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Move reference material on PyArg_Parse*() out of the Extending & Embedding

Browse source 
document to the C API reference.  Move some instructional text from the API
reference to the Extending & Embedding manual.

Fix the descriptions of the es and es# formats for PyArg_Parse*().
This closes SF bug #536516.
This commit is contained in:
Fred Drake 2002-04-05 23:01:14 +00:00
parent 6b8ab74c8a
commit 68304ccce3
5 changed files with 530 additions and 485 deletions
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112
Doc/api/newtypes.tex
View file

@ -1,4 +1,8 @@
\chapter{Defining New Object Types \label{newTypes}}
\chapter{Object Implementation Support \label{newTypes}}
This chapter describes the functions, types, and macros used when
defining new object types.
\section{Allocating Objects on the Heap
@ -388,6 +392,12 @@ which do not store references to other objects, or which only store
references to atomic types (such as numbers or strings), do not need
to provide any explicit support for garbage collection.
An example showing the use of these interfaces can be found in
``\ulink{Supporting the Cycle
Collector}{../ext/example-cycle-support.html}'' in
\citetitle[../ext/ext.html]{Extending and Embedding the Python
Interpreter}.
To create a container type, the \member{tp_flags} field of the type
object must include the \constant{Py_TPFLAGS_HAVE_GC} and provide an
implementation of the \member{tp_traverse} handler. If instances of the
@ -504,103 +514,3 @@ The \member{tp_clear} handler must be of the \ctype{inquiry} type, or
this method if it detects that this object is involved in a
reference cycle.
\end{ctypedesc}
\subsection{Example Cycle Collector Support
\label{example-cycle-support}}
This example shows only enough of the implementation of an extension
type to show how the garbage collector support needs to be added. It
shows the definition of the object structure, the
\member{tp_traverse}, \member{tp_clear} and \member{tp_dealloc}
implementations, the type structure, and a constructor --- the module
initialization needed to export the constructor to Python is not shown
as there are no special considerations there for the collector. To
make this interesting, assume that the module exposes ways for the
\member{container} field of the object to be modified. Note that
since no checks are made on the type of the object used to initialize
\member{container}, we have to assume that it may be a container.
\begin{verbatim}
#include "Python.h"
typedef struct {
PyObject_HEAD
PyObject *container;
} MyObject;
static int
my_traverse(MyObject *self, visitproc visit, void *arg)
{
if (self->container != NULL)
return visit(self->container, arg);
else
return 0;
}
static int
my_clear(MyObject *self)
{
Py_XDECREF(self->container);
self->container = NULL;
return 0;
}
static void
my_dealloc(MyObject *self)
{
PyObject_GC_UnTrack((PyObject *) self);
Py_XDECREF(self->container);
PyObject_GC_Del(self);
}
\end{verbatim}
\begin{verbatim}
statichere PyTypeObject
MyObject_Type = {
PyObject_HEAD_INIT(NULL)
0,
"MyObject",
sizeof(MyObject),
0,
(destructor)my_dealloc, /* tp_dealloc */
0, /* tp_print */
0, /* tp_getattr */
0, /* tp_setattr */
0, /* tp_compare */
0, /* tp_repr */
0, /* tp_as_number */
0, /* tp_as_sequence */
0, /* tp_as_mapping */
0, /* tp_hash */
0, /* tp_call */
0, /* tp_str */
0, /* tp_getattro */
0, /* tp_setattro */
0, /* tp_as_buffer */
Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC,
0, /* tp_doc */
(traverseproc)my_traverse, /* tp_traverse */
(inquiry)my_clear, /* tp_clear */
0, /* tp_richcompare */
0, /* tp_weaklistoffset */
};
/* This constructor should be made accessible from Python. */
static PyObject *
new_object(PyObject *unused, PyObject *args)
{
PyObject *container = NULL;
MyObject *result = NULL;
if (PyArg_ParseTuple(args, "|O:new_object", &container)) {
result = PyObject_GC_New(MyObject, &MyObject_Type);
if (result != NULL) {
result->container = container;
PyObject_GC_Track(result);
}
}
return (PyObject *) result;
}
\end{verbatim}

413
Doc/api/utilities.tex
View file

@ -357,13 +357,291 @@ and methods. Additional information and examples are available in
\citetitle[../ext/ext.html]{Extending and Embedding the Python
Interpreter}.
