Can't read a F-contiguous memoryview in physical order

[pitrou]

BPO	35845
Nosy	@pitrou, @skrah, @jakirkham
PRs	bpo-35845: Add order={'C', 'F', 'A'} parameter to memoryview.tobytes(). #11730 bpo-35845: Add order={'C', 'F', 'A'} parameter to memoryview.tobytes(). #11730 bpo-35845: Add order={'C', 'F', 'A'} parameter to memoryview.tobytes(). #11730

^{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 = 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']

[pitrou]

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]

Yes, it's modeled after NumPy's tobytes():

>>> x = np.array(list(range(6)), dtype="int8").reshape(2,3)
>>> x.tobytes()
b'\x00\x01\x02\x03\x04\x05'
>>> x.T.tobytes()
b'\x00\x03\x01\x04\x02\x05'
>>> 
>>> 
>>> memoryview(x).tobytes()
b'\x00\x01\x02\x03\x04\x05'
>>> memoryview(x.T).tobytes()
b'\x00\x03\x01\x04\x02\x05'

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]

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]

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:

>>> a = np.arange(12, dtype='int8').reshape((3,4))                                                                             
>>> a.tobytes('A')                                                                                                             
b'\x00\x01\x02\x03\x04\x05\x06\x07\x08\t\n\x0b'
>>> a.tobytes('A') == a.T.tobytes('A')                                                                                         
True

[pitrou]

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]

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]

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()).

Yes, it would probably be a simpler way of writing .cast('B', shape=(...), order='A').

[skrah]

New changeset d08ea70 by Stefan Krah in branch 'master':
bpo-35845: Add order={'C', 'F', 'A'} parameter to memoryview.tobytes(). (bpo-11730)
d08ea70

[jakirkham]

Sorry if I'm just misunderstanding the discussion here. Would it make sense to have an order keyword argument to cast as well? This seems useful when interpreting a flatten F-order bytes object (say on the receiving end of a transmission).

[serhiy-storchaka]

Resolved by #11730 (Python 3.8), which added the order={'C', 'F', 'A'} parameter to memoryview.tobytes().

The order argument for memoryview.cast() suggested above is a separate feature — please open a new issue if it's still wanted.

Closing as resolved.