Analyse des arguments et construction des valeurs

Ces fonctions sont utiles pour créer vos propres fonctions et méthodes d’extensions. Des informations supplémentaires et des exemples sont disponibles ici: Extending and Embedding the Python Interpreter.

Dans Les trois premières de ces fonctions décrites, PyArg_ParseTuple(), PyArg_ParseTupleAndKeywords(), et PyArg_Parse(), toutes utilisent des chaînes de format qui sont utilisées pour indiquer à la fonction les arguments attendus. Les chaînes de format utilise la même syntaxe pour chacune de ces fonctions.

Analyse des arguments

Une chaîne de format se compose de zéro ou plusieurs « unités de format ». Une unité de format décrit un objet Python, elle est généralement composée d’un seul caractère ou d’une séquence d’unités de format entre parenthèses. À quelques exceptions près, une unité de format qui n’est pas une séquence entre parenthèses correspond normalement à un argument d’une seule adresse pour ces fonctions. Dans la description qui suit, la forme entre guillemets est l’unité de format, l’entrée entre parenthèses est le type d’objet Python qui correspond à l’unité de format, et l’entrée entre crochets est le type de la variable C (ou des variables) dont l’adresse doit être donnée.

Chaînes et tampons

Ces formats permettent d’accéder à un objet sous forme d’un fragment de mémoire contigüe. Il n’est pas nécessaire d’allouer la mémoire pour l”unicode ou le bytes renvoyé.

In general, when a format sets a pointer to a buffer, the buffer is managed by the corresponding Python object, and the buffer shares the lifetime of this object. You won’t have to release any memory yourself. The only exceptions are es, es#, et and et#.

Néanmoins, quand une structure Py_buffer est en cours de remplissage, le buffer sous-jacent est verrouillé pour permettre à l’appelant d’utiliser le buffer par la suite, même à l’intérieur d’un bloc Py_BEGIN_ALLOW_THREADS. Ceci sans le risque pour les données muables de voir leur taille changée ou d’être supprimées. En conséquence, il vous appartient d’appeler PyBuffer_Release() après que vous ayez terminé de traiter les données (ou après une interruption prémataturée du traitement de ces données).

Sauf indication contraire, les tampons ne se terminent pas par NUL.

Some formats require a read-only bytes-like object, and set a pointer instead of a buffer structure. They work by checking that the object’s PyBufferProcs.bf_releasebuffer field is NULL, which disallows mutable objects such as bytearray.

Note

Pour toutes les variantes de formats # (s#, y#, etc), le type de l’argument length (int ou Py_ssize_t) est contrôlé en définissant la macro PY_SSIZE_T_CLEAN avant d’inclure le fichier Python.h. Si la macro est définie, la longueur est de type Py_ssize_t au lieu d’être de type int. Ce comportement changera dans une future version de Python, qui supportera seulement Py_ssize_t a la place de int. Il est préférable de toujours définir PY_SSIZE_T_CLEAN.

s (str) [const char *]

Convert a Unicode object to a C pointer to a character string. A pointer to an existing string is stored in the character pointer variable whose address you pass. The C string is NUL-terminated. The Python string must not contain embedded null code points; if it does, a ValueError exception is raised. Unicode objects are converted to C strings using 'utf-8' encoding. If this conversion fails, a UnicodeError is raised.

Note

This format does not accept bytes-like objects. If you want to accept filesystem paths and convert them to C character strings, it is preferable to use the O& format with PyUnicode_FSConverter() as converter.

