Buffer Objects

Python objects implemented in C can export a “buffer interface.” These functions can be used by an object to expose its data in a raw, byte-oriented format. Clients of the object can use the buffer interface to access the object data directly, without needing to copy it first.

Two examples of objects that support the buffer interface are bytes and arrays. The bytes object exposes the character contents in the buffer interface’s byte-oriented form. An array can also expose its contents, but it should be noted that array elements may be multi-byte values.

An example user of the buffer interface is the file object’s write() method. Any object that can export a series of bytes through the buffer interface can be written to a file. There are a number of format codes to PyArg_ParseTuple that operate against an object’s buffer interface, returning data from the target object.

More information on the buffer interface is provided in the section Buffer Object Structures, under the description for PyBufferProcs.

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.

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).

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.

Buffer related functions

int PyObject_CheckBuffer(PyObject *obj)

Return 1 if obj supports the buffer interface otherwise 0.

int PyObject_GetBuffer(PyObject *obj, Py_buffer *view, int flags)

Export obj into a Py_buffer, view. These arguments must never be NULL. The flags argument is a bit field indicating what kind of buffer the caller is prepared to deal with and therefore what kind of buffer the exporter is allowed to return. The buffer interface allows for complicated memory sharing possibilities, but some caller may not be able to handle all the complexibity but may want to see if the exporter will let them take a simpler view to its memory.

Some exporters may not be able to share memory in every possible way and may need to raise errors to signal to some consumers that something is just not possible. These errors should be a BufferError unless there is another error that is actually causing the problem. The exporter can use flags information to simplify how much of the Py_buffer structure is filled in with non-default values and/or raise an error if the object can’t support a simpler view of its memory.

0 is returned on success and -1 on error.

The following table gives possible values to the flags arguments.

Flag	Description
PyBUF_SIMPLE	This is the default flag state. The returned buffer may or may not have writable memory. The format of the data will be assumed to be unsigned bytes. This is a “stand-alone” flag constant. It never needs to be ‘|’d to the others. The exporter will raise an error if it cannot provide such a contiguous buffer of bytes.
PyBUF_WRITABLE	The returned buffer must be writable. If it is not writable, then raise an error.
PyBUF_STRIDES	This implies PyBUF_ND. The returned buffer must provide strides information (i.e. the strides cannot be NULL). This would be used when the consumer can handle strided, discontiguous arrays. Handling strides automatically assumes you can handle shape. The exporter can raise an error if a strided representation of the data is not possible (i.e. without the suboffsets).
PyBUF_ND	The returned buffer must provide shape information. The memory will be assumed C-style contiguous (last dimension varies the fastest). The exporter may raise an error if it cannot provide this kind of contiguous buffer. If this is not given then shape will be NULL.
PyBUF_C_CONTIGUOUS PyBUF_F_CONTIGUOUS PyBUF_ANY_CONTIGUOUS	These flags indicate that the contiguity returned buffer must be respectively, C-contiguous (last dimension varies the fastest), Fortran contiguous (first dimension varies the fastest) or either one. All of these flags imply PyBUF_STRIDES and guarantee that the strides buffer info structure will be filled in correctly.
PyBUF_INDIRECT	This flag indicates the returned buffer must have suboffsets information (which can be NULL if no suboffsets are needed). This can be used when the consumer can handle indirect array referencing implied by these suboffsets. This implies PyBUF_STRIDES.
PyBUF_FORMAT	The returned buffer must have true format information if this flag is provided. This would be used when the consumer is going to be checking for what ‘kind’ of data is actually stored. An exporter should always be able to provide this information if requested. If format is not explicitly requested then the format must be returned as NULL (which means 'B', or unsigned bytes)
PyBUF_STRIDED	This is equivalent to (PyBUF_STRIDES | PyBUF_WRITABLE).
PyBUF_STRIDED_RO	This is equivalent to (PyBUF_STRIDES).
PyBUF_RECORDS	This is equivalent to (PyBUF_STRIDES | PyBUF_FORMAT | PyBUF_WRITABLE).
PyBUF_RECORDS_RO	This is equivalent to (PyBUF_STRIDES | PyBUF_FORMAT).
PyBUF_FULL	This is equivalent to (PyBUF_INDIRECT | PyBUF_FORMAT | PyBUF_WRITABLE).
PyBUF_FULL_RO	This is equivalent to (PyBUF_INDIRECT | PyBUF_FORMAT).
PyBUF_CONTIG	This is equivalent to (PyBUF_ND | PyBUF_WRITABLE).
PyBUF_CONTIG_RO	This is equivalent to (PyBUF_ND).

void PyBuffer_Release(PyObject *obj, Py_buffer *view)

Release the buffer view over obj. This should be called when the buffer is no longer being used as it may free memory from it.

Py_ssize_t PyBuffer_SizeFromFormat(const char *)

Return the implied ~Py_buffer.itemsize from the struct-stype ~Py_buffer.format.

int PyObject_CopyToObject(PyObject *obj, void *buf, Py_ssize_t len, char fortran)

Copy len bytes of data pointed to by the contiguous chunk of memory pointed to by buf into the buffer exported by obj. The buffer must of course be writable. Return 0 on success and return -1 and raise an error on failure. If the object does not have a writable buffer, then an error is raised. If fortran is 'F', then if the object is multi-dimensional, then the data will be copied into the array in Fortran-style (first dimension varies the fastest). If fortran is 'C', then the data will be copied into the array in C-style (last dimension varies the fastest). If fortran is 'A', then it does not matter and the copy will be made in whatever way is more efficient.

int PyBuffer_IsContiguous(Py_buffer *view, char fortran)

Return 1 if the memory defined by the view is C-style (fortran is 'C') or Fortran-style (fortran is 'F') contiguous or either one (fortran is 'A'). Return 0 otherwise.

void PyBuffer_FillContiguousStrides(int ndim, Py_ssize_t *shape, Py_ssize_t *strides, Py_ssize_t itemsize, char fortran)

Fill the strides array with byte-strides of a contiguous (C-style if fortran is 'C' or Fortran-style if fortran is 'F' array of the given shape with the given number of bytes per element.

int PyBuffer_FillInfo(Py_buffer *view, void *buf, Py_ssize_t len, int readonly, int infoflags)

Fill in a buffer-info structure, view, correctly for an exporter that can only share a contiguous chunk of memory of “unsigned bytes” of the given length. Return 0 on success and -1 (with raising an error) on error.

MemoryView objects

A memoryview object is an extended buffer object that could replace the buffer object (but doesn’t have to as that could be kept as a simple 1-d memoryview object). It, unlike Py_buffer, is a Python object (exposed as memoryview in builtins), so it can be used with Python code.

PyObject* PyMemoryView_FromObject(PyObject *obj)

Return a memoryview object from an object that defines the buffer interface.