f} (float) {[float]}]

Convert a Python floating point number to a C \ctype{float}.

\item[\samp{d} (float) {[double]}]

Convert a Python floating point number to a C \ctype{double}.

\item[\samp{D} (complex) {[Py_complex]}]

Convert a Python complex number to a C \ctype{Py_complex} structure.

\item[\samp{O} (object) {[PyObject *]}]

Store a Python object (without any conversion) in a C object pointer.

The C program thus receives the actual object that was passed. The

object's reference count is not increased. The pointer stored is not

\NULL{}.

\item[\samp{O!} (object) {[\var{typeobject}, PyObject *]}]

Store a Python object in a C object pointer. This is similar to

\samp{O}, but takes two C arguments: the first is the address of a

Python type object, the second is the address of the C variable (of

type \ctype{PyObject *}) into which the object pointer is stored.

If the Python object does not have the required type,

\exception{TypeError} is raised.

\item[\samp{O\&} (object) {[\var{converter}, \var{anything}]}]

Convert a Python object to a C variable through a \var{converter}

function. This takes two arguments: the first is a function, the

second is the address of a C variable (of arbitrary type), converted

to \ctype{void *}. The \var{converter} function in turn is called as

follows:

\var{status}\code{ = }\var{converter}\code{(}\var{object}, \var{address}\code{);}

where \var{object} is the Python object to be converted and

\var{address} is the \ctype{void *} argument that was passed to

\cfunction{PyArg_ConvertTuple()}. The returned \var{status} should be

\code{1} for a successful conversion and \code{0} if the conversion

has failed. When the conversion fails, the \var{converter} function

should raise an exception.

\item[\samp{S} (string) {[PyStringObject *]}]

Like \samp{O} but requires that the Python object is a string object.

Raises \exception{TypeError} if the object is not a string object.

The C variable may also be declared as \ctype{PyObject *}.

\item[\samp{U} (Unicode string) {[PyUnicodeObject *]}]

Like \samp{O} but requires that the Python object is a Unicode object.

Raises \exception{TypeError} if the object is not a Unicode object.

The C variable may also be declared as \ctype{PyObject *}.

\item[\samp{t\#} (read-only character buffer) {[char *, int]}]

Like \samp{s\#}, but accepts any object which implements the read-only

buffer interface. The \ctype{char *} variable is set to point to the

first byte of the buffer, and the \ctype{int} is set to the length of

the buffer. Only single-segment buffer objects are accepted;

\exception{TypeError} is raised for all others.

\item[\samp{w} (read-write character buffer) {[char *]}]

Similar to \samp{s}, but accepts any object which implements the

read-write buffer interface. The caller must determine the length of

the buffer by other means, or use \samp{w\#} instead. Only

single-segment buffer objects are accepted; \exception{TypeError} is

raised for all others.

\item[\samp{w\#} (read-write character buffer) {[char *, int]}]

Like \samp{s\#}, but accepts any object which implements the

read-write buffer interface. The \ctype{char *} variable is set to

point to the first byte of the buffer, and the \ctype{int} is set to

the length of the buffer. Only single-segment buffer objects are

accepted; \exception{TypeError} is raised for all others.

\item[\samp{(\var{items})} (tuple) {[\var{matching-items}]}]

The object must be a Python sequence whose length is the number of

format units in \var{items}. The C arguments must correspond to the

individual format units in \var{items}. Format units for sequences

may be nested.

\strong{Note:} Prior to Python version 1.5.2, this format specifier

only accepted a tuple containing the individual parameters, not an

arbitrary sequence. Code which previously caused

\exception{TypeError} to be raised here may now proceed without an

exception. This is not expected to be a problem for existing code.

\end{description}

It is possible to pass Python long integers where integers are

requested; however no proper range checking is done --- the most

significant bits are silently truncated when the receiving field is

too small to receive the value (actually, the semantics are inherited

from downcasts in C --- your mileage may vary).

A few other characters have a meaning in a format string. These may

not occur inside nested parentheses. They are:

\begin{description}

\item[\samp{|}]

Indicates that the remaining arguments in the Python argument list are

optional. The C variables corresponding to optional arguments should

be initialized to their default value --- when an optional argument is

not specified, \cfunction{PyArg_ParseTuple()} does not touch the contents

of the corresponding C variable(s).

\item[\samp{:}]

The list of format units ends here; the string after the colon is used

as the function name in error messages (the ``associated value'' of

the exception that \cfunction{PyArg_ParseTuple()} raises).

\item[\samp{;}]

The list of format units ends here; the string after the semicolon is

used as the error message \emph{instead} of the default error message.

Clearly, \samp{:} and \samp{;} mutually exclude each other.

\end{description}

Some example calls:

\begin{verbatim}

int ok;

int i, j;

long k, l;

char *s;

int size;

ok = PyArg_ParseTuple(args, ""); /* No arguments */

/* Python call: f() */

\end{verbatim}

\begin{verbatim}

ok = PyArg_ParseTuple(args, "s", &s); /* A string */

/* Possible Python call: f('whoops!') */

\end{verbatim}

\begin{verbatim}

ok = PyArg_ParseTuple(args, "lls", &k, &l, &s); /* Two longs and a string */

/* Possible Python call: f(1, 2, 'three') */

\end{verbatim}

\begin{verbatim}

ok = PyArg_ParseTuple(args, "(ii)s#", &i, &j, &s, &size);

/* A pair of ints and a string, whose size is also returned */

/* Possible Python call: f((1, 2), 'three') */

\end{verbatim}

\begin{verbatim}

{

char *file;

char *mode = "r";

int bufsize = 0;

ok = PyArg_ParseTuple(args, "s|si", &file, &mode, &bufsize);

/* A string, and optionally another string and an integer */

/* Possible Python calls:

f('spam')

f('spam', 'w')

f('spam', 'wb', 100000) */

}

\end{verbatim}

\begin{verbatim}

{

int left, top, right, bottom, h, v;

ok = PyArg_ParseTuple(args, "((ii)(ii))(ii)",

&left, &top, &right, &bottom, &h, &v);

/* A rectangle and a point */

/* Possible Python call:

f(((0, 0), (400, 300)), (10, 10)) */

}

\end{verbatim}

\begin{verbatim}

{

Py_complex c;

ok = PyArg_ParseTuple(args, "D:myfunction", &c);

/* a complex, also providing a function name for errors */

/* Possible Python call: myfunction(1+2j) */

}

\end{verbatim}

\section{Keyword Parameters for Extension Functions

\label{parseTupleAndKeywords}}

The \cfunction{PyArg_ParseTupleAndKeywords()} function is declared as

follows:

\begin{verbatim}

int PyArg_ParseTupleAndKeywords(PyObject *arg, PyObject *kwdict,

char *format, char **kwlist, ...);

\end{verbatim}

The \var{arg} and \var{format} parameters are identical to those of the

\cfunction{PyArg_ParseTuple()} function. The \var{kwdict} parameter

is the dictionary of keywords received as the third parameter from the

Python runtime. The \var{kwlist} parameter is a \NULL{}-terminated

list of strings which identify the parameters; the names are matched

with the type information from \var{format} from left to right.

\strong{Note:} Nested tuples cannot be parsed when using keyword

arguments! Keyword parameters passed in which are not present in the

\var{kwlist} will cause \exception{TypeError} to be raised.

Here is an example module which uses keywords, based on an example by

Geoff Philbrick (\email{[email protected]}):%

\index{Philbrick, Geoff}