This pre-PEP proposes enhancing the buffer protocol in Python 3000 to implement the array interface (protocol).

This Wiki will serve as a place to develop the PEP until it is assigned a number and committed to the Python development tree.

Abstract

This PEP proposes re-designing the buffer API (PyBufferProcs function pointers) to improve the way Python allows memory sharing in Python 3.0

In particular, it is proposed that the multiple-segment and character buffer portions of the buffer API are eliminated and additional function pointers are provided to allow sharing any multi-dimensional character of the memory and what the memory contains.

Rational

The buffer protocol allows different Python types to exchange a pointer to a sequence of internal buffers. This functionality is extremely useful for sharing large segments of memory between different high-level objects, but it's too limited and has issues.

1. There is the little (never?) used "sequence-of-segments" option (bf_getsegcount)
2. There is the apparently redundant character-buffer option (bf_getcharbuffer)
3. There is no way for a consumer to tell the buffer-API-exporting object it is "finished" with its view of the memory and therefore no way for the expoerting object to be sure that it is safe to reallocate the pointer to the memory that it owns (the array object reallocating its memory after sharing it with the buffer object which held the original pointer led to the infamous buffer-object problem).
4. Memory is just a pointer with a length. There is no way to describe what's "in" the memory (float, int, C-structure, etc.)

5. There is no shape information provided for the memory. But, several array-like Python types could make use of a standard way to describe the shape-interpretation of the memory (!wxPython, GTK, pyQT, CVXOPT, PyVox, Audio and Video Libraries, ctypes, NumPy)

General Proposal

1. Get rid of the char-buffer and multiple-segment sections of the buffer-protocol.
2. Unify the read/write versions of getting the buffer.
3. Add a new function to the protocol that should be called when the consumer object is "done" with the view.
4. Add a new function to allow the protocol to describe what is in memory (unifying what is currently done now in struct and array)
5. Add a new function to allow the protocol to share shape information

Specification

• Change the PyBufferProcs structure to

typedef struct {
     getbufferproc bf_getbuffer
     releasebufferproc bf_releasebuffer
     formatbufferproc bf_getbufferformat
     shapebufferproc bf_getbuffershape 
}

The signatures and purposes of these function pointers are provided here:

typedef Py_ssize_t (*getbufferproc)(PyObject *obj, void **buf, int writeable)

A pointer to the memory is returned in buf and the length of that memory buffer is the function return value. If writeable is 1, then a writeable buffer is needed, otherwise a read-only buffer is sufficient. A -1 is returned if an error occurs.

typedef int (*releasebufferproc)(PyObject *obj)

This function is called when a view of memory previously acquired from the object is no longer needed. It is up to the exporter of the API to make sure all views have been released before eliminating a reference to a previously returned pointer. It is up to consumers of the API to call this function on the object whose view is obtained when it is no longer needed. A -1 is returned on error and 0 on success.

typedef char *(*formatbufferproc)(PyObject *obj)

Get the format-string of the memory using the struct-string syntax with the addition of 'g' to describe complex data, a prefix '&' to describe a pointer, '(' and ')' to describe a nested structure, and 'F','D','G' to describe complex-valued data. If the implied size of this string is smaller than the length of the buffer then it is assumed that the string is repeated.

typedef PyObject *(*shapebufferproc)(PyObject *obj)

Return a 3-tuple of lists containing shape information: (shape, strides, offsets). The strides and offsets objects can be None if the memory is C-style contiguous with 0 offsets in each dimension).

Some C-API calls should also be made available.

All of these routines are optional (but the last three make no sense unless at least one of the first two is implemented).

Questions