Memoryview for column-major (f_contiguous) arrays from bytes impossible to achieve

@ncoghlan

BPO	34778
Nosy	@ncoghlan, @pitrou, @skrah, @mattip

^{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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GitHub fields:

assignee = 'https://github.com/skrah'
closed_at = None
created_at = <Date 2018-09-23.17:51:38.368>
labels = ['extension-modules', '3.8', 'type-feature', 'library', '3.7']
title = 'Memoryview for column-major (f_contiguous) arrays from bytes impossible to achieve'
updated_at = <Date 2021-10-03.13:34:19.879>
user = 'https://bugs.python.org/lgautier'

bugs.python.org fields:

activity = <Date 2021-10-03.13:34:19.879>
actor = 'lgautier'
assignee = 'skrah'
closed = False
closed_date = None
closer = None
components = ['Extension Modules', 'Library (Lib)']
creation = <Date 2018-09-23.17:51:38.368>
creator = 'lgautier'
dependencies = []
files = []
hgrepos = []
issue_num = 34778
keywords = []
message_count = 16.0
messages = ['326167', '326276', '326660', '326684', '326689', '332743', '332751', '332766', '332805', '334519', '334520', '334622', '334742', '334783', '350933', '403086']
nosy_count = 5.0
nosy_names = ['ncoghlan', 'pitrou', 'lgautier', 'skrah', 'mattip']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue34778'
versions = ['Python 3.4', 'Python 3.5', 'Python 3.6', 'Python 3.7', 'Python 3.8']

Linked PRs

gh-78959: Add order parameter to memoryview.cast() #153663

The buffer protocol is accounting for the row-major or column-major arrays, and that information is shown in the attributes, c_contiguous and f_contiguous respectively, of a memoryview object.

Using the method cast allows one to specify a shape but does not allow
to specify whether row or column major:

# column-major 3x2 array of bytes that was serialized
b = bytearray([1,2,3,4,5,6])

mv = memoryview(b)
mv_b = mv.cast('b', shape=(3,2))

The result object is believed to be row-major and little can be done
to correct it:

>>> mv_int.c_contiguous
True
>>> mv_int.c_contiguous = False
AttributeError: attribute 'c_contiguous' of 'memoryview' objects is not writable

This could be done via a shape kwarg to cast

@mattip

@mattip : do you mean that it can currently be achieved by calling cast with a specific shape parameter ? If the case, how so ?

Sorry, I meant a "strides" keyword. "shape" is already a valid keyword

Wouldn't a contiguity argument ('C' or 'F') be simpler ?

(Independently, an argument strides is likely also missing from "cast").

Do you know what are the next possible steps here ? Bring this to the python-dev list ? Submit a patch ?

Bump.

What are the next steps here ?

memoryview.cast() was originally meant to be a faster version of tobytes(), which always converts to C-contiguous.

The 'shape' keyword was added because it is odd if you can cast from ND-C to 1D-Bytes but not back.

I'm not sure if we should introduce that feature, just pointing out that the original decision to exclude non 'C' views was deliberate.

Wait. Isn't a memoryview memerely a Python object for a buffer inferface, whatever its valid attributes or flags might be ?

The perceived oddness that lead to the addition of the keyword 'shape' was a good initial instinct that something was off, but this is an incomplete workaround .

If the rationale was to follow what tobytes is doing, this delegates the justification for excluding non 'C' views it. Then I do not understand the rationale behind memoryview.tobytes's exclusive relationshop to C-contiguous arrays. A memmoryview is a window on a memory region (a Python buffer), and one would expect tobytes to just return bytes for it (in whatever bytes/strides) the memoryview is originally in.

Bump.

Is there anything I could do to move this forward (as in write and submit a patch for review) ?

Bump.

I am still stumbling on that issue. I have a workaround by importing numpy but that's a rather large dependency for that one little thing.

I see that the related issue https://bugs.python.org/issue35845 opened later has later discussion entries.

Note the bpo issue 35845 is now #80026 on github

Opened #153663.

It adds a keyword-only order parameter to memoryview.cast(); with order='F' you get a zero-copy Fortran-contiguous (column-major) view of a flat buffer, mirroring memoryview.tobytes(order=...).

gh-78959: Add order parameter to memoryview.cast() (GH-153663)

pythongh-78959: Add order parameter to memoryview.cast() (pythonGH-15…

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Memoryview for column-major (f_contiguous) arrays from bytes impossible to achieve #78959

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Memoryview for column-major (f_contiguous) arrays from bytes impossible to achieve #78959

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lgautier commented on Sep 23, 2018

mattip commented on Sep 24, 2018

lgautier commented on Sep 28, 2018

mattip commented on Sep 29, 2018

lgautier commented on Sep 29, 2018

lgautier commented on Dec 30, 2018

skrah commented on Dec 30, 2018

lgautier commented on Dec 30, 2018

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lgautier commented on Aug 31, 2019

lgautier commented on Oct 3, 2021

mattip commented on May 13, 2022

serhiy-storchaka commented on Jul 13, 2026

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