• Home
  • Line#
  • Scopes#
  • Navigate#
  • Raw
  • Download
1.. highlightlang:: c
2
3.. _bufferobjects:
4
5Buffers and Memoryview Objects
6------------------------------
7
8.. sectionauthor:: Greg Stein <gstein@lyra.org>
9.. sectionauthor:: Benjamin Peterson
10
11
12.. index::
13   object: buffer
14   single: buffer interface
15
16Python objects implemented in C can export a group of functions called the
17"buffer interface."  These functions can be used by an object to expose its
18data in a raw, byte-oriented format. Clients of the object can use the buffer
19interface to access the object data directly, without needing to copy it
20first.
21
22Two examples of objects that support the buffer interface are strings and
23arrays. The string object exposes the character contents in the buffer
24interface's byte-oriented form. An array can only expose its contents via the
25old-style buffer interface. This limitation does not apply to Python 3,
26where :class:`memoryview` objects can be constructed from arrays, too.
27Array elements may be multi-byte values.
28
29An example user of the buffer interface is the file object's :meth:`write`
30method. Any object that can export a series of bytes through the buffer
31interface can be written to a file. There are a number of format codes to
32:c:func:`PyArg_ParseTuple` that operate against an object's buffer interface,
33returning data from the target object.
34
35Starting from version 1.6, Python has been providing Python-level buffer
36objects and a C-level buffer API so that any built-in or used-defined type can
37expose its characteristics. Both, however, have been deprecated because of
38various shortcomings, and have been officially removed in Python 3 in favour
39of a new C-level buffer API and a new Python-level object named
40:class:`memoryview`.
41
42The new buffer API has been backported to Python 2.6, and the
43:class:`memoryview` object has been backported to Python 2.7. It is strongly
44advised to use them rather than the old APIs, unless you are blocked from
45doing so for compatibility reasons.
46
47
48The new-style Py_buffer struct
49==============================
50
51
52.. c:type:: Py_buffer
53
54   .. c:member:: void *buf
55
56      A pointer to the start of the memory for the object.
57
58   .. c:member:: Py_ssize_t len
59      :noindex:
60
61      The total length of the memory in bytes.
62
63   .. c:member:: int readonly
64
65      An indicator of whether the buffer is read only.
66
67   .. c:member:: const char *format
68      :noindex:
69
70      A *NULL* terminated string in :mod:`struct` module style syntax giving
71      the contents of the elements available through the buffer.  If this is
72      *NULL*, ``"B"`` (unsigned bytes) is assumed.
73
74   .. c:member:: int ndim
75
76      The number of dimensions the memory represents as a multi-dimensional
77      array.  If it is ``0``, :c:data:`strides` and :c:data:`suboffsets` must be
78      *NULL*.
79
80   .. c:member:: Py_ssize_t *shape
81
82      An array of :c:type:`Py_ssize_t`\s the length of :c:data:`ndim` giving the
83      shape of the memory as a multi-dimensional array.  Note that
84      ``((*shape)[0] * ... * (*shape)[ndims-1])*itemsize`` should be equal to
85      :c:data:`len`.
86
87   .. c:member:: Py_ssize_t *strides
88
89      An array of :c:type:`Py_ssize_t`\s the length of :c:data:`ndim` giving the
90      number of bytes to skip to get to a new element in each dimension.
91
92   .. c:member:: Py_ssize_t *suboffsets
93
94      An array of :c:type:`Py_ssize_t`\s the length of :c:data:`ndim`.  If these
95      suboffset numbers are greater than or equal to 0, then the value stored
96      along the indicated dimension is a pointer and the suboffset value
97      dictates how many bytes to add to the pointer after de-referencing. A
98      suboffset value that it negative indicates that no de-referencing should
99      occur (striding in a contiguous memory block).
100
101      If all suboffsets are negative (i.e. no de-referencing is needed, then
102      this field must be NULL (the default value).
