[issue41226] Supporting `strides` in `memoryview.cast`

[jakirkham]

jakirkham  added the comment:

The 2nd argument is the `strides`. IOW it is just specifying how to traverse 
the buffer in memory to visit each of the dimensions.

For the first example where `strides` is not specified, Python makes them 
C-ordered. IOW `m2.strides` would be `(3, 1)`. Effectively this is represented 
like this:

```
[[ b"a", b"c", b"e"],
 [ b"b", b"d', b"f"]]
```

For the second case where strides are overridden (so `m2.strides` would be `(1, 
2)`), we get something like this:

```
[[b"a", b"b", b"c"],
 [b"d", b"e", b"f"]]
```

In either case the `1` here has specified which dimension is fastest to 
traverse along. IOW that content is adjacent in memory.

Should add the reason it is `1` is that for `uint8_t` (or format "B"), this is 
that type's size. If we had a different format, this would be the size of that 
format.

HTH

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[issue41226] Supporting `strides` in `memoryview.cast`

[Vedran Čačić]

Vedran Čačić  added the comment:

I surely don't understand what the third argument means. What's (1, 2) there, 
and what stride does it produce?

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nosy: +veky

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[issue41226] Supporting `strides` in `memoryview.cast`

[jakirkham]

Change by jakirkham :


--
components: +Library (Lib)
type:  -> enhancement

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[issue41226] Supporting `strides` in `memoryview.cast`

[jakirkham]

Change by jakirkham :


--
versions: +Python 3.11

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[issue41226] Supporting `strides` in `memoryview.cast`

[jakirkham]

New submission from jakirkham :

Currently one can reshape a `memoryview` using `.cast(...)` like so...

```
In [1]: m = memoryview(b"abcdef")

In [2]: m2 = m.cast("B", (2, 3))
```

However it is not currently possible to specify the `strides` when reshaping 
the `memoryview`. This would be useful if the `memoryview` should be F-order or 
otherwise strided. To that end, syntax like this would be useful...

```
In [1]: m = memoryview(b"abcdef")

In [2]: m2 = m.cast("B", (2, 3), (1, 2))
```

--
messages: 373202
nosy: jakirkham
priority: normal
severity: normal
status: open
title: Supporting `strides` in `memoryview.cast`
versions: Python 3.10, Python 3.9

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