Issue #2396: backport the memoryview object.

5 files changed, 407 insertions, 7 deletions

diff --git a/Doc/c-api/buffer.rst b/Doc/c-api/buffer.rst
index 6b80ad6..ce9a4e4 100644
--- a/Doc/c-api/buffer.rst
+++ b/Doc/c-api/buffer.rst
@@ -2,10 +2,11 @@
 
 .. _bufferobjects:
 
-Buffer Objects
---------------
+Buffers and Memoryview Objects
+------------------------------
 
 .. sectionauthor:: Greg Stein <gstein@lyra.org>
+.. sectionauthor:: Benjamin Peterson
 
 
 .. index::
@@ -28,9 +29,296 @@ interface can be written to a file. There are a number of format codes to
 :cfunc:`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 builtin or used-defined type
+can expose its characteristics. Both, however, have been deprecated because
+of various shortcomings, and have been officially removed in Python 3.0 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
+==============================
+
+
+.. ctype:: Py_buffer
+
+   .. cmember:: void *buf
+
+      A pointer to the start of the memory for the object.
+
+   .. cmember:: Py_ssize_t len
+      :noindex:
+
+      The total length of the memory in bytes.
+
+   .. cmember:: int readonly
+
+      An indicator of whether the buffer is read only.
+
+   .. cmember:: 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.
+
+   .. cmember:: int ndim
+
+      The number of dimensions the memory represents as a multi-dimensional
+      array.  If it is 0, :cdata:`strides` and :cdata:`suboffsets` must be
+      *NULL*.
+
+   .. cmember:: Py_ssize_t *shape
+
+      An array of :ctype:`Py_ssize_t`\s the length of :cdata:`ndim` giving the
+      shape of the memory as a multi-dimensional array.  Note that
+      ``((*shape)[0] * ... * (*shape)[ndims-1])*itemsize`` should be equal to
+      :cdata:`len`.
+
+   .. cmember:: Py_ssize_t *strides
+
+      An array of :ctype:`Py_ssize_t`\s the length of :cdata:`ndim` giving the
+      number of bytes to skip to get to a new element in each dimension.
+
+   .. cmember:: Py_ssize_t *suboffsets
+
+      An array of :ctype:`Py_ssize_t`\s the length of :cdata:`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;
+           }
+
+
+   .. cmember:: 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
+      :cfunc:`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.
+
+   .. cmember:: 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
+========================
+
+
+.. cfunction:: int PyObject_CheckBuffer(PyObject *obj)
+
+   Return 1 if *obj* supports the buffer interface otherwise 0.
+
+
+.. cfunction:: int PyObject_GetBuffer(PyObject *obj, PyObject *view, int flags)
+
+      Export *obj* into a :ctype:`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 :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 :cdata:`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                                       |
+      +==============================+===================================================+
+      | :cmacro:`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.                       |
+      |                              |                                                   |
+      +------------------------------+---------------------------------------------------+
+      | :cmacro:`PyBUF_WRITABLE`     | The returned buffer must be writable.  If it is   |
+      |                              | not writable, then raise an error.                |
+      +------------------------------+---------------------------------------------------+
+      | :cmacro:`PyBUF_STRIDES`      | This implies :cmacro:`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).       |
+      |                              |                                                   |
+      +------------------------------+---------------------------------------------------+
+      | :cmacro:`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*.      |
+      |                              |                                                   |
+      |                              |                                                   |
+      |                              |                                                   |
+      +------------------------------+---------------------------------------------------+
+      |:cmacro:`PyBUF_C_CONTIGUOUS`  | These flags indicate that the contiguity returned |
+      |:cmacro:`PyBUF_F_CONTIGUOUS`  | buffer must be respectively, C-contiguous (last   |
+      |:cmacro:`PyBUF_ANY_CONTIGUOUS`| dimension varies the fastest), Fortran contiguous |
+      |                              | (first dimension varies the fastest) or either    |
+      |                              | one.  All of these flags imply                    |
+      |                              | :cmacro:`PyBUF_STRIDES` and guarantee that the    |
+      |                              | strides buffer info structure will be filled in   |
+      |                              | correctly.                                        |
+      |                              |                                                   |
+      +------------------------------+---------------------------------------------------+
+      | :cmacro:`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 :cmacro:`PyBUF_STRIDES`.                  |
+      |                              |                                                   |
+      |                              |                                                   |
+      |                              |                                                   |
+      +------------------------------+---------------------------------------------------+
+      | :cmacro:`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)                                   |
+      +------------------------------+---------------------------------------------------+
+      | :cmacro:`PyBUF_STRIDED`      | This is equivalent to ``(PyBUF_STRIDES |          |
+      |                              | PyBUF_WRITABLE)``.                                |
+      +------------------------------+---------------------------------------------------+
+      | :cmacro:`PyBUF_STRIDED_RO`   | This is equivalent to ``(PyBUF_STRIDES)``.        |
+      |                              |                                                   |
+      +------------------------------+---------------------------------------------------+
+      | :cmacro:`PyBUF_RECORDS`      | This is equivalent to ``(PyBUF_STRIDES |          |
+      |                              | PyBUF_FORMAT | PyBUF_WRITABLE)``.                 |
+      +------------------------------+---------------------------------------------------+
+      | :cmacro:`PyBUF_RECORDS_RO`   | This is equivalent to ``(PyBUF_STRIDES |          |
+      |                              | PyBUF_FORMAT)``.                                  |
+      +------------------------------+---------------------------------------------------+
+      | :cmacro:`PyBUF_FULL`         | This is equivalent to ``(PyBUF_INDIRECT |         |
+      |                              | PyBUF_FORMAT | PyBUF_WRITABLE)``.                 |
+      +------------------------------+---------------------------------------------------+
+      | :cmacro:`PyBUF_FULL_RO``     | This is equivalent to ``(PyBUF_INDIRECT |         |
+      |                              | PyBUF_FORMAT)``.                                  |
+      +------------------------------+---------------------------------------------------+
+      | :cmacro:`PyBUF_CONTIG`       | This is equivalent to ``(PyBUF_ND |               |
+      |                              | PyBUF_WRITABLE)``.                                |
+      +------------------------------+---------------------------------------------------+
+      | :cmacro:`PyBUF_CONTIG_RO`    | This is equivalent to ``(PyBUF_ND)``.             |
+      |                              |                                                   |
+      +------------------------------+---------------------------------------------------+
+
+
+.. cfunction:: void PyBuffer_Release(PyObject *obj, Py_buffer *view)
+
+   Release the buffer *view* over *obj*.  This shouldd be called when the buffer
+   is no longer being used as it may free memory from it.
+
+
+.. cfunction:: Py_ssize_t PyBuffer_SizeFromFormat(const char *)
+
+   Return the implied :cdata:`~Py_buffer.itemsize` from the struct-stype
+   :cdata:`~Py_buffer.format`.
+
+
+.. cfunction:: 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.
+
+
+.. cfunction:: 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.
+
+
+.. cfunction:: 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.
+
+
+.. cfunction:: 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 :ctype:`Py_buffer`, is a Python object (exposed as
+:class:`memoryview` in :mod:`builtins`), so it can be used with Python code.
+
+.. cfunction:: PyObject* PyMemoryView_FromObject(PyObject *obj)
+
+   Return a memoryview object from an object that defines the buffer interface.
+
+
+Old-style buffer objects
+========================
+
 .. index:: single: PyBufferProcs
 