The first three of these functions described,
\cfunction{PyArg_ParseTuple()},
\cfunction{PyArg_ParseTupleAndKeywords()}, and
\cfunction{PyArg_Parse()}, all use \emph{format strings} which are
used to tell the function about the expected arguments. The format
strings use the same syntax for each of these functions.
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 these functions. 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.
\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
NUL-terminated. The Python string must not contain embedded NUL
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, a \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 NUL-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 \ctype{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.
Non-Unicode objects are handled by interpreting their read-buffer
pointer as pointer to a \ctype{Py_UNICODE} array.
\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 NUL bytes.
This format requires two arguments. The first is only used as
input, and must be a \ctype{char*} which points to the name of an
encoding as a NUL-terminated string, or \NULL, in which case the
default encoding is used. An exception is raised if the named
encoding is not known to Python. The second argument must be a
\ctype{char**}; the value of the pointer it references will be set
to a buffer with the contents of the argument text. The text will
be encoded in the encoding specified by the first argument.
\cfunction{PyArg_ParseTuple()} will allocate a buffer of the needed
size, 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 use.
\item[\samp{et} (string, Unicode object or character buffer
compatible object) {[const char *encoding, char **buffer]}]
Same as \samp{es} except that 8-bit 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. Unlike the
\samp{es} format, this variant allows input data which contains NUL
characters.
It requires three arguments. The first is only used as input, and
must be a \ctype{char*} which points to the name of an encoding as a
NUL-terminated string, or \NULL, in which case the default encoding
is used. An exception is raised if the named encoding is not known
to Python. The second argument must be a \ctype{char**}; the value
of the pointer it references will be set to a buffer with the
contents of the argument text. The text will be encoded in the
encoding specified by the first argument. The third argument must
be a pointer to an integer; the referenced integer will be set to
the number of bytes in the output buffer.
There are two modes of operation:
If \var{*buffer} points a \NULL{} pointer, the function will
allocate a buffer of the needed size, copy the encoded data into
this buffer and set \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 the buffer and interpret the initial value of
\var{*buffer_length} as the buffer size. It will then copy the
encoded data into the buffer and NUL-terminate it. If the buffer
is not large enough, a \exception{ValueError} will be set.
In both cases, \var{*buffer_length} is set to the length of the
encoded data without the trailing NUL 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{L} (integer) {[LONG_LONG]}]
Convert a Python integer to a C \ctype{long long}. This format is
only available on platforms that support \ctype{long long} (or
\ctype{_int64} on Windows).
\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 the
\cfunction{PyArg_Parse*()} function. 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.
\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}
Note that any Python object references which are provided to the
caller are \emph{borrowed} references; do not decrement their
reference count!
Additional arguments passed to these functions must be addresses of
variables whose type is determined by the format string; these are
used to store values from the input tuple. There are a few cases, as
described in the list of format units above, where these parameters
are used as input values; they should match what is specified for the
corresponding format unit in that case.
For the conversion to succeed, the \var{arg} object must match the
format and the format must be exhausted. On success, the
\cfunction{PyArg_Parse*()} functions return true, otherwise they
return false and raise an appropriate exception.
\begin{cfuncdesc}{int}{PyArg_ParseTuple}{PyObject *args, char *format,
\moreargs}
Parse the parameters of a function that takes only positional
parameters into local variables. Returns true on success; on
failure, it returns false and raises the appropriate exception. See
\citetitle[../ext/parseTuple.html]{Extending and Embedding the
Python Interpreter} for more information.
failure, it returns false and raises the appropriate exception.