103
104      Here is a function that returns a pointer to the element in an N-D array
105      pointed to by an N-dimensional index when there are both non-NULL strides
106      and suboffsets::
107
108          void *get_item_pointer(int ndim, void *buf, Py_ssize_t *strides,
109              Py_ssize_t *suboffsets, Py_ssize_t *indices) {
110              char *pointer = (char*)buf;
111              int i;
112              for (i = 0; i < ndim; i++) {
113                  pointer += strides[i] * indices[i];
114                  if (suboffsets[i] >=0 ) {
115                      pointer = *((char**)pointer) + suboffsets[i];
116                  }
117              }
118              return (void*)pointer;
119           }
120
121
122   .. c:member:: Py_ssize_t itemsize
123
124      This is a storage for the itemsize (in bytes) of each element of the
125      shared memory. It is technically un-necessary as it can be obtained
126      using :c:func:`PyBuffer_SizeFromFormat`, however an exporter may know
127      this information without parsing the format string and it is necessary
128      to know the itemsize for proper interpretation of striding. Therefore,
129      storing it is more convenient and faster.
130
131   .. c:member:: void *internal
132
133      This is for use internally by the exporting object. For example, this
134      might be re-cast as an integer by the exporter and used to store flags
135      about whether or not the shape, strides, and suboffsets arrays must be
136      freed when the buffer is released. The consumer should never alter this
137      value.
138
139
140Buffer related functions
141========================
142
143
144.. c:function:: int PyObject_CheckBuffer(PyObject *obj)
145
146   Return ``1`` if *obj* supports the buffer interface otherwise ``0``.
147
148
149.. c:function:: int PyObject_GetBuffer(PyObject *obj, Py_buffer *view, int flags)
150
151      Export *obj* into a :c:type:`Py_buffer`, *view*.  These arguments must
152      never be *NULL*.  The *flags* argument is a bit field indicating what
153      kind of buffer the caller is prepared to deal with and therefore what
154      kind of buffer the exporter is allowed to return.  The buffer interface
155      allows for complicated memory sharing possibilities, but some caller may
156      not be able to handle all the complexity but may want to see if the
157      exporter will let them take a simpler view to its memory.
158
159      Some exporters may not be able to share memory in every possible way and
160      may need to raise errors to signal to some consumers that something is
161      just not possible. These errors should be a :exc:`BufferError` unless
162      there is another error that is actually causing the problem. The
163      exporter can use flags information to simplify how much of the
164      :c:data:`Py_buffer` structure is filled in with non-default values and/or
165      raise an error if the object can't support a simpler view of its memory.
166
167      ``0`` is returned on success and ``-1`` on error.
168
169      The following table gives possible values to the *flags* arguments.
170
171      +-------------------------------+---------------------------------------------------+
172      | Flag                          | Description                                       |
173      +===============================+===================================================+
174      | :c:macro:`PyBUF_SIMPLE`       | This is the default flag state.  The returned     |
175      |                               | buffer may or may not have writable memory.  The  |
176      |                               | format of the data will be assumed to be unsigned |
177      |                               | bytes.  This is a "stand-alone" flag constant. It |
178      |                               | never needs to be '|'d to the others. The exporter|
179      |                               | will raise an error if it cannot provide such a   |
180      |                               | contiguous buffer of bytes.                       |
181      |                               |                                                   |
182      +-------------------------------+---------------------------------------------------+
183      | :c:macro:`PyBUF_WRITABLE`     | The returned buffer must be writable.  If it is   |
184      |                               | not writable, then raise an error.                |
185      +-------------------------------+---------------------------------------------------+
186      | :c:macro:`PyBUF_STRIDES`      | This implies :c:macro:`PyBUF_ND`. The returned    |
187      |                               | buffer must provide strides information (i.e. the |
188      |                               | strides cannot be NULL). This would be used when  |
189      |                               | the consumer can handle strided, discontiguous    |
190      |                               | arrays.  Handling strides automatically assumes   |
191      |                               | you can handle shape.  The exporter can raise an  |
192      |                               | error if a strided representation of the data is  |
193      |                               | not possible (i.e. without the suboffsets).       |
194      |                               |                                                   |
195      +-------------------------------+---------------------------------------------------+
196      | :c:macro:`PyBUF_ND`           | The returned buffer must provide shape            |
197      |                               | information. The memory will be assumed C-style   |
198      |                               | contiguous (last dimension varies the             |
199      |                               | fastest). The exporter may raise an error if it   |
