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gh-148431: Clarify format and item size in buffer requests #159832

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17 changes: 12 additions & 5 deletions Doc/c-api/buffer.rst
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Expand Up @@ -151,8 +151,12 @@ a buffer, see :c:func:`PyObject_GetBuffer`.
.. c:member:: char *format

A *NULL* terminated string in :mod:`struct` module style syntax describing
the contents of a single item. If this is ``NULL``, ``"B"`` (unsigned bytes)
is assumed.
the contents of a single item. If this is ``NULL``, the original format
is not available to the consumer. For a :c:macro:`PyBUF_SIMPLE` or
:c:macro:`PyBUF_WRITABLE` request, the buffer is treated as unsigned bytes
(``"B"``). For other requests without :c:macro:`PyBUF_FORMAT`,
:c:member:`~Py_buffer.itemsize` still describes the size of each item,
even though its format is unknown.

This field is controlled by the :c:macro:`PyBUF_FORMAT` flag.

Expand Down Expand Up @@ -272,9 +276,12 @@ readonly, format
Since :c:macro:`PyBUF_SIMPLE` is defined as 0, :c:macro:`PyBUF_WRITABLE`
can be used as a stand-alone flag to request a simple writable buffer.

:c:macro:`PyBUF_FORMAT` must be \|'d to any of the flags except :c:macro:`PyBUF_SIMPLE`, because
the latter already implies format ``B`` (unsigned bytes). :c:macro:`!PyBUF_FORMAT` cannot be
used on its own.
:c:macro:`PyBUF_FORMAT` is optional and can be \|'d to any of the flags in the
next section except :c:macro:`PyBUF_SIMPLE`. Without it, the exporter sets
:c:member:`~Py_buffer.format` to ``NULL``, even when shape information is
requested. To request the original format, combine :c:macro:`!PyBUF_FORMAT`
with :c:macro:`PyBUF_ND` or a flag that includes it. It cannot be used on its
own or combined only with :c:macro:`PyBUF_WRITABLE`.


shape, strides, suboffsets
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