What exactly is the point of memoryview in Python

Question

Checking the documentation on memoryview:

memoryview objects allow Python code to access the internal data of an object that suppor

Answer 1

Let me make plain where lies the glitch in understanding here.

The questioner, like myself, expected to be able to create a memoryview that selects a slice of an existing array (for example a bytes or bytearray). We therefore expected something like:

desired_slice_view = memoryview(existing_array, start_index, end_index)

Alas, there is no such constructor, and the docs don't make a point of what to do instead.

The key is that you have to first make a memoryview that covers the entire existing array. From that memoryview you can create a second memoryview that covers a slice of the existing array, like this:

whole_view = memoryview(existing_array)
desired_slice_view = whole_view[10:20]

In short, the purpose of the first line is simply to provide an object whose slice implementation (dunder-getitem) returns a memoryview.

That might seem untidy, but one can rationalize it a couple of ways:

1. Our desired output is a memoryview that is a slice of something. Normally we get a sliced object from an object of that same type, by using the slice operator [10:20] on it. So there's some reason to expect that we need to get our desired_slice_view from a memoryview, and that therefore the first step is to get a memoryview of the whole underlying array.

2. The naive expection of a memoryview constructor with start and end arguments fails to consider that the slice specification really needs all the expressivity of the usual slice operator (including things like [3::2] or [:-4] etc). There is no way to just use the existing (and understood) operator in that one-liner constructor. You can't attach it to the existing_array argument, as that will make a slice of that array, instead of telling the memoryview constructor some slice parameters. And you can't use the operator itself as an argument, because it's an operator and not a value or object.

Conceivably, a memoryview constructor could take a slice object:

desired_slice_view = memoryview(existing_array, slice(1, 5, 2) )

... but that's not very satisfactory, since users would have to learn about the slice object and what its constructor's parameters mean, when they already think in terms of the slice operator's notation.