The future of buffer interfaces

[jim-easterbrook]

The exiv2 classes DataBuf, PreviewImage, and BasicIo are all wrappers around potentially large blocks of memory. python-exiv2 gives each of these classes a "buffer interface", which allows many Python functions to access the data directly without making a copy.

At some time in the future I'd like to use Python's "stable API" for python-exiv2 so that one binary wheel can work with multiple versions of Python. Unfortunately the buffer interface wasn't added to Python's stable API until Python 3.11, so any wheel compatible with Python < 3.11 won't be able to have buffer interfaces.

Another potential problem is that the next release of swig may not allow buffer interfaces in Python < 3.9. This is a consequence of changes in the way Python types are initialised. I've raised this as an issue in swig: swig/swig#3211

The three exiv2 classes also have methods that (in C++) return a pointer to the data. In Python these return a memoryview object, and that also has a buffer interface. It should be just as efficient to use these data() methods as to use the buffer interface, so I'd like to deprecate using buffer interfaces from now on.