Various improvements to the docs of the buffer API
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Antoine Pitrou
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@ -2,8 +2,8 @@
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.. _bufferobjects:
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Buffer Objects
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--------------
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Buffer API
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----------
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.. sectionauthor:: Greg Stein <gstein@lyra.org>
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.. sectionauthor:: Benjamin Peterson
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@ -17,30 +17,56 @@ functions can be used by an object to expose its data in a raw, byte-oriented
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format. Clients of the object can use the buffer interface to access the
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object data directly, without needing to copy it first.
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Two examples of objects that support the buffer interface are bytes and
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arrays. The bytes object exposes the character contents in the buffer
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interface's byte-oriented form. An array can also expose its contents, but it
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should be noted that array elements may be multi-byte values.
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Examples of objects that support the buffer interface are :class:`bytes`,
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:class:`bytearray` and :class:`array.array`. The bytes and bytearray objects
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exposes their bytes contents in the buffer interface's byte-oriented form.
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An :class:`array.array` can also expose its contents, but it should be noted
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that array elements may be multi-byte values.
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An example consumer of the buffer interface is the :meth:`~io.BufferedIOBase.write`
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method of file objects: any object that can export a series of bytes through
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the buffer interface can be written to a file. While :meth:`write` only
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needs read-only access to the internal contents of the object passed to it,
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other methods such as :meth:`~io.BufferedIOBase.readinto` need write access
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to the contents of their argument. The buffer interface allows objects to
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selectively allow or reject exporting of read-write and read-only buffers.
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There are two ways for a consumer of the buffer interface to acquire a buffer
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over a target object:
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* call :cfunc:`PyObject_GetBuffer` with the right parameters;
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* call :cfunc:`PyArg_ParseTuple` (or one of its siblings) with one of the
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``y*``, ``w*`` or ``s*`` :ref:`format codes <arg-parsing>`.
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In both cases, :cfunc:`PyBuffer_Release` must be called when the buffer
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isn't needed anymore. Failure to do so could lead to various issues such as
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resource leaks.
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An example user of the buffer interface is the file object's :meth:`write`
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method. Any object that can export a series of bytes through the buffer
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interface can be written to a file. There are a number of format codes to
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:cfunc:`PyArg_ParseTuple` that operate against an object's buffer interface,
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returning data from the target object.
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.. index:: single: PyBufferProcs
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More information on the buffer interface is provided in the section
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:ref:`buffer-structs`, under the description for :ctype:`PyBufferProcs`.
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How the buffer interface is exposed by a type object is described in the
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section :ref:`buffer-structs`, under the description for :ctype:`PyBufferProcs`.
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Buffer objects are useful as a way to expose the data from another object's
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buffer interface to the Python programmer. They can also be used as a zero-copy
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Buffer objects
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==============
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Buffer objects are useful as a way to expose the binary data from another
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object to the Python programmer. They can also be used as a zero-copy
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slicing mechanism. Using their ability to reference a block of memory, it is
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possible to expose any data to the Python programmer quite easily. The memory
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could be a large, constant array in a C extension, it could be a raw block of
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memory for manipulation before passing to an operating system library, or it
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could be used to pass around structured data in its native, in-memory format.
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Contrary to most data types exposed by the Python interpreter, buffer objects
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are not :ctype:`PyObject` pointers but rather simple C structures. This
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allows them to be created and copied very simply. When a generic wrapper
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around a buffer object is needed, a :ref:`memoryview <memoryviewobjects>` object
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can be created.
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.. ctype:: Py_buffer
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@ -133,18 +159,23 @@ Buffer related functions
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.. cfunction:: int PyObject_CheckBuffer(PyObject *obj)
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Return 1 if *obj* supports the buffer interface otherwise 0.
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Return 1 if *obj* supports the buffer interface otherwise 0. When 1 is
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returned, it doesn't guarantee that :cfunc:`PyObject_GetBuffer` will
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succeed.
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.. cfunction:: int PyObject_GetBuffer(PyObject *obj, Py_buffer *view, int flags)
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Export *obj* into a :ctype:`Py_buffer`, *view*. These arguments must
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never be *NULL*. The *flags* argument is a bit field indicating what
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kind of buffer the caller is prepared to deal with and therefore what
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kind of buffer the exporter is allowed to return. The buffer interface
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allows for complicated memory sharing possibilities, but some caller may
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not be able to handle all the complexity but may want to see if the
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exporter will let them take a simpler view to its memory.
