Objets de type MemoryView

Les objets Python implémentés en C peuvent exporter une « interface sur des tampons ». Ces fonctions peuvent être utilisées par un objets pour rendre publiques ses données, dans un format brut orienté octets. Les clients de ces objets peuvent utiliser l’interface sur les tampons pour accéder directement aux données de l’objet, sans nécessiter une copie préalable.

Deux exemples d’objets qui supportent l’interfaces sur les tampons sont les octets et les tableaux. Les objets octets exposent leur contenu en tant que caractètres, dans une interface sur tampon orientée octets. Un tableau peut également exposer son contenu, mais il doit être remarqué que les éléments du tableau peuvent être des valeurs multi-octets.

Un exemple d’utilisation de l’interface sur les tampons est la méthode write() de l’objet fichier. Tout objet qui peut exporter une série d’octets en utilisant l’interface tampons peut être écrit dans un fichier. Il y a un nombre de codes de format pour PyArg_ParseTuple() qui contredisent l’interface tampon de l’objet, en retournant les données de l’objet cible.

Starting from version 1.6, Python has been providing Python-level buffer objects and a C-level buffer API so that any built-in or used-defined type can expose its characteristics. Both, however, have been deprecated because of various shortcomings, and have been officially removed in Python 3 in favour of a new C-level buffer API and a new Python-level object named memoryview.

The new buffer API has been backported to Python 2.6, and the memoryview object has been backported to Python 2.7. It is strongly advised to use them rather than the old APIs, unless you are blocked from doing so for compatibility reasons.

The new-style Py_buffer struct

Py_buffer
void *buf

A pointer to the start of the memory for the object.

Py_ssize_t len

The total length of the memory in bytes.

int readonly

An indicator of whether the buffer is read only.

const char *format

A NULL terminated string in struct module style syntax giving the contents of the elements available through the buffer. If this is NULL, "B" (unsigned bytes) is assumed.

int ndim

The number of dimensions the memory represents as a multi-dimensional array. If it is 0, strides and suboffsets must be NULL.

Py_ssize_t *shape

An array of Py_ssize_ts the length of ndim giving the shape of the memory as a multi-dimensional array. Note that ((*shape)[0] * ... * (*shape)[ndims-1])*itemsize should be equal to len.

Py_ssize_t *strides

An array of Py_ssize_ts the length of ndim giving the number of bytes to skip to get to a new element in each dimension.

Py_ssize_t *suboffsets

An array of Py_ssize_ts the length of ndim. If these suboffset numbers are greater than or equal to 0, then the value stored along the indicated dimension is a pointer and the suboffset value dictates how many bytes to add to the pointer after de-referencing. A suboffset value that it negative indicates that no de-referencing should occur (striding in a contiguous memory block).

If all suboffsets are negative (i.e. no de-referencing is needed, then this field must be NULL (the default value).

Here is a function that returns a pointer to the element in an N-D array pointed to by an N-dimensional index when there are both non-NULL strides and suboffsets:

void *get_item_pointer(int ndim, void *buf, Py_ssize_t *strides,
    Py_ssize_t *suboffsets, Py_ssize_t *indices) {
    char *pointer = (char*)buf;
    int i;
    for (i = 0; i < ndim; i++) {
        pointer += strides[i] * indices[i];
        if (suboffsets[i] >=0 ) {
            pointer = *((char**)pointer) + suboffsets[i];
        }
    }
    return (void*)pointer;
 }
Py_ssize_t itemsize

This is a storage for the itemsize (in bytes) of each element of the shared memory. It is technically un-necessary as it can be obtained using PyBuffer_SizeFromFormat(), however an exporter may know this information without parsing the format string and it is necessary to know the itemsize for proper interpretation of striding. Therefore, storing it is more convenient and faster.

void *internal

This is for use internally by the exporting object. For example, this might be re-cast as an integer by the exporter and used to store flags about whether or not the shape, strides, and suboffsets arrays must be freed when the buffer is released. The consumer should never alter this value.

Objets de type MemoryView

Nouveau dans la version 2.7.

A memoryview object exposes the new C level buffer interface as a Python object which can then be passed around like any other object.

PyObject *PyMemoryView_FromObject(PyObject *obj)

Un objet MemoryView expose l’interface tampon au niveau C à Python.

