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: Extension et intégration de l’interpréteur Python.

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.

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.

Ces formats permettent d’accéder à un objet dans un espace mémoire contigu. Vous n’avez pas à fournir un stockage pour les chaînes ou octets renvoyés. Alors, vous n’aurez pas à libérer la mémoire vous-même, sauf pour les formats es, es#, et et et#.

s (string or Unicode) [const 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 TypeError exception is raised. Unicode objects are converted to C strings using the default encoding. If this conversion fails, a UnicodeError is raised.

s# (string, Unicode or any read buffer compatible object) [const char *, int (or Py_ssize_t, see below)]

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.

Starting with Python 2.5 the type of the length argument can be controlled by defining the macro PY_SSIZE_T_CLEAN before including Python.h. If the macro is defined, length is a Py_ssize_t rather than an int.

s* (string, Unicode, or any buffer compatible object) [Py_buffer]

Similar to s#, this code fills a Py_buffer structure provided by the caller. The buffer gets locked, so that the caller can subsequently use the buffer even inside a Py_BEGIN_ALLOW_THREADS block; the caller is responsible for calling PyBuffer_Release with the structure after it has processed the data.

Nouveau dans la version 2.6.

z (string, Unicode or None) [const char *]

Comme s, mais l’objet Python peut aussi être None, auquel cas le pointeur C devient NULL.

z# (string, Unicode, None or any read buffer compatible object) [const char *, int]

This is to s# as z is to s.

z* (string, Unicode, None or any buffer compatible object) [Py_buffer]

This is to s* as z is to s.

Nouveau dans la version 2.6.

u (Unicode) [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 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) [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. Non-Unicode objects are handled by interpreting their read-buffer pointer as pointer to a Py_UNICODE array.

es (string, Unicode 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 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 the default 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 (string, Unicode or character buffer compatible object) [const char *encoding, char **buffer]

Same as 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.

es# (string, Unicode 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. 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 the default 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 TypeError will be set. Note: starting from Python 3.6 a ValueError will be set.

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

et# (string, Unicode or character buffer compatible object) [const char *encoding, char **buffer, int *buffer_length]

Same as 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.

b (integer) [unsigned char]

Convertit un entier Python positif ou nul en un unsigned tiny int, stocké dans un unsigned char C.

B (integer) [unsigned char]

Convertit un entier Python en un tiny int sans vérifier le débordement, stocké dans un unsigned char C.

Nouveau dans la version 2.3.

h (integer) [short int]

Convertit un entier Python en un short int C.

H (integer) [unsigned short int]

Convertit un entier Python en un unsigned short int C, sans contrôle de débordement.

Nouveau dans la version 2.3.

i (integer) [int]

Convertit un entier Python en un int C.

I (integer) [unsigned int]

Convertit un entier Python en un unsigned int C, sans contrôle de le débordement.

Nouveau dans la version 2.3.

l (integer) [long int]

Convertit un entier Python en un long int.

k (integer) [unsigned long]

Convertit un entier Python en un type C unsigned long sans en vérifier le débordement.

Nouveau dans la version 2.3.

L (integer) [PY_LONG_LONG]

Convertit un entier Python en un long long C. Ce format est uniquement disponible sur les plates-formes qui prennent en charge long long (ou _int64 sous Windows).

K (integer) [unsigned PY_LONG_LONG]

Convert a Python integer or long integer to a C unsigned long long without overflow checking. This format is only available on platforms that support unsigned long long (or unsigned _int64 on Windows).

Nouveau dans la version 2.3.

n (integer) [Py_ssize_t]

Convertit un entier Python en un type C Py_ssize_t.

Nouveau dans la version 2.5.

c (string of length 1) [char]

Convertit un caractère Python, représenté comme une chaîne de longneur 1 en un type C int.

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.

O (objet) [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! (objet) [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& (objet) [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.

S (string) [PyStringObject *]

Exige que l’objet Python soit un objet Unicode, sans tenter aucune conversion. Lève une TypeError si l’objet n’est pas un objet Unicode. La variable C peut également être déclarée en tant que PyObject*.

U (Unicode string) [PyUnicodeObject *]

Exige que l’objet Python soit un objet Unicode, sans tenter aucune conversion. Lève une TypeError si l’objet n’est pas un objet Unicode. La variable C peut également être déclarée en tant que 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 *, Py_ssize_t]

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 Py_ssize_t is set to the length of the buffer. Only single-segment buffer objects are accepted; TypeError is raised for all others.

w* (read-write byte-oriented buffer) [Py_buffer]

This is to w what s* is to s.

Nouveau dans la version 2.6.

(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.

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).

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).
:: 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 chaîne 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.

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. Returns true on success; on failure, it returns false and raises the appropriate exception.

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_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. 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)

Nouveau dans la version 2.2.

Modifié dans la version 2.5: This function used an int type for min and max. This might require changes in your code for properly supporting 64-bit systems.

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 (string) [char *]: Convert a null-terminated C string to a Python object. If the C string pointer is NULL, None is used.
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 None is returned.
z (string or None) [char *]: Same as s.
z# (string or None) [char *, int]: Same as s#.
u (Unicode string) [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# (Unicode string) [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.
i (integer) [int]: Convert a plain C int to a Python integer object.
b (integer) [char]: Convert a plain C char to a Python integer object.
h (integer) [short int]: Convert a plain C short int to a Python integer object.
l (integer) [long int]: Convertit un long int en un int Python.
B (integer) [unsigned char]: Convert a C unsigned char to a Python integer object.
H (integer) [unsigned short int]: Convert a C unsigned short int to a Python integer object.
I (integer/long) [unsigned int]: Convert a C unsigned int to a Python integer object or a Python long integer object, if it is larger than sys.maxint.
k (integer/long) [unsigned long]: Convert a C unsigned long to a Python integer object or a Python long integer object, if it is larger than sys.maxint.
L (long) [PY_LONG_LONG]: Convert a C long long to a Python long integer object. Only available on platforms that support long long.
K (long) [unsigned PY_LONG_LONG]: Convert a C unsigned long long to a Python long integer object. Only available on platforms that support unsigned long long.
n (int) [Py_ssize_t]: Convert a C Py_ssize_t to a Python integer or long integer.

Nouveau dans la version 2.5.
c (string of length 1) [char]: Convert a C int representing a character to a Python string of length 1.
d (float) [double]: Convert a C double to a Python floating point number.
f (float) [float]: Same as d.
D (complex) [Py_complex *]: Convert a C Py_complex structure to a Python complex number.
O (objet) [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 (objet) [PyObject *]: Same as O.
N (objet) [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& (objet) [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} (dictionary) [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.

Analyse des arguments et construction des valeurs¶

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