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Export a view over some internal data from the target object *obj*.
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*obj* must not be NULL, and *view* must point to an existing
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:ctype:`Py_buffer` structure allocated by the caller (most uses of
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this function will simply declare a local variable of type
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:ctype:`Py_buffer`). The *flags* argument is a bit field indicating
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what kind of buffer is requested. The buffer interface allows
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for complicated memory layout possibilities; however, some callers
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won't want to handle all the complexity and instead request a simple
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view of the target object (using :cmacro:`PyBUF_SIMPLE` for a read-only
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view and :cmacro:`PyBUF_WRITABLE` for a read-write view).
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Some exporters may not be able to share memory in every possible way and
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may need to raise errors to signal to some consumers that something is
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@ -154,26 +185,31 @@ Buffer related functions
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:cdata:`Py_buffer` structure is filled in with non-default values and/or
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raise an error if the object can't support a simpler view of its memory.
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0 is returned on success and -1 on error.
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On success, 0 is returned and the *view* structure is filled with useful
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values. On error, -1 is returned and an exception is raised; the *view*
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is left in an undefined state.
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The following table gives possible values to the *flags* arguments.
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+------------------------------+---------------------------------------------------+
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| Flag | Description |
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+==============================+===================================================+
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| :cmacro:`PyBUF_SIMPLE` | This is the default flag state. The returned |
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| | buffer may or may not have writable memory. The |
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| | format of the data will be assumed to be unsigned |
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| | bytes. This is a "stand-alone" flag constant. It |
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| .. cmacro:: PyBUF_SIMPLE | This is the default flag. The returned buffer |
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| | exposes a read-only memory area. The format of |
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| | data is assumed to be raw unsigned bytes, without |
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| | any particular structure. This is a "stand-alone"|
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| | flag constant. It |
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| | never needs to be '|'d to the others. The exporter|
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| | will raise an error if it cannot provide such a |
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| | contiguous buffer of bytes. |
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| | |
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+------------------------------+---------------------------------------------------+
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| :cmacro:`PyBUF_WRITABLE` | The returned buffer must be writable. If it is |
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| | not writable, then raise an error. |
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| .. cmacro:: PyBUF_WRITABLE | Like :cmacro:`PyBUF_SIMPLE`, but the returned |
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| | buffer is writable. If the exporter doesn't |
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| | support |
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| | writable buffers, an error is raised. |
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+------------------------------+---------------------------------------------------+
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| :cmacro:`PyBUF_STRIDES` | This implies :cmacro:`PyBUF_ND`. The returned |
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| .. cmacro:: PyBUF_STRIDES | This implies :cmacro:`PyBUF_ND`. The returned |
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| | buffer must provide strides information (i.e. the |
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| | strides cannot be NULL). This would be used when |
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| | the consumer can handle strided, discontiguous |
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@ -183,19 +219,17 @@ Buffer related functions
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| | not possible (i.e. without the suboffsets). |
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| | |
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+------------------------------+---------------------------------------------------+
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| :cmacro:`PyBUF_ND` | The returned buffer must provide shape |
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| .. cmacro:: PyBUF_ND | The returned buffer must provide shape |
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| | information. The memory will be assumed C-style |
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| | contiguous (last dimension varies the |
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| | fastest). The exporter may raise an error if it |
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| | cannot provide this kind of contiguous buffer. If |
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| | this is not given then shape will be *NULL*. |
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| | |
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| | |
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| | |
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+------------------------------+---------------------------------------------------+
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|:cmacro:`PyBUF_C_CONTIGUOUS` | These flags indicate that the contiguity returned |
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|:cmacro:`PyBUF_F_CONTIGUOUS` | buffer must be respectively, C-contiguous (last |