PyObject *PyMemoryView_FromBuffer(Py_buffer *view)

Create a memoryview object wrapping the given buffer-info structure view. The memoryview object then owns the buffer, which means you shouldn’t try to release it yourself: it will be released on deallocation of the memoryview object.

PyObject *PyMemoryView_GetContiguous(PyObject *obj, int buffertype, char order)

Create a memoryview object to a contiguous chunk of memory (in either “C” or “F’ortran order) from an object that defines the buffer interface. If memory is contiguous, the memoryview object points to the original memory. Otherwise copy is made and the memoryview points to a new bytes object.

int PyMemoryView_Check(PyObject *obj)

Return true if the object obj is a memoryview object. It is not currently allowed to create subclasses of memoryview.

Py_buffer *PyMemoryView_GET_BUFFER(PyObject *obj)

Return a pointer to the buffer-info structure wrapped by the given object. The object must be a memoryview instance; this macro doesn’t check its type, you must do it yourself or you will risk crashes.

Autres objets

Plus d’informations sur l’interface sur les tampons sont données dans la section Buffer Object Structures, dans la description sur PyBufferProcs.

A « buffer object » is defined in the bufferobject.h header (included by Python.h). These objects look very similar to string objects at the Python programming level: they support slicing, indexing, concatenation, and some other standard string operations. However, their data can come from one of two sources: from a block of memory, or from another object which exports the buffer interface.

Buffer objects are useful as a way to expose the data from another object’s buffer interface to the Python programmer. They can also be used as a zero-copy slicing mechanism. Using their ability to reference a block of memory, it is possible to expose any data to the Python programmer quite easily. The memory could be a large, constant array in a C extension, it could be a raw block of memory for manipulation before passing to an operating system library, or it could be used to pass around structured data in its native, in-memory format.

PyBufferObject

This subtype of PyObject represents a buffer object.

PyTypeObject PyBuffer_Type

The instance of PyTypeObject which represents the Python buffer type; it is the same object as buffer and types.BufferType in the Python layer. .

int Py_END_OF_BUFFER

This constant may be passed as the size parameter to PyBuffer_FromObject() or PyBuffer_FromReadWriteObject(). It indicates that the new PyBufferObject should refer to base object from the specified offset to the end of its exported buffer. Using this enables the caller to avoid querying the base object for its length.

int PyBuffer_Check(PyObject *p)

Return true if the argument has type PyBuffer_Type.

PyObject* PyBuffer_FromObject(PyObject *base, Py_ssize_t offset, Py_ssize_t size)
Return value: New reference.

Return a new read-only buffer object. This raises TypeError if base doesn’t support the read-only buffer protocol or doesn’t provide exactly one buffer segment, or it raises ValueError if offset is less than zero. The buffer will hold a reference to the base object, and the buffer’s contents will refer to the base object’s buffer interface, starting as position offset and extending for size bytes. If size is Py_END_OF_BUFFER, then the new buffer’s contents extend to the length of the base object’s exported buffer data.

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

PyObject* PyBuffer_FromReadWriteObject(PyObject *base, Py_ssize_t offset, Py_ssize_t size)
Return value: New reference.

Return a new writable buffer object. Parameters and exceptions are similar to those for PyBuffer_FromObject(). If the base object does not export the writeable buffer protocol, then TypeError is raised.

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

PyObject* PyBuffer_FromMemory(void *ptr, Py_ssize_t size)
Return value: New reference.

Return a new read-only buffer object that reads from a specified location in memory, with a specified size. The caller is responsible for ensuring that the memory buffer, passed in as ptr, is not deallocated while the returned buffer object exists. Raises ValueError if size is less than zero. Note that Py_END_OF_BUFFER may not be passed for the size parameter; ValueError will be raised in that case.

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

PyObject* PyBuffer_FromReadWriteMemory(void *ptr, Py_ssize_t size)
Return value: New reference.

Similar to PyBuffer_FromMemory(), but the returned buffer is writable.

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

PyObject* PyBuffer_New(Py_ssize_t size)
Return value: New reference.

Return a new writable buffer object that maintains its own memory buffer of size bytes. ValueError is returned if size is not zero or positive. Note that the memory buffer (as returned by PyObject_AsWriteBuffer()) is not specifically aligned.

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