\end{cfuncdesc}
\begin{cfuncdesc}{int}{PyArg_ParseTupleAndKeywords}{PyObject *args,
@ -372,8 +650,6 @@ Interpreter}.
Parse the parameters of a function that takes both positional and
keyword parameters into local variables. Returns true on success;
on failure, it returns false and raises the appropriate exception.
See \citetitle[../ext/parseTupleAndKeywords.html]{Extending and
Embedding the Python Interpreter} for more information.
\end{cfuncdesc}
\begin{cfuncdesc}{int}{PyArg_Parse}{PyObject *args, char *format,
@ -440,8 +716,127 @@ PyArg_ParseTuple(args, "O|O:ref", &object, &callback)
Create a new value based on a format string similar to those
accepted by the \cfunction{PyArg_Parse*()} family of functions and a
sequence of values. Returns the value or \NULL{} in the case of an
error; an exception will be raised if \NULL{} is returned. For more
information on the format string and additional parameters, see
\citetitle[../ext/buildValue.html]{Extending and Embedding the
Python Interpreter}.
error; an exception will be raised if \NULL{} is returned.
\cfunction{Py_BuildValue()} does not always build a tuple. It
builds a tuple only if its format string contains two or more format
units. If the format string is empty, it returns \code{None}; if it
contains exactly one format unit, it returns whatever object is
described by that format unit. To force it to return a tuple of
size 0 or one, parenthesize the format string.
When memory buffers are passed as parameters to supply data to build
objects, as for the \samp{s} and \samp{s\#} formats, the required
data is copied. Buffers provided by the caller are never referenced
by the objects created by \cfunction{Py_BuildValue()}. In other
words, if your code invokes \cfunction{malloc()} and passes the
allocated memory to \cfunction{Py_BuildValue()}, your code is
responsible for calling \cfunction{free()} for that memory once
\cfunction{Py_BuildValue()} returns.
In the following description, the quoted form is the format unit;
the entry in (round) parentheses is the Python object type that the
format unit will return; and the entry in [square] brackets is the
type of the C value(s) to be passed.
The characters space, tab, colon and comma are ignored in format
strings (but not within format units such as \samp{s\#}). This can
be used to make long format strings a tad more readable.
\begin{description}
\item[\samp{s} (string) {[char *]}]
Convert a null-terminated C string to a Python object. If the C
string pointer is \NULL, \code{None} is used.
\item[\samp{s\#} (string) {[char *, int]}]
Convert a C string and its length to a Python object. If the C
string pointer is \NULL, the length is ignored and \code{None} is
returned.
\item[\samp{z} (string or \code{None}) {[char *]}]
Same as \samp{s}.
\item[\samp{z\#} (string or \code{None}) {[char *, int]}]
Same as \samp{s\#}.
\item[\samp{u} (Unicode string) {[Py_UNICODE *]}]
Convert a null-terminated buffer of Unicode (UCS-2) data to a
Python Unicode object. If the Unicode buffer pointer is \NULL,
\code{None} is returned.
\item[\samp{u\#} (Unicode string) {[Py_UNICODE *, int]}]
Convert a Unicode (UCS-2) data buffer and its length to a Python
Unicode object. If the Unicode buffer pointer is \NULL, the
length is ignored and \code{None} is returned.
\item[\samp{i} (integer) {[int]}]
Convert a plain C \ctype{int} to a Python integer object.
\item[\samp{b} (integer) {[char]}]
Same as \samp{i}.
\item[\samp{h} (integer) {[short int]}]
Same as \samp{i}.
\item[\samp{l} (integer) {[long int]}]
Convert a C \ctype{long int} to a Python integer object.
\item[\samp{c} (string of length 1) {[char]}]
Convert a C \ctype{int} representing a character to a Python
string of length 1.
\item[\samp{d} (float) {[double]}]
Convert a C \ctype{double} to a Python floating point number.
\item[\samp{f} (float) {[float]}]
Same as \samp{d}.
\item[\samp{D} (complex) {[Py_complex *]}]
Convert a C \ctype{Py_complex} structure to a Python complex
number.
