8 .. sectionauthor:: Greg Stein <gstein@lyra.org>
13 single: buffer interface
15 Python objects implemented in C can export a group of functions called the
16 "buffer interface." These functions can be used by an object to expose its data
17 in a raw, byte-oriented format. Clients of the object can use the buffer
18 interface to access the object data directly, without needing to copy it first.
20 Two examples of objects that support the buffer interface are strings and
21 arrays. The string object exposes the character contents in the buffer
22 interface's byte-oriented form. An array can also expose its contents, but it
23 should be noted that array elements may be multi-byte values.
25 An example user of the buffer interface is the file object's :meth:`write`
26 method. Any object that can export a series of bytes through the buffer
27 interface can be written to a file. There are a number of format codes to
28 :cfunc:`PyArg_ParseTuple` that operate against an object's buffer interface,
29 returning data from the target object.
31 .. index:: single: PyBufferProcs
33 More information on the buffer interface is provided in the section
34 :ref:`buffer-structs`, under the description for :ctype:`PyBufferProcs`.
36 A "buffer object" is defined in the :file:`bufferobject.h` header (included by
37 :file:`Python.h`). These objects look very similar to string objects at the
38 Python programming level: they support slicing, indexing, concatenation, and
39 some other standard string operations. However, their data can come from one of
40 two sources: from a block of memory, or from another object which exports the
43 Buffer objects are useful as a way to expose the data from another object's
44 buffer interface to the Python programmer. They can also be used as a zero-copy
45 slicing mechanism. Using their ability to reference a block of memory, it is
46 possible to expose any data to the Python programmer quite easily. The memory
47 could be a large, constant array in a C extension, it could be a raw block of
48 memory for manipulation before passing to an operating system library, or it
49 could be used to pass around structured data in its native, in-memory format.
52 .. ctype:: PyBufferObject
54 This subtype of :ctype:`PyObject` represents a buffer object.
57 .. cvar:: PyTypeObject PyBuffer_Type
59 .. index:: single: BufferType (in module types)
61 The instance of :ctype:`PyTypeObject` which represents the Python buffer type;
62 it is the same object as ``buffer`` and ``types.BufferType`` in the Python
66 .. cvar:: int Py_END_OF_BUFFER
68 This constant may be passed as the *size* parameter to
69 :cfunc:`PyBuffer_FromObject` or :cfunc:`PyBuffer_FromReadWriteObject`. It
70 indicates that the new :ctype:`PyBufferObject` should refer to *base* object
71 from the specified *offset* to the end of its exported buffer. Using this
72 enables the caller to avoid querying the *base* object for its length.
75 .. cfunction:: int PyBuffer_Check(PyObject *p)
77 Return true if the argument has type :cdata:`PyBuffer_Type`.
80 .. cfunction:: PyObject* PyBuffer_FromObject(PyObject *base, Py_ssize_t offset, Py_ssize_t size)
82 Return a new read-only buffer object. This raises :exc:`TypeError` if *base*
83 doesn't support the read-only buffer protocol or doesn't provide exactly one
84 buffer segment, or it raises :exc:`ValueError` if *offset* is less than zero.
85 The buffer will hold a reference to the *base* object, and the buffer's contents
86 will refer to the *base* object's buffer interface, starting as position
87 *offset* and extending for *size* bytes. If *size* is :const:`Py_END_OF_BUFFER`,
88 then the new buffer's contents extend to the length of the *base* object's
92 .. cfunction:: PyObject* PyBuffer_FromReadWriteObject(PyObject *base, Py_ssize_t offset, Py_ssize_t size)
94 Return a new writable buffer object. Parameters and exceptions are similar to
95 those for :cfunc:`PyBuffer_FromObject`. If the *base* object does not export
96 the writeable buffer protocol, then :exc:`TypeError` is raised.
99 .. cfunction:: PyObject* PyBuffer_FromMemory(void *ptr, Py_ssize_t size)
101 Return a new read-only buffer object that reads from a specified location in
102 memory, with a specified size. The caller is responsible for ensuring that the
103 memory buffer, passed in as *ptr*, is not deallocated while the returned buffer
104 object exists. Raises :exc:`ValueError` if *size* is less than zero. Note that
105 :const:`Py_END_OF_BUFFER` may *not* be passed for the *size* parameter;
106 :exc:`ValueError` will be raised in that case.
109 .. cfunction:: PyObject* PyBuffer_FromReadWriteMemory(void *ptr, Py_ssize_t size)
111 Similar to :cfunc:`PyBuffer_FromMemory`, but the returned buffer is writable.
114 .. cfunction:: PyObject* PyBuffer_New(Py_ssize_t size)
116 Return a new writable buffer object that maintains its own memory buffer of
117 *size* bytes. :exc:`ValueError` is returned if *size* is not zero or positive.
118 Note that the memory buffer (as returned by :cfunc:`PyObject_AsWriteBuffer`) is
119 not specifically aligned.