Modifié dans la version 3.5: Previously, TypeError was raised when embedded null code points were encountered in the Python string.

s* (str ou bytes-like object) [Py_buffer]
This format accepts Unicode objects as well as bytes-like objects. It fills a Py_buffer structure provided by the caller. In this case the resulting C string may contain embedded NUL bytes. Unicode objects are converted to C strings using 'utf-8' encoding.
s# (str, read-only bytes-like object) [const char *, int or Py_ssize_t]
Like s*, except that it doesn’t accept mutable objects. The result is stored into two C variables, the first one a pointer to a C string, the second one its length. The string may contain embedded null bytes. Unicode objects are converted to C strings using 'utf-8' encoding.
z (str ou None) [const char *]
Comme s, mais l’objet Python peut aussi être None, auquel cas le pointeur C devient NULL.
z* (str, bytes-like object ou None) [Py_buffer]
Like s*, but the Python object may also be None, in which case the buf member of the Py_buffer structure is set to NULL.
z# (str, read-only bytes-like object or None) [const char *, int]
Comme s#, mais l’objet Python peut également être None, dans ce cas le pointeur C est définie à NULL.
y (read-only bytes-like object) [const char *]

This format converts a bytes-like object to a C pointer to a character string; it does not accept Unicode objects. The bytes buffer must not contain embedded null bytes; if it does, a ValueError exception is raised.

Modifié dans la version 3.5: Previously, TypeError was raised when embedded null bytes were encountered in the bytes buffer.

y* (bytes-like object) [Py_buffer]
This variant on s* doesn’t accept Unicode objects, only bytes-like objects. This is the recommended way to accept binary data.
y# (read-only bytes-like object) [const char *, int]
Cette variante de s# n’accepte pas les objets Unicode, uniquement des objets assimilés à des octets.
S (bytes) [PyBytesObject *]
Requires that the Python object is a bytes object, without attempting any conversion. Raises TypeError if the object is not a bytes object. The C variable may also be declared as PyObject*.
Y (bytearray) [PyByteArrayObject *]
Requires that the Python object is a bytearray object, without attempting any conversion. Raises TypeError if the object is not a bytearray object. The C variable may also be declared as PyObject*.
u (str) [Py_UNICODE *]

Convert a Python Unicode object to a C pointer to a NUL-terminated buffer of Unicode characters. You must pass the address of a Py_UNICODE pointer variable, which will be filled with the pointer to an existing Unicode buffer. Please note that the width of a Py_UNICODE character depends on compilation options (it is either 16 or 32 bits). The Python string must not contain embedded null code points; if it does, a ValueError exception is raised.

Modifié dans la version 3.5: Previously, TypeError was raised when embedded null code points were encountered in the Python string.

u# (str) [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. This variant allows null code points.
Z (str ou None) [Py_UNICODE *]
Comme u, mais l’objet Python peut aussi être None, auquel cas le pointeur Py_UNICODE vaut NULL.
Z# (str ou None) [Py_UNICODE *, int]
Comme u#, mais l’objet Python peut également être None, auquel cas le pointeur Py_UNICODE vaut NULL.
U (str) [PyObject *]
Requires that the Python object is a Unicode object, without attempting any conversion. Raises TypeError if the object is not a Unicode object. The C variable may also be declared as PyObject*.
w* (read-write bytes-like object) [Py_buffer]
This format accepts any object which implements the read-write buffer interface. It fills a Py_buffer structure provided by the caller. The buffer may contain embedded null bytes. The caller have to call PyBuffer_Release() when it is done with the buffer.
es (str) [const char *encoding, char **buffer]

This variant on s is used for encoding 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 const char* which points to the name of an encoding as a NUL-terminated string, or NULL, in which case 'utf-8' encoding is used. An exception is raised if the named encoding is not known to Python. The second argument must be a 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.

PyArg_ParseTuple() will allocate a buffer of the needed size, 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 use.

et (str, bytes or bytearray) [const char *encoding, char **buffer]
Same as es except that byte string objects are passed through without recoding them. Instead, the implementation assumes that the byte string object uses the encoding passed in as parameter.
es# (str) [const char *encoding, char **buffer, int *buffer_length]

This variant on s# is used for encoding Unicode into a character buffer. Unlike the 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 const char* which points to the name of an encoding as a NUL-terminated string, or NULL, in which case 'utf-8' encoding is used. An exception is raised if the named encoding is not known to Python. The second argument must be a 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.