\item[\samp{O} (object) {[PyObject *]}]
Pass a Python object untouched (except for its reference count,
which is incremented by one). If the object passed in is a
\NULL{} pointer, it is assumed that this was caused because the
call producing the argument found an error and set an exception.
Therefore, \cfunction{Py_BuildValue()} will return \NULL{} but
won't raise an exception. If no exception has been raised yet,
\exception{SystemError} is set.
\item[\samp{S} (object) {[PyObject *]}]
Same as \samp{O}.
\item[\samp{U} (object) {[PyObject *]}]
Same as \samp{O}.
\item[\samp{N} (object) {[PyObject *]}]
Same as \samp{O}, except it doesn't increment the reference count
on the object. Useful when the object is created by a call to an
object constructor in the argument list.
\item[\samp{O\&} (object) {[\var{converter}, \var{anything}]}]
Convert \var{anything} to a Python object through a
\var{converter} function. The function is called with
\var{anything} (which should be compatible with \ctype{void *}) as
its argument and should return a ``new'' Python object, or \NULL{}
if an error occurred.
\item[\samp{(\var{items})} (tuple) {[\var{matching-items}]}]
Convert a sequence of C values to a Python tuple with the same
number of items.
\item[\samp{[\var{items}]} (list) {[\var{matching-items}]}]
Convert a sequence of C values to a Python list with the same
number of items.
\item[\samp{\{\var{items}\}} (dictionary) {[\var{matching-items}]}]
Convert a sequence of C values to a Python dictionary. Each pair
of consecutive C values adds one item to the dictionary, serving
as key and value, respectively.
\end{description}
If there is an error in the format string, the
\exception{SystemError} exception is set and \NULL{} returned.
\end{cfuncdesc}

79
Doc/ext/cycle-gc.c Normal file
View file

@ -0,0 +1,79 @@
#include "Python.h"
typedef struct {
PyObject_HEAD
PyObject *container;
} MyObject;
static int
my_traverse(MyObject *self, visitproc visit, void *arg)
{
if (self->container != NULL)
return visit(self->container, arg);
else
return 0;
}
static int
my_clear(MyObject *self)
{
Py_XDECREF(self->container);
self->container = NULL;
return 0;
}
static void
my_dealloc(MyObject *self)
{
PyObject_GC_UnTrack((PyObject *) self);
Py_XDECREF(self->container);
PyObject_GC_Del(self);
}
static PyTypeObject
MyObject_Type = {
PyObject_HEAD_INIT(NULL)
0,
"MyObject",
sizeof(MyObject),
0,
(destructor)my_dealloc, /* tp_dealloc */
0, /* tp_print */
0, /* tp_getattr */
0, /* tp_setattr */
0, /* tp_compare */
0, /* tp_repr */
0, /* tp_as_number */
0, /* tp_as_sequence */
0, /* tp_as_mapping */
0, /* tp_hash */
0, /* tp_call */
0, /* tp_str */
0, /* tp_getattro */
0, /* tp_setattro */
0, /* tp_as_buffer */
Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC,
0, /* tp_doc */
(traverseproc)my_traverse, /* tp_traverse */
(inquiry)my_clear, /* tp_clear */
0, /* tp_richcompare */
0, /* tp_weaklistoffset */
};
/* This constructor should be made accessible from Python. */
static PyObject *
new_object(PyObject *unused, PyObject *args)
{
PyObject *container = NULL;
MyObject *result = NULL;
if (PyArg_ParseTuple(args, "|O:new_object", &container)) {
result = PyObject_GC_New(MyObject, &MyObject_Type);
if (result != NULL) {
result->container = container;
PyObject_GC_Track(result);
}
}
return (PyObject *) result;
}

377
Doc/ext/extending.tex
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@ -602,12 +602,12 @@ int PyArg_ParseTuple(PyObject *arg, char *format, ...);
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
must be a format string, whose syntax is explained in
``\ulink{Parsing arguments and building
values}{../api/arg-parsing.html}'' in the
\citetitle[../api/api.html]{Python/C API Reference Manual}. 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. On success, \cfunction{PyArg_ParseTuple()} returns true,
otherwise it returns false and raises an appropriate exception.
determined by the format string.
