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Antoine Pitrou <report@bugs.python.org> wrote:
> > I thought the whole Py_buffer API was only temporarily removed from the
> > limited API until the issues were sorted out:
> > 
> > http://bugs.python.org/issue10181#msg125462
> 
> I'm talking about the memoryview access macros. It's like
> PyList_GET_SIZE, it isn't part of the limited API and you have to use
> PyList_GetSize instead.
You're right: I presumed that the macros were excluded temporarily
when in fact that had already happened in 62b61abd02b8.
> > The flags are primarily for the memoryview itself to avoid repeated calls
> > to PyBuffer_IsContiguous(). So when 3rd-party uses PyMemoryView_GET_BUFFER
> > to get the view and also needs to determine the contiguity type, that
> > code could also use the flags.
> 
> But why would 3rd-party code use PyMemoryView_GET_BUFFER instead of, for
> example, PyObject_GetBuffer? You seldom have APIs which *expect* a
> memoryview, I think. Instead, they would expect a buffer-compatible
> object.
That's a good question. It looks to me that the macro was present as
PyMemoryView() initially. You renamed it in #3560 (with Guido's approval),
and later PyMemoryView_GET_BUFFER appeared in the docs.
I think 3rd-party code uses the macros mainly because they are
present and, in the case of PyMemoryView_GET_BUFFER, documented.
In most situations PyObject_GetBuffer() could be used indeed.
Most use cases I can think of would also involve having access to
the managed buffer API. As an example, here's a technique that is
similar to what goes on in PyMemoryView_GetContiguous():
Suppose you have an initialized bytes object that you want to
wrap as a multi-dimensional exporter. Then:
 - Base the memoryview on the bytes object and keep exactly one
 reference to it.
 - Adjust the shape, strides etc. to get the structure you want.
 - Return the view: You now have a fully compliant exporter that
 only needs a single Py_DECREF() to disappear and do all cleanup.
Of course this could also be exposed as a function, e.g.:
 /* stealing a reference to bytes */
 PyMemoryView_FromBytesAndInfo(PyObject *bytes, Py_buffer *info);
So let's make the flags private. What do you prefer?
 1) Leave them in memoryview.h, but with a leading underscore:
 _Py_MEMORYVIEW_C
 [...]
 2) Move them to memoryobject.c, with a leading underscore.
 3) Move them to memoryobject.c, without a leading underscore (I find
 this more readable).
 4) Move them to memoryobject.c as MV_C, MV_FORTRAN, etc.
Also, I'll add a note to the docs that PyMemoryView_GET_BUFFER can
probably be avoided by using PyObject_GetBuffer() directly.