Assignment Into Python 3.x Buffers With Itemsize > 1

I am trying to expose a buffer of image pixel information (32 bit RGBA) through the Python 3.x buffer interface. After quite a bit of playing around, I was able to get this working

Solution 1:

I found this in the python code (in memoryobject.c in Objects) in the function memory_ass_sub:

/* XXX should we allow assignment of different item sizes
   as long as the byte length is the same?
   (e.g. assign 2 shorts to a 4-byte slice) */if (srcview.itemsize != view->itemsize) {
    PyErr_Format(PyExc_TypeError,
        "mismatching item sizes for \"%.200s\" and \"%.200s\"", 
        view->obj->ob_type->tp_name, srcview.obj->ob_type->tp_name);
    goto _error;
}

that's the source of the latter two errors. It looks like the itemsize for even mv[0] is still not equal to itself.

Update

Here's what I think is going on. When you try to assign something in mv, it calls memory_ass_sub in Objects/memoryobject.c, but that function takes only a PyObject as input. This object is then changed into a buffer inside using the PyObject_GetBuffer function even though in the case of mv[0] it is already a buffer (and the buffer you want!). My guess is that this function takes the object and makes it into a simple buffer of itemsize=1 regardless of whether it is already a buffer or not. That is why you get the mismatching item sizes even for

mv[0] = mv[0]

The problem with the first assignment,

mv[0] = 0xFFFFFFFF

stems (I think) from checking if the int is able to be used as a buffer, which currently it isn't set-up for from what I understand.

In other words, the buffer system isn't currently able to handle item sizes bigger from 1. It doesn't look like it is so far off, but it would take a bit more work on your end. If you do get it working, you should probably submit the changes back to the main Python distribution.

Another Update

The error code from your first try at assigning mv[0] stems from the int failing the PyObject_CheckBuffer when PyObject_CheckBuffer is called on it. Apparently the system only handles copies from bufferable objects. This seems like it should be changed too.

Conclusion

Currently the Python buffer system can't handle items with itemsize > 1 as you guessed. Also, it can't handle assignments to a buffer from non-bufferable objects such as ints.