Note that while \cfunction{PyArg_ParseTuple()} checks that the Python
arguments have the required types, it cannot check the validity of the
@ -615,263 +615,10 @@ 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 \ctype{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.
Non-Unicode objects are handled by interpreting their read buffer
pointer as pointer to a \ctype{Py_UNICODE} array.
\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{L} (integer) {[LONG_LONG]}]
Convert a Python integer to a C \ctype{long long}. This format is only
available on platforms that support \ctype{long long} (or \ctype{_int64}
on Windows).
\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_ParseTuple()}. 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.
\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}
@ -1042,120 +789,6 @@ exactly one format unit, it returns whatever object is described by
that format unit. To force it to return a tuple of size 0 or one,
parenthesize the format string.
When memory buffers are passed as parameters to supply data to build
objects, as for the \samp{s} and \samp{s\#} formats, the required data
is copied. Buffers provided by the caller are never referenced by the
objects created by \cfunction{Py_BuildValue()}. In other words, if
your code invokes \cfunction{malloc()} and passes the allocated memory
to \cfunction{Py_BuildValue()}, your code is responsible for
calling \cfunction{free()} for that memory once
\cfunction{Py_BuildValue()} returns.
In the following description, the quoted form is the format unit; the
entry in (round) parentheses is the Python object type that the format
unit will return; and the entry in [square] brackets is the type of
the C value(s) to be passed.
The characters space, tab, colon and comma are ignored in format
strings (but not within format units such as \samp{s\#}). This can be
used to make long format strings a tad more readable.
\begin{description}
\item[\samp{s} (string) {[char *]}]
Convert a null-terminated C string to a Python object. If the C
string pointer is \NULL, \code{None} is used.
\item[\samp{s\#} (string) {[char *, int]}]
Convert a C string and its length to a Python object. If the C string
pointer is \NULL, the length is ignored and \code{None} is
returned.
\item[\samp{z} (string or \code{None}) {[char *]}]
Same as \samp{s}.
\item[\samp{z\#} (string or \code{None}) {[char *, int]}]
Same as \samp{s\#}.
\item[\samp{u} (Unicode string) {[Py_UNICODE *]}]
Convert a null-terminated buffer of Unicode (UCS-2) data to a Python
Unicode object. If the Unicode buffer pointer is \NULL,
\code{None} is returned.
\item[\samp{u\#} (Unicode string) {[Py_UNICODE *, int]}]
Convert a Unicode (UCS-2) data buffer and its length to a Python
Unicode object. If the Unicode buffer pointer is \NULL, the length
is ignored and \code{None} is returned.
\item[\samp{i} (integer) {[int]}]
Convert a plain C \ctype{int} to a Python integer object.
\item[\samp{b} (integer) {[char]}]
Same as \samp{i}.
\item[\samp{h} (integer) {[short int]}]
Same as \samp{i}.
\item[\samp{l} (integer) {[long int]}]
Convert a C \ctype{long int} to a Python integer object.
\item[\samp{c} (string of length 1) {[char]}]
Convert a C \ctype{int} representing a character to a Python string of
length 1.
\item[\samp{d} (float) {[double]}]
Convert a C \ctype{double} to a Python floating point number.
\item[\samp{f} (float) {[float]}]
Same as \samp{d}.
\item[\samp{D} (complex) {[Py_complex *]}]
Convert a C \ctype{Py_complex} structure to a Python complex number.
\item[\samp{O} (object) {[PyObject *]}]
Pass a Python object untouched (except for its reference count, which
is incremented by one). If the object passed in is a \NULL{}
pointer, it is assumed that this was caused because the call producing
the argument found an error and set an exception. Therefore,
\cfunction{Py_BuildValue()} will return \NULL{} but won't raise an
exception. If no exception has been raised yet,
\cdata{PyExc_SystemError} is set.
