Edit File: buffer.txt
.. highlightlang:: c .. _bufferobjects: Buffers and Memoryview Objects ------------------------------ .. sectionauthor:: Greg Stein <gstein@lyra.org> .. sectionauthor:: Benjamin Peterson .. index:: object: buffer single: buffer interface Python objects implemented in C can export a group of functions called the "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 strings and arrays. The string 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 :meth:`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 :c:func:`PyArg_ParseTuple` that operate against an object's buffer interface, returning data from the target object. 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 :class:`memoryview`. The new buffer API has been backported to Python 2.6, and the :class:`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 ============================== .. c:type:: Py_buffer .. c:member:: void *buf A pointer to the start of the memory for the object. .. c:member:: Py_ssize_t len :noindex: The total length of the memory in bytes. .. c:member:: int readonly An indicator of whether the buffer is read only. .. c:member:: const char *format :noindex: A *NULL* terminated string in :mod:`struct` module style syntax giving the contents of the elements available through the buffer. If this is *NULL*, ``"B"`` (unsigned bytes) is assumed. .. c:member:: int ndim The number of dimensions the memory represents as a multi-dimensional array. If it is 0, :c:data:`strides` and :c:data:`suboffsets` must be *NULL*. .. c:member:: Py_ssize_t *shape An array of :c:type:`Py_ssize_t`\s the length of :c:data:`ndim` giving the shape of the memory as a multi-dimensional array. Note that ``((*shape)[0] * ... * (*shape)[ndims-1])*itemsize`` should be equal to :c:data:`len`. .. c:member:: Py_ssize_t *strides An array of :c:type:`Py_ssize_t`\s the length of :c:data:`ndim` giving the number of bytes to skip to get to a new element in each dimension. .. c:member:: Py_ssize_t *suboffsets An array of :c:type:`Py_ssize_t`\s the length of :c:data:`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-dimesional 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; } .. c:member:: 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 :c:func:`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. .. c:member:: 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 ======================== .. c:function:: int PyObject_CheckBuffer(PyObject *obj) Return 1 if *obj* supports the buffer interface otherwise 0. .. c:function:: int PyObject_GetBuffer(PyObject *obj, Py_buffer *view, int flags) Export *obj* into a :c:type:`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 complexity 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 :exc:`BufferError` unless there is another error that is actually causing the problem. The exporter can use flags information to simplify how much of the :c:data:`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 | +===============================+===================================================+ | :c:macro:`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. | | | | +-------------------------------+---------------------------------------------------+ | :c:macro:`PyBUF_WRITABLE` | The returned buffer must be writable. If it is | | | not writable, then raise an error. | +-------------------------------+---------------------------------------------------+ | :c:macro:`PyBUF_STRIDES` | This implies :c:macro:`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). | | | | +-------------------------------+---------------------------------------------------+ | :c:macro:`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*. | | | | | | | | | | +-------------------------------+---------------------------------------------------+ |:c:macro:`PyBUF_C_CONTIGUOUS` | These flags indicate that the contiguity returned | |:c:macro:`PyBUF_F_CONTIGUOUS` | buffer must be respectively, C-contiguous (last | |:c:macro:`PyBUF_ANY_CONTIGUOUS`| dimension varies the fastest), Fortran contiguous | | | (first dimension varies the fastest) or either | | | one. All of these flags imply | | | :c:macro:`PyBUF_STRIDES` and guarantee that the | | | strides buffer info structure will be filled in | | | correctly. | | | | +-------------------------------+---------------------------------------------------+ | :c:macro:`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 :c:macro:`PyBUF_STRIDES`. | | | | | | | | | | +-------------------------------+---------------------------------------------------+ | :c:macro:`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) | +-------------------------------+---------------------------------------------------+ | :c:macro:`PyBUF_STRIDED` | This is equivalent to ``(PyBUF_STRIDES | | | | PyBUF_WRITABLE)``. | +-------------------------------+---------------------------------------------------+ | :c:macro:`PyBUF_STRIDED_RO` | This is equivalent to ``(PyBUF_STRIDES)``. | | | | +-------------------------------+---------------------------------------------------+ | :c:macro:`PyBUF_RECORDS` | This is equivalent to ``(PyBUF_STRIDES | | | | PyBUF_FORMAT | PyBUF_WRITABLE)``. | +-------------------------------+---------------------------------------------------+ | :c:macro:`PyBUF_RECORDS_RO` | This is equivalent to ``(PyBUF_STRIDES | | | | PyBUF_FORMAT)``. | +-------------------------------+---------------------------------------------------+ | :c:macro:`PyBUF_FULL` | This