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(python2.1-ext.info)Extracting Parameters in Extension Functions
Extracting Parameters in Extension Functions
============================================
The `PyArg_ParseTuple()' function is declared as follows:
int PyArg_ParseTuple(PyObject *arg, char *format, ...);
The ARG argument must be a tuple object containing an argument list
passed from Python to a C function. The 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 ARG object must
match the format and the format must be exhausted.
Note that while `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 `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 `&' operator to pass a variable's
address.)
Note that any Python object references which are provided to the caller
are _borrowed_ references; do not decrement their reference count!
``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 `TypeError' exception is raised. Unicode
objects are converted to C strings using the default encoding. If
this conversion fails, an `UnicodeError' is raised.
``s#' (string, Unicode or any read buffer compatible object)'
{[char *, int }] This variant on `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.
``z' (string or `None') {[char * }]'
Like `s', but the Python object may also be `None', in which case
the C pointer is set to `NULL'.
``z#' (string or `None' or any read buffer compatible object)'
{[char *, int }] This is to `s#' as `z' is to `s'.
``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 `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.
``u#' (Unicode object) {[Py_UNICODE *, int }]'
This variant on `u' stores into two C variables, the first one a
pointer to a Unicode data buffer, the second one its length.
``es' (string, Unicode object or character buffer compatible'
object) {[const char *encoding, char **buffer }] This variant on
`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 (ENCODING), and
the second a pointer to a pointer to a character buffer (**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.
`PyArg_ParseTuple()' will allocate a buffer of the needed size
using `PyMem_NEW()', copy the encoded data into this buffer and
adjust *BUFFER to reference the newly allocated storage. The
caller is responsible for calling `PyMem_Free()' to free the
allocated buffer after usage.
``es#' (string, Unicode object or character buffer compatible'
object) {[const char *encoding, char **buffer, int *buffer_length
}] This variant on `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 (ENCODING), the second a
pointer to a pointer to a character buffer (**BUFFER, the buffer
used for storing the encoded data) and the third one a pointer to
an integer (*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 *BUFFER points a `NULL' pointer, `PyArg_ParseTuple()' will
allocate a buffer of the needed size using `PyMem_NEW()', copy the
encoded data into this buffer and adjust *BUFFER to reference the
newly allocated storage. The caller is responsible for calling
`PyMem_Free()' to free the allocated buffer after usage.
If *BUFFER points to a non-`NULL' pointer (an already allocated
buffer), `PyArg_ParseTuple()' will use this location as buffer and
interpret *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, *BUFFER_LENGTH is set to the length of the encoded
data without the trailing 0-byte.
``b' (integer) {[char }]'
Convert a Python integer to a tiny int, stored in a C `char'.
``h' (integer) {[short int }]'
Convert a Python integer to a C `short int'.
``i' (integer) {[int }]'
Convert a Python integer to a plain C `int'.
``l' (integer) {[long int }]'
Convert a Python integer to a C `long int'.
``c' (string of length 1) {[char }]'
Convert a Python character, represented as a string of length 1,
to a C `char'.
``f' (float) {[float }]'
Convert a Python floating point number to a C `float'.
``d' (float) {[double }]'
Convert a Python floating point number to a C `double'.
``D' (complex) {[Py_complex }]'
Convert a Python complex number to a C `Py_complex' structure.
``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'.
``O!' (object) {[TYPEOBJECT, PyObject * }]'
Store a Python object in a C object pointer. This is similar to
`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 `PyObject *') into which the object pointer is stored. If
the Python object does not have the required type, `TypeError' is
raised.
``O&' (object) {[CONVERTER, ANYTHING }]'
Convert a Python object to a C variable through a 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 `void *'. The CONVERTER function in turn is called as
follows:
STATUS` = 'CONVERTER`('OBJECT, ADDRESS`);'
where OBJECT is the Python object to be converted and ADDRESS is
the `void *' argument that was passed to `PyArg_ParseTuple()'.
The returned STATUS should be `1' for a successful conversion and
`0' if the conversion has failed. When the conversion fails, the
CONVERTER function should raise an exception.
``S' (string) {[PyStringObject * }]'
Like `O' but requires that the Python object is a string object.
Raises `TypeError' if the object is not a string object. The C
variable may also be declared as `PyObject *'.
``U' (Unicode string) {[PyUnicodeObject * }]'
Like `O' but requires that the Python object is a Unicode object.
Raises `TypeError' if the object is not a Unicode object. The C
variable may also be declared as `PyObject *'.
``t#' (read-only character buffer) {[char *, int }]'
Like `s#', but accepts any object which implements the read-only
buffer interface. The `char *' variable is set to point to the
first byte of the buffer, and the `int' is set to the length of
the buffer. Only single-segment buffer objects are accepted;
`TypeError' is raised for all others.
``w' (read-write character buffer) {[char * }]'
Similar to `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 `w#' instead. Only
single-segment buffer objects are accepted; `TypeError' is raised
for all others.
``w#' (read-write character buffer) {[char *, int }]'
Like `s#', but accepts any object which implements the read-write
buffer interface. The `char *' variable is set to point to the
first byte of the buffer, and the `int' is set to the length of
the buffer. Only single-segment buffer objects are accepted;
`TypeError' is raised for all others.
``(ITEMS)' (tuple) {[MATCHING-ITEMS }]'
The object must be a Python sequence whose length is the number of
format units in ITEMS. The C arguments must correspond to the
individual format units in 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 `TypeError' to
be raised here may now proceed without an exception. This is not
expected to be a problem for existing code.
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:
``|''
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, `PyArg_ParseTuple()' does not touch the
contents of the corresponding C variable(s).
``:''
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 `PyArg_ParseTuple()' raises).
``;''
The list of format units ends here; the string after the semicolon
is used as the error message _instead_ of the default error
message. Clearly, `:' and `;' mutually exclude each other.
Some example calls:
int ok;
int i, j;
long k, l;
char *s;
int size;
ok = PyArg_ParseTuple(args, ""); /* No arguments */
/* Python call: f() */
ok = PyArg_ParseTuple(args, "s", &s); /* A string */
/* Possible Python call: f('whoops!') */
ok = PyArg_ParseTuple(args, "lls", &k, &l, &s); /* Two longs and a string */
/* Possible Python call: f(1, 2, 'three') */
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') */
{
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) */
}
{
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)) */
}
{
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) */
}
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