\item[\samp{S} (object) {[PyObject *]}]
Same as \samp{O}.
\item[\samp{U} (object) {[PyObject *]}]
Same as \samp{O}.
\item[\samp{N} (object) {[PyObject *]}]
Same as \samp{O}, except it doesn't increment the reference count on
the object. Useful when the object is created by a call to an object
constructor in the argument list.
\item[\samp{O\&} (object) {[\var{converter}, \var{anything}]}]
Convert \var{anything} to a Python object through a \var{converter}
function. The function is called with \var{anything} (which should be
compatible with \ctype{void *}) as its argument and should return a
``new'' Python object, or \NULL{} if an error occurred.
\item[\samp{(\var{items})} (tuple) {[\var{matching-items}]}]
Convert a sequence of C values to a Python tuple with the same number
of items.
\item[\samp{[\var{items}]} (list) {[\var{matching-items}]}]
Convert a sequence of C values to a Python list with the same number
of items.
\item[\samp{\{\var{items}\}} (dictionary) {[\var{matching-items}]}]
Convert a sequence of C values to a Python dictionary. Each pair of
consecutive C values adds one item to the dictionary, serving as key
and value, respectively.
\end{description}
If there is an error in the format string, the
\cdata{PyExc_SystemError} exception is raised and \NULL{} returned.
Examples (to the left the call, to the right the resulting Python value):
\begin{verbatim}

34
Doc/ext/newtypes.tex
View file

@ -47,7 +47,9 @@ functions that have no yet been defined, but we need to be able to
refer to it, hence the declaration.
The \code{staticforward} is required to placate various brain dead
compilers.
compilers. The actual definition of the object declared using
\code{staticforward} should use \code{statichere} instead of
\keyword{static}.
\begin{verbatim}
typedef struct {
@ -154,7 +156,7 @@ Python objects, one would decref them here.
Moving on, we come to the crunch --- the type object.
\begin{verbatim}
static PyTypeObject noddy_NoddyType = {
statichere PyTypeObject noddy_NoddyType = {
PyObject_HEAD_INIT(NULL)
0, /* ob_size */
"Noddy", /* tp_name */
@ -173,6 +175,9 @@ static PyTypeObject noddy_NoddyType = {
};
\end{verbatim}
(Note the use of \code{statichere} instead of \keyword{static}, since
we used \code{staticforward} in the declaration.)
Now if you go and look up the definition of \ctype{PyTypeObject} in
\file{object.h} you'll see that it has many, many more fields that the
definition above. The remaining fields will be filled with zeros by
@ -404,7 +409,7 @@ my_dealloc(PyObject *obj)
\end{verbatim}
\subsection{Object Representation}
\subsection{Object Presentation}
In Python, there are three ways to generate a textual representation
of an object: the \function{repr()}\bifuncindex{repr} function (or
@ -913,6 +918,29 @@ avoiding the exception can yield slightly better performance. If an
actual error occurs, it should set an exception and return \NULL.
\subsection{Cycle Collector Support
\label{example-cycle-support}}
This example shows only enough of the implementation of an extension
type to show how the garbage collector support needs to be added. It
shows the definition of the object structure, the
\member{tp_traverse}, \member{tp_clear} and \member{tp_dealloc}
implementations, the type structure, and a constructor --- the module
initialization needed to export the constructor to Python is not shown
as there are no special considerations there for the collector. To
make this interesting, assume that the module exposes ways for the
\member{container} field of the object to be modified. Note that
since no checks are made on the type of the object used to initialize
\member{container}, we have to assume that it may be a container.
\verbatiminput{cycle-gc.c}
Full details on the APIs related to the cycle detector are in
\ulink{Supporting Cyclic Garbarge
Collection}{../api/supporting-cycle-detection.html} in the
\citetitle[../api/api.html]{Python/C API Reference Manual}.
\subsection{More Suggestions}
Remember that you can omit most of these functions, in which case you