is equivalent to ``(PyBUF_INDIRECT | | | | PyBUF_FORMAT | PyBUF_WRITABLE)``. | +-------------------------------+---------------------------------------------------+ | :c:macro:`PyBUF_FULL_RO` | This is equivalent to ``(PyBUF_INDIRECT | | | | PyBUF_FORMAT)``. | +-------------------------------+---------------------------------------------------+ | :c:macro:`PyBUF_CONTIG` | This is equivalent to ``(PyBUF_ND | | | | PyBUF_WRITABLE)``. | +-------------------------------+---------------------------------------------------+ | :c:macro:`PyBUF_CONTIG_RO` | This is equivalent to ``(PyBUF_ND)``. | | | | +-------------------------------+---------------------------------------------------+ .. c:function:: void PyBuffer_Release(Py_buffer *view) Release the buffer *view*. This should be called when the buffer is no longer being used as it may free memory from it. .. c:function:: Py_ssize_t PyBuffer_SizeFromFormat(const char *) Return the implied :c:data:`~Py_buffer.itemsize` from the struct-stype :c:data:`~Py_buffer.format`. .. c:function:: 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. .. c:function:: 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. .. c:function:: int PyBuffer_FillInfo(Py_buffer *view, PyObject *obj, 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 ================== .. versionadded:: 2.7 A :class:`memoryview` object exposes the new C level buffer interface as a Python object which can then be passed around like any other object. .. c:function:: PyObject *PyMemoryView_FromObject(PyObject *obj) Create a memoryview object from an object that defines the new buffer interface. .. c:function:: 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. .. c:function:: 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. .. c:function:: int PyMemoryView_Check(PyObject *obj) Return true if the object *obj* is a memoryview object. It is not currently allowed to create subclasses of :class:`memoryview`. .. c:function:: 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. Old-style buffer objects ======================== .. index:: single: PyBufferProcs More information on the old buffer interface is provided in the section :ref:`buffer-structs`, under the description for :c:type:`PyBufferProcs`. A "buffer object" is defined in the :file:`bufferobject.h` header (included by :file:`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. .. c:type:: PyBufferObject This subtype of :c:type:`PyObject` represents a buffer object. .. c:var:: PyTypeObject PyBuffer_Type .. index:: single: BufferType (in module types) The instance of :c:type:`PyTypeObject` which represents the Python buffer type; it is the same object as ``buffer`` and ``types.BufferType`` in the Python layer. . .. c:var:: int Py_END_OF_BUFFER This constant may be passed as the *size* parameter to :c:func:`PyBuffer_FromObject` or :c:func:`PyBuffer_FromReadWriteObject`. It indicates that the new :c:type:`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. .. c:function:: int PyBuffer_Check(PyObject *p) Return true if the argument has type :c:data:`PyBuffer_Type`. .. c:function:: PyObject* PyBuffer_FromObject(PyObject *base, Py_ssize_t offset, Py_ssize_t size) Return a new read-only buffer object. This raises :exc:`TypeError` if *base* doesn't support the read-only buffer protocol or doesn't provide exactly one buffer segment, or it raises :exc:`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 :const:`Py_END_OF_BUFFER`, then the new buffer's contents extend to the length of the *base* object's exported buffer data. .. versionchanged:: 2.5 This function used an :c:type:`int` type for *offset* and *size*. This might require changes in your code for properly supporting 64-bit systems. .. c:function:: PyObject* PyBuffer_FromReadWriteObject(PyObject *base, Py_ssize_t offset, Py_ssize_t size) Return a new writable buffer object. Parameters and exceptions are similar to those for :c:func:`PyBuffer_FromObject`. If the *base* object does not export the writeable buffer protocol, then :exc:`TypeError` is raised. .. versionchanged:: 2.5 This function used an :c:type:`int` type for *offset* and *size*. This might require changes in your code for properly supporting 64-bit systems. .. c:function:: PyObject* PyBuffer_FromMemory(void *ptr, Py_ssize_t size) 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 :exc:`ValueError` if *size* is less than zero. Note that :const:`Py_END_OF_BUFFER` may *not* be passed for the *size* parameter; :exc:`ValueError` will be raised in that case. .. versionchanged:: 2.5 This function used an :c:type:`int` type for *size*. This might require changes in your code for properly supporting 64-bit systems. .. c:function:: PyObject* PyBuffer_FromReadWriteMemory(void *ptr, Py_ssize_t size) Similar to :c:func:`PyBuffer_FromMemory`, but the returned buffer is writable. .. versionchanged:: 2.5 This function used an :c:type:`int` type for *size*. This might require changes in your code for properly supporting 64-bit systems. .. c:function:: PyObject* PyBuffer_New(Py_ssize_t size) Return a new writable buffer object that maintains its own memory buffer of *size* bytes. :exc:`ValueError` is returned if *size* is not zero or positive. Note that the memory buffer (as returned by :c:func:`PyObject_AsWriteBuffer`) is not specifically aligned. .. versionchanged:: 2.5 This function used an :c:type:`int` type for *size*. This might require changes in your code for properly supporting 64-bit systems.