Supporting `strides` in `memoryview.cast`

[jakirkham]

BPO	41226
Nosy	@vedgar, @jakirkham

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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assignee = None
closed_at = None
created_at = <Date 2020-07-07.02:01:27.687>
labels = ['type-feature', 'library', '3.9', '3.10', '3.11']
title = 'Supporting `strides` in `memoryview.cast`'
updated_at = <Date 2022-01-13.19:54:52.107>
user = 'https://github.com/jakirkham'

bugs.python.org fields:

activity = <Date 2022-01-13.19:54:52.107>
actor = 'jakirkham'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2020-07-07.02:01:27.687>
creator = 'jakirkham'
dependencies = []
files = []
hgrepos = []
issue_num = 41226
keywords = []
message_count = 3.0
messages = ['373202', '402944', '410515']
nosy_count = 2.0
nosy_names = ['veky', 'jakirkham']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue41226'
versions = ['Python 3.9', 'Python 3.10', 'Python 3.11']

[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))

[vedgar]

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

[jakirkham]

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