Il existe deux modes de fonctionnement :

If *buffer points a NULL pointer, the function will allocate a buffer of the needed size, copy the encoded data into this buffer and set *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 the buffer and interpret the initial value of *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 ValueError will be set.

Dans les deux cas, *buffer_length est la longueur des données encodées, sans l’octet NUL de fin.

et# (str, bytes or bytearray) [const char *encoding, char **buffer, int *buffer_length]
Same as es# except that byte string objects are passed through without recoding them. Instead, the implementation assumes that the byte string object uses the encoding passed in as parameter.

Les nombres

b (int) [unsigned char]
Convertit un entier Python positif ou nul en un unsigned tiny int, stocké dans un unsigned char C.
B (int) [unsigned char]
Convertit un entier Python en un tiny int sans vérifier le débordement, stocké dans un unsigned char C.
h (int) [short int]
Convertit un entier Python en un short int C.
H (int) [unsigned short int]
Convertit un entier Python en un unsigned short int C, sans contrôle de débordement.
i (int) [int]
Convertit un entier Python en un int C.
I (int) [unsigned int]
Convertit un entier Python en un unsigned int C, sans contrôle de le débordement.
l (int) [long int]
Convertit un entier Python en un long int.
k (int) [unsigned long]
Convertit un entier Python en un unsigned long C sans en vérifier le débordement.
L (int) [long long]
Convert a Python integer to a C long long.
K (int) [unsigned long long]
Convert a Python integer to a C unsigned long long without overflow checking.
n (int) [Py_ssize_t]
Convertit un entier Python en un short int C.
c (bytes ou bytearray of length 1) [char]

Convertit un octet Python, représenté comme un objet bytes ou bytearray de longueur 1, en un char C.

Modifié dans la version 3.3: Allow bytearray objects.

C (str of length 1) [int]
Convertit un caractère Python, représenté comme un objet str de longueur 1, en un int C.
f (float) [float]
Convertit un nombre flottant Python vers un float.
d (float) [double]
Convertit un nombre flottant Python vers un double C.
D (complex) [Py_complex]
Convertit un nombre complexe Python vers une structure Py_complex C.

Autres Objets

O (object) [PyObject *]
Stocke un objet Python (sans aucune conversion) en un pointeur sur un objet C. Ainsi, Le programme C reçoit l’objet réel qui a été passé. Le compteur de référence sur l’objet n’est pas incrémenté. Le pointeur stocké n’est pas 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 the PyArg_Parse*() function. 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 and leave the content of address unmodified.

If the converter returns Py_CLEANUP_SUPPORTED, it may get called a second time if the argument parsing eventually fails, giving the converter a chance to release any memory that it had already allocated. In this second call, the object parameter will be NULL; address will have the same value as in the original call.

Modifié dans la version 3.1: Py_CLEANUP_SUPPORTED à été ajouté.

p (bool) [int]

Tests the value passed in for truth (a boolean predicate) and converts the result to its equivalent C true/false integer value. Sets the int to 1 if the expression was true and 0 if it was false. This accepts any valid Python value. See Valeurs booléennes for more information about how Python tests values for truth.

Nouveau dans la version 3.3.

(items) (tuple) [matching-items]
L’objet doit être une séquence Python dont la longueur est le nombre d’unités de formats dans articles. Les arguments C doivent correspondre à chaque unité de format particulière dans articles. Les unités de formats pour les séquences peuvent être imbriquées.

It is possible to pass « long » integers (integers whose value exceeds the platform’s LONG_MAX) 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).

Quelques autres caractères ont un sens dans une chaîne de format. On ne doit pas les trouvées dans des parenthèses imbriquées. Ce sont :

|
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).
$

PyArg_ParseTupleAndKeywords() only: Indicates that the remaining arguments in the Python argument list are keyword-only. Currently, all keyword-only arguments must also be optional arguments, so | must always be specified before $ in the format string.