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|:cmacro:`PyBUF_ANY_CONTIGUOUS`| dimension varies the fastest), Fortran contiguous |
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|.. cmacro:: PyBUF_C_CONTIGUOUS| These flags indicate that the contiguity returned |
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| PyBUF_F_CONTIGUOUS| buffer must be respectively, C-contiguous (last |
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| PyBUF_ANY_CONTIGUOUS| dimension varies the fastest), Fortran contiguous |
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| | (first dimension varies the fastest) or either |
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| | one. All of these flags imply |
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| | :cmacro:`PyBUF_STRIDES` and guarantee that the |
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@ -203,7 +237,7 @@ Buffer related functions
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| | correctly. |
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| | |
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+------------------------------+---------------------------------------------------+
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| :cmacro:`PyBUF_INDIRECT` | This flag indicates the returned buffer must have |
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| .. cmacro:: PyBUF_INDIRECT | This flag indicates the returned buffer must have |
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| | suboffsets information (which can be NULL if no |
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| | suboffsets are needed). This can be used when |
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| | the consumer can handle indirect array |
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@ -213,7 +247,7 @@ Buffer related functions
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| | |
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| | |
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+------------------------------+---------------------------------------------------+
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| :cmacro:`PyBUF_FORMAT` | The returned buffer must have true format |
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| .. cmacro:: PyBUF_FORMAT | The returned buffer must have true format |
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| | information if this flag is provided. This would |
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| | be used when the consumer is going to be checking |
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| | for what 'kind' of data is actually stored. An |
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@ -223,28 +257,28 @@ Buffer related functions
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| | returned as *NULL* (which means ``'B'``, or |
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| | unsigned bytes) |
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+------------------------------+---------------------------------------------------+
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| :cmacro:`PyBUF_STRIDED` | This is equivalent to ``(PyBUF_STRIDES | |
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| .. cmacro:: PyBUF_STRIDED | This is equivalent to ``(PyBUF_STRIDES | |
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| | PyBUF_WRITABLE)``. |
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+------------------------------+---------------------------------------------------+
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| :cmacro:`PyBUF_STRIDED_RO` | This is equivalent to ``(PyBUF_STRIDES)``. |
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| .. cmacro:: PyBUF_STRIDED_RO | This is equivalent to ``(PyBUF_STRIDES)``. |
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| | |
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+------------------------------+---------------------------------------------------+
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| :cmacro:`PyBUF_RECORDS` | This is equivalent to ``(PyBUF_STRIDES | |
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| .. cmacro:: PyBUF_RECORDS | This is equivalent to ``(PyBUF_STRIDES | |
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| | PyBUF_FORMAT | PyBUF_WRITABLE)``. |
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+------------------------------+---------------------------------------------------+
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| :cmacro:`PyBUF_RECORDS_RO` | This is equivalent to ``(PyBUF_STRIDES | |
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| .. cmacro:: PyBUF_RECORDS_RO | This is equivalent to ``(PyBUF_STRIDES | |
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| | PyBUF_FORMAT)``. |
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+------------------------------+---------------------------------------------------+
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| :cmacro:`PyBUF_FULL` | This is equivalent to ``(PyBUF_INDIRECT | |
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| .. cmacro:: PyBUF_FULL | This is equivalent to ``(PyBUF_INDIRECT | |
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| | PyBUF_FORMAT | PyBUF_WRITABLE)``. |
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+------------------------------+---------------------------------------------------+
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| :cmacro:`PyBUF_FULL_RO` | This is equivalent to ``(PyBUF_INDIRECT | |
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| .. cmacro:: PyBUF_FULL_RO | This is equivalent to ``(PyBUF_INDIRECT | |
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| | PyBUF_FORMAT)``. |
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+------------------------------+---------------------------------------------------+
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| :cmacro:`PyBUF_CONTIG` | This is equivalent to ``(PyBUF_ND | |
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| .. cmacro:: PyBUF_CONTIG | This is equivalent to ``(PyBUF_ND | |
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| | PyBUF_WRITABLE)``. |
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+------------------------------+---------------------------------------------------+
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| :cmacro:`PyBUF_CONTIG_RO` | This is equivalent to ``(PyBUF_ND)``. |
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| .. cmacro:: PyBUF_CONTIG_RO | This is equivalent to ``(PyBUF_ND)``. |
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| | |
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+------------------------------+---------------------------------------------------+
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@ -299,6 +333,7 @@ Buffer related functions
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.. index::
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object: memoryview
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.. _memoryviewobjects:
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MemoryView objects
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==================
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