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<H2><A NAME="SECTION009330000000000000000"></A><A NAME="bufferObjects"></A>
<BR>
7.3.3 Buffer Objects
</H2>

<P>
<a id='l2h-548' xml:id='l2h-548'></a>Python objects implemented in C can export a group of functions called
the ``buffer<a id='l2h-560' xml:id='l2h-560'></a> 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.

<P>
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.

<P>
An example user of the buffer interface is the file object's
<tt class="method">write()</tt> 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 <tt class="cfunction">PyArg_ParseTuple()</tt> that operate
against an object's buffer interface, returning data from the target
object.

<P>
More information on the buffer interface is provided in the section
``Buffer Object Structures'' (section&nbsp;<A href="buffer-structs.html#buffer-structs">10.7</A>), under
the description for <tt class="ctype">PyBufferProcs</tt><a id='l2h-561' xml:id='l2h-561'></a>.

<P>
A ``buffer object'' is defined in the <span class="file">bufferobject.h</span> header
(included by <span class="file">Python.h</span>). 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.

<P>
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.

<P>
<dl><dt><b><tt class="ctype"><a id='l2h-549' xml:id='l2h-549'>PyBufferObject</a></tt></b></dt>
<dd>
  This subtype of <tt class="ctype">PyObject</tt> represents a buffer object.
</dl>

<P>
<dl><dt>PyTypeObject <b><tt id='l2h-550' xml:id='l2h-550' class="cdata">PyBuffer_Type</tt></b></dt>
<dd>
  The instance of <tt class="ctype">PyTypeObject</tt> which represents the Python
  buffer type; it is the same object as <code>types.BufferType</code> in the
  Python layer.<a id='l2h-552' xml:id='l2h-552'></a>.
</dd></dl>

<P>
<dl><dt>int <b><tt id='l2h-553' xml:id='l2h-553' class="cdata">Py_END_OF_BUFFER</tt></b></dt>
<dd>
  This constant may be passed as the <var>size</var> parameter to
  <tt class="cfunction">PyBuffer_FromObject()</tt> or
  <tt class="cfunction">PyBuffer_FromReadWriteObject()</tt>.  It indicates that the
  new <tt class="ctype">PyBufferObject</tt> should refer to <var>base</var> object from
  the specified <var>offset</var> to the end of its exported buffer.  Using
  this enables the caller to avoid querying the <var>base</var> object for
  its length.
</dd></dl>

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"><td><nobr>int&nbsp;<b><tt id='l2h-554' xml:id='l2h-554' class="cfunction">PyBuffer_Check</tt></b>(</nobr></td><td>PyObject *<var>p</var>)</td></tr></table></dt>
<dd>
  Return true if the argument has type <tt class="cdata">PyBuffer_Type</tt>.
</dd></dl>

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"><td><nobr>PyObject*&nbsp;<b><tt id='l2h-555' xml:id='l2h-555' class="cfunction">PyBuffer_FromObject</tt></b>(</nobr></td><td>PyObject *<var>base</var>,
                                                  int <var>offset</var>, int <var>size</var>)</td></tr></table></dt>
<dd>
<div class="refcount-info">
  <span class="label">Return value:</span>
  <span class="value">New reference.</span>
</div>
  Return a new read-only buffer object.  This raises
  <tt class="exception">TypeError</tt> if <var>base</var> doesn't support the read-only
  buffer protocol or doesn't provide exactly one buffer segment, or it
  raises <tt class="exception">ValueError</tt> if <var>offset</var> is less than zero. The
  buffer will hold a reference to the <var>base</var> object, and the
  buffer's contents will refer to the <var>base</var> object's buffer
  interface, starting as position <var>offset</var> and extending for
  <var>size</var> bytes. If <var>size</var> is <tt class="constant">Py_END_OF_BUFFER</tt>, then
  the new buffer's contents extend to the length of the <var>base</var>
  object's exported buffer data.
</dd></dl>

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"><td><nobr>PyObject*&nbsp;<b><tt id='l2h-556' xml:id='l2h-556' class="cfunction">PyBuffer_FromReadWriteObject</tt></b>(</nobr></td><td>PyObject *<var>base</var>,
                                                           int <var>offset</var>,
                                                           int <var>size</var>)</td></tr></table></dt>
<dd>
<div class="refcount-info">
  <span class="label">Return value:</span>
  <span class="value">New reference.</span>
</div>
  Return a new writable buffer object.  Parameters and exceptions are
  similar to those for <tt class="cfunction">PyBuffer_FromObject()</tt>.  If the
  <var>base</var> object does not export the writeable buffer protocol,
  then <tt class="exception">TypeError</tt> is raised.
</dd></dl>

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"><td><nobr>PyObject*&nbsp;<b><tt id='l2h-557' xml:id='l2h-557' class="cfunction">PyBuffer_FromMemory</tt></b>(</nobr></td><td>void *<var>ptr</var>, int <var>size</var>)</td></tr></table></dt>
<dd>
<div class="refcount-info">
  <span class="label">Return value:</span>
  <span class="value">New reference.</span>
</div>
  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
  <var>ptr</var>, is not deallocated while the returned buffer object
  exists.  Raises <tt class="exception">ValueError</tt> if <var>size</var> is less than
  zero.  Note that <tt class="constant">Py_END_OF_BUFFER</tt> may <em>not</em> be
  passed for the <var>size</var> parameter; <tt class="exception">ValueError</tt> will be
  raised in that case.
</dd></dl>

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"><td><nobr>PyObject*&nbsp;<b><tt id='l2h-558' xml:id='l2h-558' class="cfunction">PyBuffer_FromReadWriteMemory</tt></b>(</nobr></td><td>void *<var>ptr</var>, int <var>size</var>)</td></tr></table></dt>
<dd>
<div class="refcount-info">
  <span class="label">Return value:</span>
  <span class="value">New reference.</span>
</div>
  Similar to <tt class="cfunction">PyBuffer_FromMemory()</tt>, but the returned
  buffer is writable.
</dd></dl>

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"><td><nobr>PyObject*&nbsp;<b><tt id='l2h-559' xml:id='l2h-559' class="cfunction">PyBuffer_New</tt></b>(</nobr></td><td>int <var>size</var>)</td></tr></table></dt>
<dd>
<div class="refcount-info">
  <span class="label">Return value:</span>
  <span class="value">New reference.</span>
</div>
  Return a new writable buffer object that maintains its own memory
  buffer of <var>size</var> bytes.  <tt class="exception">ValueError</tt> is returned if
  <var>size</var> is not zero or positive.  Note that the memory buffer (as
  returned by <tt class="cfunction">PyObject_AsWriteBuffer()</tt>) is not specifically
  aligned.
</dd></dl>

<P>

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<span class="release-info">Release 2.4.2, documentation updated on 28 September 2005.</span>
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