Nouveau dans la version 3.3.

:
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).
;
La liste des unités de format s’arrête ici ; la chaîne après le point-virgule est utilise comme message d’erreur au lieu du message d’erreur par défaut. : et ; sont mutuellement exclusifs.

Notez que n’importe quelles références sur un objet Python qui sont données à l’appelant sont des références empruntées ; ne décrémentez pas leur compteur de références !

Les arguments additionnels qui sont donnés à ces fonctions doivent être des adresses de variables dont le type est déterminé par la chaîine de format. Elles sont utilisées pour stocker les valeurs du n-uplet d’entrée. Il y a quelques cas, comme décrit précédemment dans le liste des unités de formats, où ces paramètres sont utilisés comme valeurs d’entrée. Dans ce cas, ils devraient correspondre à ce qui est spécifié pour l’unité de format correspondante.

For the conversion to succeed, the arg object must match the format and the format must be exhausted. On success, the PyArg_Parse*() functions return true, otherwise they return false and raise an appropriate exception. When the PyArg_Parse*() functions fail due to conversion failure in one of the format units, the variables at the addresses corresponding to that and the following format units are left untouched.

Fonction de l’API

int PyArg_ParseTuple(PyObject *args, const char *format, ...)

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.

int PyArg_VaParse(PyObject *args, const char *format, va_list vargs)

Identical to PyArg_ParseTuple(), except that it accepts a va_list rather than a variable number of arguments.

int PyArg_ParseTupleAndKeywords(PyObject *args, PyObject *kw, const char *format, char *keywords[], ...)

Parse the parameters of a function that takes both positional and keyword parameters into local variables. The keywords argument is a NULL-terminated array of keyword parameter names. Empty names denote positional-only parameters. Returns true on success; on failure, it returns false and raises the appropriate exception.

Modifié dans la version 3.6: Added support for positional-only parameters.

int PyArg_VaParseTupleAndKeywords(PyObject *args, PyObject *kw, const char *format, char *keywords[], va_list vargs)

Identical to PyArg_ParseTupleAndKeywords(), except that it accepts a va_list rather than a variable number of arguments.

int PyArg_ValidateKeywordArguments(PyObject *)

Ensure that the keys in the keywords argument dictionary are strings. This is only needed if PyArg_ParseTupleAndKeywords() is not used, since the latter already does this check.

Nouveau dans la version 3.2.

int PyArg_Parse(PyObject *args, const char *format, ...)

Function used to deconstruct the argument lists of « old-style » functions — these are functions which use the METH_OLDARGS parameter parsing method, which has been removed in Python 3. This is not recommended for use in parameter parsing in new code, and most code in the standard interpreter has been modified to no longer use this for that purpose. It does remain a convenient way to decompose other tuples, however, and may continue to be used for that purpose.

int PyArg_UnpackTuple(PyObject *args, const char *name, Py_ssize_t min, Py_ssize_t max, ...)

A simpler form of parameter retrieval which does not use a format string to specify the types of the arguments. Functions which use this method to retrieve their parameters should be declared as METH_VARARGS in function or method tables. The tuple containing the actual parameters should be passed as args; it must actually be a tuple. The length of the tuple must be at least min and no more than max; min and max may be equal. Additional arguments must be passed to the function, each of which should be a pointer to a PyObject* variable; these will be filled in with the values from args; they will contain borrowed references. The variables which correspond to optional parameters not given by args will not be filled in; these should be initialized by the caller. This function returns true on success and false if args is not a tuple or contains the wrong number of elements; an exception will be set if there was a failure.

This is an example of the use of this function, taken from the sources for the _weakref helper module for weak references:

static PyObject *
weakref_ref(PyObject *self, PyObject *args)
{
    PyObject *object;
    PyObject *callback = NULL;
    PyObject *result = NULL;

    if (PyArg_UnpackTuple(args, "ref", 1, 2, &object, &callback)) {
        result = PyWeakref_NewRef(object, callback);
    }
    return result;
}

The call to PyArg_UnpackTuple() in this example is entirely equivalent to this call to PyArg_ParseTuple():

PyArg_ParseTuple(args, "O|O:ref", &object, &callback)

Construction des valeurs

PyObject* Py_BuildValue(const char *format, ...)
Return value: New reference.

