[FEAT] Wrap bytearray with pybind11::bytearray

[fritzr]

It would be nice to have a pybind11::bytearray type for directly manipulating bytearray objects, similar to pybind11::bytes. This way a wrapper function could allocate and return a native mutable Python buffer. A toy example:

#include <pybind11/pybind11.h>
namespace py = pybind11;

extern size_t decompressed_size(const void *in, size_t in_size);
extern int decompress(const void *in, size_t in_size, void *out, size_t out_size);

PYBIND11_MODULE(mycompression, m) {
    m.def("decompress_mutable", [](py::buffer compressed_data) {
        py::buffer_info in_inf = compressed_data.request();
        py::bytearray out(decompressed_size(in_inf.ptr, in_inf.len));
        (void) decompress(in_inf.ptr, in_inf.size, out.data(), out.size());
        return out;
    });
}

This example borrows the idea of exposing data() and size() methods on the py::bytearray type which was suggested for py::bytes in #2517. These could use PyByteArray_AsString and PyByteArray_GetSize with appropriate error checking under the hood.

If deemed worthwhile, this should be simple to implement using the PyByteArray_* family of functions. These functions mirror the PyBytes_* family currently in use for pybind11::bytes, the definition of which is only some 30 lines of code.

[YannickJadoul]

This seems viable, indeed :-) Do you know how similar Python 2 and 3 are when it comes to bytearray?

[in42]

Reading this answer and the following documentation:
https://stackoverflow.com/questions/1740696/bytes-vs-bytearray-in-python-2-6-and-3
https://docs.python.org/2.7/library/functions.html?highlight=bytearray#bytearray
https://docs.python.org/3/library/stdtypes.html?highlight=bytearray#bytearray
it seems to me that the python 2 and 3 bytearray are exactly similar.

Documentation for the c api functions:
https://docs.python.org/3/c-api/bytearray.html
https://docs.python.org/2.7/c-api/bytearray.html

[YannickJadoul]

@in42 Great! Then this is definitely something that's possible to add to pybind11 :-)

[YannickJadoul]

Added by @in42 in #2799!