-More information on the buffer interface is provided in the section
+More information on the old buffer interface is provided in the section
 :ref:`buffer-structs`, under the description for :ctype:`PyBufferProcs`.
 
 A "buffer object" is defined in the :file:`bufferobject.h` header (included by
diff --git a/Doc/c-api/objbuffer.rst b/Doc/c-api/objbuffer.rst
index 7d66db2..ba6e38f 100644
--- a/Doc/c-api/objbuffer.rst
+++ b/Doc/c-api/objbuffer.rst
@@ -2,8 +2,15 @@
 
 .. _abstract-buffer:
 
-Buffer Protocol
-===============
+
+Old Buffer Protocol
+===================
+
+This section describes the legacy buffer protocol, which has been introduced
+in Python 1.6. It is still supported but deprecated in the Python 2.x series.
+Python 3.0 introduces a new buffer protocol which fixes weaknesses and
+shortcomings of the protocol, and has been backported to Python 2.6.
+See :ref:`bufferobjects` for more information.
 
 
 .. cfunction:: int PyObject_AsCharBuffer(PyObject *obj, const char **buffer, Py_ssize_t *buffer_len)
diff --git a/Doc/library/functions.rst b/Doc/library/functions.rst
index c19462c..5ca6e42 100644
--- a/Doc/library/functions.rst
+++ b/Doc/library/functions.rst
@@ -680,6 +680,13 @@ available.  They are listed here in alphabetical order.
       Added support for the optional *key* argument.
 