200      |                               | cannot provide this kind of contiguous buffer. If |
201      |                               | this is not given then shape will be *NULL*.      |
202      |                               |                                                   |
203      |                               |                                                   |
204      |                               |                                                   |
205      +-------------------------------+---------------------------------------------------+
206      |:c:macro:`PyBUF_C_CONTIGUOUS`  | These flags indicate that the contiguity returned |
207      |:c:macro:`PyBUF_F_CONTIGUOUS`  | buffer must be respectively, C-contiguous (last   |
208      |:c:macro:`PyBUF_ANY_CONTIGUOUS`| dimension varies the fastest), Fortran contiguous |
209      |                               | (first dimension varies the fastest) or either    |
210      |                               | one.  All of these flags imply                    |
211      |                               | :c:macro:`PyBUF_STRIDES` and guarantee that the   |
212      |                               | strides buffer info structure will be filled in   |
213      |                               | correctly.                                        |
214      |                               |                                                   |
215      +-------------------------------+---------------------------------------------------+
216      | :c:macro:`PyBUF_INDIRECT`     | This flag indicates the returned buffer must have |
217      |                               | suboffsets information (which can be NULL if no   |
218      |                               | suboffsets are needed).  This can be used when    |
219      |                               | the consumer can handle indirect array            |
220      |                               | referencing implied by these suboffsets. This     |
221      |                               | implies :c:macro:`PyBUF_STRIDES`.                 |
222      |                               |                                                   |
223      |                               |                                                   |
224      |                               |                                                   |
225      +-------------------------------+---------------------------------------------------+
226      | :c:macro:`PyBUF_FORMAT`       | The returned buffer must have true format         |
227      |                               | information if this flag is provided. This would  |
228      |                               | be used when the consumer is going to be checking |
229      |                               | for what 'kind' of data is actually stored. An    |
230      |                               | exporter should always be able to provide this    |
231      |                               | information if requested. If format is not        |
232      |                               | explicitly requested then the format must be      |
233      |                               | returned as *NULL* (which means ``'B'``, or       |
234      |                               | unsigned bytes)                                   |
235      +-------------------------------+---------------------------------------------------+
236      | :c:macro:`PyBUF_STRIDED`      | This is equivalent to ``(PyBUF_STRIDES |          |
237      |                               | PyBUF_WRITABLE)``.                                |
238      +-------------------------------+---------------------------------------------------+
239      | :c:macro:`PyBUF_STRIDED_RO`   | This is equivalent to ``(PyBUF_STRIDES)``.        |
240      |                               |                                                   |
241      +-------------------------------+---------------------------------------------------+
242      | :c:macro:`PyBUF_RECORDS`      | This is equivalent to ``(PyBUF_STRIDES |          |
243      |                               | PyBUF_FORMAT | PyBUF_WRITABLE)``.                 |
244      +-------------------------------+---------------------------------------------------+
245      | :c:macro:`PyBUF_RECORDS_RO`   | This is equivalent to ``(PyBUF_STRIDES |          |
246      |                               | PyBUF_FORMAT)``.                                  |
247      +-------------------------------+---------------------------------------------------+
248      | :c:macro:`PyBUF_FULL`         | This is equivalent to ``(PyBUF_INDIRECT |         |
249      |                               | PyBUF_FORMAT | PyBUF_WRITABLE)``.                 |
250      +-------------------------------+---------------------------------------------------+
251      | :c:macro:`PyBUF_FULL_RO`      | This is equivalent to ``(PyBUF_INDIRECT |         |
252      |                               | PyBUF_FORMAT)``.                                  |
253      +-------------------------------+---------------------------------------------------+
254      | :c:macro:`PyBUF_CONTIG`       | This is equivalent to ``(PyBUF_ND |               |
255      |                               | PyBUF_WRITABLE)``.                                |
256      +-------------------------------+---------------------------------------------------+
257      | :c:macro:`PyBUF_CONTIG_RO`    | This is equivalent to ``(PyBUF_ND)``.             |
258      |                               |                                                   |
259      +-------------------------------+---------------------------------------------------+
260
261
262.. c:function:: void PyBuffer_Release(Py_buffer *view)
263
264   Release the buffer *view*.  This should be called when the buffer
265   is no longer being used as it may free memory from it.
