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Can't read a F-contiguous memoryview in physical order #80026
Description
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GitHub fields:
assignee = None
closed_at = None
created_at = <Date 2019-01-28.20:51:37.338>
labels = ['interpreter-core', 'type-feature', '3.8']
title = "Can't read a F-contiguous memoryview in physical order"
updated_at = <Date 2020-06-05.16:33:50.845>
user = 'https://github.com/pitrou'bugs.python.org fields:
activity = <Date 2020-06-05.16:33:50.845>
actor = 'jakirkham'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Interpreter Core']
creation = <Date 2019-01-28.20:51:37.338>
creator = 'pitrou'
dependencies = []
files = []
hgrepos = []
issue_num = 35845
keywords = ['patch', 'patch', 'patch']
message_count = 9.0
messages = ['334491', '334495', '334496', '334497', '334498', '334739', '334743', '334759', '370767']
nosy_count = 3.0
nosy_names = ['pitrou', 'skrah', 'jakirkham']
pr_nums = ['11730', '11730', '11730']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue35845'
versions = ['Python 3.8']
Activity
pitrou commentedon Jan 28, 2019
This request is motivated in detail here:
python/peps#883 (comment)
In short: in C, when you have a Py_buffer, you can directly read the memory in whatever order you want (including physical order). It is not possible in pure Python, though. Somewhat unintuitively, memoryview.tobytes() as well as bytes(memoryview) read bytes in *logical* order, even though it flattens the dimensions and doesn't keep the original type. Logical order is different from physical order for Fortran-contiguous arrays.
One possible way of alleviating this would be to offer a memoryview.transpose() method, similar to the Numpy transpose() method (see https://docs.scipy.org/doc/numpy/reference/generated/numpy.ndarray.transpose.html).
One could also imagine a memoryview.to_c_contiguous() method.
Or even: a memoryview.raw_memory() method, that would 1) flatten dimensions 2) cast to 'B' format 3) keep physical order.
skrah commentedon Jan 28, 2019
Yes, it's modeled after NumPy's tobytes():
I guess the reason is that without a type it's easier to serialize the logical array by default, so you can always assume C when you read back.
NumPy also has an 'F' parameter though that flips the order:
>>> x.tobytes('F') b'\x00\x03\x01\x04\x02\x05'It would be possible to add this to memoryview as well.
skrah commentedon Jan 28, 2019
raw_bytes() is also possible of course. I assume it would do nothing and just dump the memory.
Or tobytes('F') AND tobytes('raw').
pitrou commentedon Jan 28, 2019
Well, raw_memory() would avoid a copy, which is useful.
As for tobytes(), if we want to follow NumPy, we can have 'F' mean if F-contiguous, 'C' otherwise:
pitrou commentedon Jan 28, 2019
Sorry, my fingers slipped. Let me try again:
As for tobytes(), if we want to follow NumPy, we can have 'A' mean 'F' if F-contiguous, 'C' otherwise: [...]
skrah commentedon Feb 2, 2019
Yes, following NumPy looks like the sanest option for tobytes(), so I
went ahead and implemented that signature.
memory.raw() is of course complicated by the fact that things like
m[::-1] move buf.ptr to the end of the buffer.
So we'd need to restrict to contiguous views anyway, which makes
the method less appealing (IOW, it doesn't offer more than an
augmented memoryview.cast()).
pitrou commentedon Feb 2, 2019
Yes, it would probably be a simpler way of writing
.cast('B', shape=(...), order='A').skrah commentedon Feb 2, 2019
New changeset d08ea70 by Stefan Krah in branch 'master':
bpo-35845: Add order={'C', 'F', 'A'} parameter to memoryview.tobytes(). (bpo-11730)
d08ea70
jakirkham commentedon Jun 5, 2020
Sorry if I'm just misunderstanding the discussion here. Would it make sense to have an
orderkeyword argument tocastas well? This seems useful when interpreting a flatten F-orderbytesobject (say on the receiving end of a transmission).serhiy-storchaka commentedon Jul 13, 2026
Resolved by #11730 (Python 3.8), which added the
order={'C', 'F', 'A'}parameter tomemoryview.tobytes().The
orderargument formemoryview.cast()suggested above is a separate feature — please open a new issue if it's still wanted.Closing as resolved.
serhiy-storchaka commentedon Jul 13, 2026
The
cast(order=)follow-up mentioned above is already tracked in #78959.