 
+.. function:: memoryview(obj)
+   :noindex:
+
+   Return a "memory view" object created from the given argument.  See
+   :ref:`typememoryview` for more information.
+
+
 .. function:: min(iterable[, args...][key])
 
    With a single argument *iterable*, return the smallest item of a non-empty
diff --git a/Doc/library/stdtypes.rst b/Doc/library/stdtypes.rst
index 03565b0..af84f4f 100644
--- a/Doc/library/stdtypes.rst
+++ b/Doc/library/stdtypes.rst
@@ -2354,6 +2354,104 @@ the particular object.
       state.
 
 
+.. _typememoryview:
+
+memoryview Types
+================
+
+:class:`memoryview`\s allow Python code to access the internal data of an object
+that supports the buffer protocol without copying.  Memory can be interpreted as
+simple bytes or complex data structures.
+
+.. class:: memoryview(obj)
+
+   Create a :class:`memoryview` that references *obj*.  *obj* must support the
+   buffer protocol.  Builtin objects that support the buffer protocol include
+   :class:`str` and :class:`bytearray` (but not :class:`unicode`).
+
+   ``len(view)`` returns the total number of bytes in the memoryview, *view*.
+
+   A :class:`memoryview` supports slicing to expose its data.  Taking a single
+   index will return a single byte.  Full slicing will result in a subview::
+
+      >>> v = memoryview('abcefg')
+      >>> v[1]
+      'b'
+      >>> v[-1]
+      'g'
+      >>> v[1:4]
+      <memory at 0x77ab28>
+      >>> str(v[1:4])
+      'bce'
+      >>> v[3:-1]
+      <memory at 0x744f18>
+      >>> str(v[4:-1])
+      'f'
+
+   If the object the memory view is over supports changing its data, the
+   memoryview supports slice assignment::
+
+      >>> data = bytearray('abcefg')
+      >>> v = memoryview(data)
+      >>> v.readonly
+      False
+      >>> v[0] = 'z'
+      >>> data
+      bytearray(b'zbcefg')
+      >>> v[1:4] = '123'
+      >>> data
+      bytearray(b'z123fg')
+      >>> v[2] = 'spam'
+      Traceback (most recent call last):
+        File "<stdin>", line 1, in <module>
+      ValueError: cannot modify size of memoryview object
+
+   Notice how the size of the memoryview object can not be changed.
+
+
+   :class:`memoryview` has two methods:
+
+   .. method:: tobytes()
+
+      Return the data in the buffer as a bytestring (an object of class
+      :class:`str`).
+
+   .. method:: tolist()
+
+      Return the data in the buffer as a list of integers. ::
+
+         >>> memoryview(b'abc').tolist()
+         [97, 98, 99]
+
+   There are also several readonly attributes available:
+
+   .. attribute:: format
+
+      A string containing the format (in :mod:`struct` module style) for each
+      element in the view.  This defaults to ``'B'``, a simple bytestring.
+
+   .. attribute:: itemsize
+
+      The size in bytes of each element of the memoryview.
+
+   .. attribute:: shape
+
+      A tuple of integers the length of :attr:`ndim` giving the shape of the
+      memory as a N-dimensional array.
+
+   .. attribute:: ndim
+
+      An integer indicating how many dimensions of a multi-dimensional array the
+      memory represents.
+
+   .. attribute:: strides
+
+      A tuple of integers the length of :attr:`ndim` giving the size in bytes to
+      access each element for each dimension of the array.
+
+   .. memoryview.suboffsets isn't documented because it only seems useful for C
+
+
 .. _typecontextmanager:
 
 Context Manager Types
diff --git a/Doc/tutorial/modules.rst b/Doc/tutorial/modules.rst
index f147cea..c1ff982 100644
--- a/Doc/tutorial/modules.rst
+++ b/Doc/tutorial/modules.rst
@@ -328,8 +328,8 @@ want a list of those, they are defined in the standard module
     'enumerate', 'eval', 'execfile', 'exit', 'file', 'filter', 'float',
     'frozenset', 'getattr', 'globals', 'hasattr', 'hash', 'help', 'hex',
     'id', 'input', 'int', 'intern', 'isinstance', 'issubclass', 'iter',
-    'len', 'license', 'list', 'locals', 'long', 'map', 'max', 'min',
-    'object', 'oct', 'open', 'ord', 'pow', 'property', 'quit', 'range',
+    'len', 'license', 'list', 'locals', 'long', 'map', 'max', 'memoryview',
+    'min', 'object', 'oct', 'open', 'ord', 'pow', 'property', 'quit', 'range',
     'raw_input', 'reduce', 'reload', 'repr', 'reversed', 'round', 'set',
     'setattr', 'slice', 'sorted', 'staticmethod', 'str', 'sum', 'super',
     'tuple', 'type', 'unichr', 'unicode', 'vars', 'xrange', 'zip']