266
267
268.. c:function:: Py_ssize_t PyBuffer_SizeFromFormat(const char *)
269
270   Return the implied :c:data:`~Py_buffer.itemsize` from the struct-stype
271   :c:data:`~Py_buffer.format`.
272
273
274.. c:function:: int PyBuffer_IsContiguous(Py_buffer *view, char fortran)
275
276   Return ``1`` if the memory defined by the *view* is C-style (*fortran* is
277   ``'C'``) or Fortran-style (*fortran* is ``'F'``) contiguous or either one
278   (*fortran* is ``'A'``).  Return ``0`` otherwise.
279
280
281.. c:function:: void PyBuffer_FillContiguousStrides(int ndims, Py_ssize_t *shape, Py_ssize_t *strides, int itemsize, char fortran)
282
283   Fill the *strides* array with byte-strides of a contiguous (C-style if
284   *fortran* is ``'C'`` or Fortran-style if *fortran* is ``'F'``) array of the
285   given shape with the given number of bytes per element.
286
287
288.. c:function:: int PyBuffer_FillInfo(Py_buffer *view, PyObject *obj, void *buf, Py_ssize_t len, int readonly, int infoflags)
289
290   Fill in a buffer-info structure, *view*, correctly for an exporter that can
291   only share a contiguous chunk of memory of "unsigned bytes" of the given
292   length.  Return ``0`` on success and ``-1`` (with raising an error) on error.
293
294
295MemoryView objects
296==================
297
298.. versionadded:: 2.7
299
300A :class:`memoryview` object exposes the new C level buffer interface as a
301Python object which can then be passed around like any other object.
302
303.. c:function:: PyObject *PyMemoryView_FromObject(PyObject *obj)
304
305   Create a memoryview object from an object that defines the new buffer
306   interface.
307
308
309.. c:function:: PyObject *PyMemoryView_FromBuffer(Py_buffer *view)
310
311   Create a memoryview object wrapping the given buffer-info structure *view*.
312   The memoryview object then owns the buffer, which means you shouldn't
313   try to release it yourself: it will be released on deallocation of the
314   memoryview object.
315
316
317.. c:function:: PyObject *PyMemoryView_GetContiguous(PyObject *obj, int buffertype, char order)
318
319   Create a memoryview object to a contiguous chunk of memory (in either
320   'C' or 'F'ortran *order*) from an object that defines the buffer
321   interface. If memory is contiguous, the memoryview object points to the
322   original memory. Otherwise copy is made and the memoryview points to a
323   new bytes object.
324
325
326.. c:function:: int PyMemoryView_Check(PyObject *obj)
327
328   Return true if the object *obj* is a memoryview object.  It is not
329   currently allowed to create subclasses of :class:`memoryview`.
330
331
332.. c:function:: Py_buffer *PyMemoryView_GET_BUFFER(PyObject *obj)
333
334   Return a pointer to the buffer-info structure wrapped by the given
335   object.  The object **must** be a memoryview instance; this macro doesn't
336   check its type, you must do it yourself or you will risk crashes.
337
338
339Old-style buffer objects
340========================
341
342.. index:: single: PyBufferProcs
343
344More information on the old buffer interface is provided in the section
345:ref:`buffer-structs`, under the description for :c:type:`PyBufferProcs`.
346
347A "buffer object" is defined in the :file:`bufferobject.h` header (included by
348:file:`Python.h`). These objects look very similar to string objects at the
349Python programming level: they support slicing, indexing, concatenation, and
350some other standard string operations. However, their data can come from one
351of two sources: from a block of memory, or from another object which exports
352the buffer interface.
353
354Buffer objects are useful as a way to expose the data from another object's
355buffer interface to the Python programmer. They can also be used as a
356zero-copy slicing mechanism. Using their ability to reference a block of
357memory, it is possible to expose any data to the Python programmer quite
358easily. The memory could be a large, constant array in a C extension, it could
359be a raw block of memory for manipulation before passing to an operating
360system library, or it could be used to pass around structured data in its
361native, in-memory format.
