> memoryview is converted to bytes because len() for memoryview returns a size of first dimension (a number of items for one-dimension view), not a number of bytes.
> 
>>>> m = memoryview(array.array('I', [1, 2, 3]))
>>>> len(m)
> 3
>>>> len(m.tobytes())
> 12
>>>> len(m.cast('B'))
> 12

Right, I was aware of this. But are you saying that my proposed solution (using memoryview.nbytes) is wrong ? If so, then cast is certainly an option and should still outperform tobytes.