Create a new value based on a format string similar to those accepted by the 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.

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 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 s and s# formats, the required data is copied. Buffers provided by the caller are never referenced by the objects created by Py_BuildValue(). In other words, if your code invokes malloc() and passes the allocated memory to Py_BuildValue(), your code is responsible for calling free() for that memory once 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 s#). This can be used to make long format strings a tad more readable.

s (str ou None) [char *]
Convert a null-terminated C string to a Python str object using 'utf-8' encoding. If the C string pointer is NULL, None is used.
s# (str ou None) [char *, int]
Convert a C string and its length to a Python str object using 'utf-8' encoding. If the C string pointer is NULL, the length is ignored and None is returned.
y (bytes) [char *]
This converts a C string to a Python bytes object. If the C string pointer is NULL, None is returned.
y# (bytes) [char *, int]
This converts a C string and its lengths to a Python object. If the C string pointer is NULL, None is returned.
z (str ou None) [char *]
Same as s.
z# (str ou None) [char *, int]
Same as s#.
u (str) [Py_UNICODE *]
Convert a null-terminated buffer of Unicode (UCS-2 or UCS-4) data to a Python Unicode object. If the Unicode buffer pointer is NULL, None is returned.
u# (str) [Py_UNICODE *, int]
Convert a Unicode (UCS-2 or UCS-4) data buffer and its length to a Python Unicode object. If the Unicode buffer pointer is NULL, the length is ignored and None is returned.
U (str ou None) [char *]
Same as s.
U# (str ou None) [char *, int]
Same as s#.
i (int) [int]
Convert a plain C int to a Python integer object.
b (int) [char]
Convert a plain C char to a Python integer object.
h (int) [short int]
Convert a plain C short int to a Python integer object.
l (int) [long int]
Convertit un long int en un int Python.
B (int) [unsigned char]
Convert a C unsigned char to a Python integer object.
H (int) [unsigned short int]
Convert a C unsigned short int to a Python integer object.
I (int) [unsigned int]
Convert a C unsigned int to a Python integer object.
k (int) [unsigned long]
Convert a C unsigned long to a Python integer object.
L (int) [long long]
Convert a C long long to a Python integer object.
K (int) [unsigned long long]
Convert a C unsigned long long to a Python integer object.
n (int) [Py_ssize_t]
Convert a C Py_ssize_t to a Python integer.
c (bytes de taille 1) [char]
Convert a C int representing a byte to a Python bytes object of length 1.
C (str of length 1) [int]
Convert a C int representing a character to Python str object of length 1.
d (float) [double]
Convert a C double to a Python floating point number.
f (float) [float]
Convert a C float to a Python floating point number.
D (complex) [Py_complex *]
Convert a C Py_complex structure to a Python complex number.
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, Py_BuildValue() will return NULL but won’t raise an exception. If no exception has been raised yet, SystemError is set.
S (object) [PyObject *]
Same as O.
N (object) [PyObject *]
Same as 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.
O& (object) [converter, anything]
Convert anything to a Python object through a converter function. The function is called with anything (which should be compatible with void *) as its argument and should return a « new » Python object, or NULL if an error occurred.
(items) (tuple) [matching-items]
Convert a sequence of C values to a Python tuple with the same number of items.
[items] (list) [matching-items]
Convert a sequence of C values to a Python list with the same number of items.
{items} (dict) [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.

If there is an error in the format string, the SystemError exception is set and NULL returned.

PyObject* Py_VaBuildValue(const char *format, va_list vargs)

Identical to Py_BuildValue(), except that it accepts a va_list rather than a variable number of arguments.