362
363
364.. c:type:: PyBufferObject
365
366   This subtype of :c:type:`PyObject` represents a buffer object.
367
368
369.. c:var:: PyTypeObject PyBuffer_Type
370
371   .. index:: single: BufferType (in module types)
372
373   The instance of :c:type:`PyTypeObject` which represents the Python buffer type;
374   it is the same object as ``buffer`` and  ``types.BufferType`` in the Python
375   layer. .
376
377
378.. c:var:: int Py_END_OF_BUFFER
379
380   This constant may be passed as the *size* parameter to
381   :c:func:`PyBuffer_FromObject` or :c:func:`PyBuffer_FromReadWriteObject`.  It
382   indicates that the new :c:type:`PyBufferObject` should refer to *base*
383   object from the specified *offset* to the end of its exported buffer.
384   Using this enables the caller to avoid querying the *base* object for its
385   length.
386
387
388.. c:function:: int PyBuffer_Check(PyObject *p)
389
390   Return true if the argument has type :c:data:`PyBuffer_Type`.
391
392
393.. c:function:: PyObject* PyBuffer_FromObject(PyObject *base, Py_ssize_t offset, Py_ssize_t size)
394
395   Return a new read-only buffer object.  This raises :exc:`TypeError` if
396   *base* doesn't support the read-only buffer protocol or doesn't provide
397   exactly one buffer segment, or it raises :exc:`ValueError` if *offset* is
398   less than zero.  The buffer will hold a reference to the *base* object, and
399   the buffer's contents will refer to the *base* object's buffer interface,
400   starting as position *offset* and extending for *size* bytes. If *size* is
401   :const:`Py_END_OF_BUFFER`, then the new buffer's contents extend to the
402   length of the *base* object's exported buffer data.
403
404   .. versionchanged:: 2.5
405      This function used an :c:type:`int` type for *offset* and *size*. This
406      might require changes in your code for properly supporting 64-bit
407      systems.
408
409
410.. c:function:: PyObject* PyBuffer_FromReadWriteObject(PyObject *base, Py_ssize_t offset, Py_ssize_t size)
411
412   Return a new writable buffer object.  Parameters and exceptions are similar
413   to those for :c:func:`PyBuffer_FromObject`.  If the *base* object does not
414   export the writeable buffer protocol, then :exc:`TypeError` is raised.
415
416   .. versionchanged:: 2.5
417      This function used an :c:type:`int` type for *offset* and *size*. This
418      might require changes in your code for properly supporting 64-bit
419      systems.
420
421
422.. c:function:: PyObject* PyBuffer_FromMemory(void *ptr, Py_ssize_t size)
423
424   Return a new read-only buffer object that reads from a specified location
425   in memory, with a specified size.  The caller is responsible for ensuring
426   that the memory buffer, passed in as *ptr*, is not deallocated while the
427   returned buffer object exists.  Raises :exc:`ValueError` if *size* is less
428   than zero.  Note that :const:`Py_END_OF_BUFFER` may *not* be passed for the
429   *size* parameter; :exc:`ValueError` will be raised in that case.
430
431   .. versionchanged:: 2.5
432      This function used an :c:type:`int` type for *size*. This might require
433      changes in your code for properly supporting 64-bit systems.
434
435
436.. c:function:: PyObject* PyBuffer_FromReadWriteMemory(void *ptr, Py_ssize_t size)
437
438   Similar to :c:func:`PyBuffer_FromMemory`, but the returned buffer is
439   writable.
440
441   .. versionchanged:: 2.5
442      This function used an :c:type:`int` type for *size*. This might require
443      changes in your code for properly supporting 64-bit systems.
444
445
446.. c:function:: PyObject* PyBuffer_New(Py_ssize_t size)
447
448   Return a new writable buffer object that maintains its own memory buffer of
449   *size* bytes.  :exc:`ValueError` is returned if *size* is not zero or
450   positive.  Note that the memory buffer (as returned by
451   :c:func:`PyObject_AsWriteBuffer`) is not specifically aligned.
452
453   .. versionchanged:: 2.5
454      This function used an :c:type:`int` type for *size*. This might require
455      changes in your code for properly supporting 64-bit systems.
456