[issue32780] ctypes: memoryview gives incorrect PEP3118 format strings for both packed and unpacked structs

[Ned Deily]

Change by Ned Deily :


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nosy: +vinay.sajip

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[issue32780] ctypes: memoryview gives incorrect PEP3118 format strings for both packed and unpacked structs

[mattip]

mattip  added the comment:

Is there a ctypes or PEP-3118 core-dev who could review the issue and the PR 
solution?

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nosy: +mattip

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[issue32780] ctypes: memoryview gives incorrect PEP3118 format strings for both packed and unpacked structs

[Stefan Krah]

Stefan Krah  added the comment:

Since Terry added me: Yes, this is clearly a bug, but it is a ctypes issue and 
not a memoryview issue.

ctypes issues unfortunately tend to take some time until someone reviews.

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[issue32780] ctypes: memoryview gives incorrect PEP3118 format strings for both packed and unpacked structs

[Terry J. Reedy]

Terry J. Reedy  added the comment:

Fixing one case is better than fixing no cases.

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nosy: +skrah, terry.reedy

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[issue32780] ctypes: memoryview gives incorrect PEP3118 format strings for both packed and unpacked structs

[Terry J. Reedy]

Change by Terry J. Reedy :


--
versions:  -Python 3.6

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[issue32780] ctypes: memoryview gives incorrect PEP3118 format strings for both packed and unpacked structs

[Eric Wieser]

Eric Wieser  added the comment:

> It should be fairly simple to modify the code to use a format of "B" 
> for unions, so that it at least matches the itemsize

Seems reasonable, although:

* I think it should be "B" or "()B"
* I'd rather leave that for a later patch. While it would be correct, it's 
still not correct enough to be that useful, since ultimately the union layout 
is still lost. I'd prefer to focus on fixing the part of the PEPE3118 
implementation that is most useful, rather than committing to fixing the whole 
thing all at once.

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[issue32780] ctypes: memoryview gives incorrect PEP3118 format strings for both packed and unpacked structs

[Carl Banks]

Carl Banks  added the comment:

I guess I'll weigh in since I was pinged.

I agree with the approach in the patch.  All the memoryview does is to use the 
format field verbatim from the underlying buffer, so if the format field is 
inaccurate then the only thing to do is to fix the object providing the buffer.

Since the format field is only there for interpretation of the data, and is not 
used to calculate of itemsizes or strides anywhere as far as I know, it's a 
fairly low-risk change.

However, the patch still leaves ctypes inaccurate for the case of unions.  It 
should be fairly simple to modify the code to use a format of "B" for 
unions, so that it at least matches the itemsize, even if the type information 
is lost.

(As an aside, let me point out that I did not actually write or advocate for 
the PEP; for some reason my name was added to it even though I all I did was to 
provide feedback.)

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[issue32780] ctypes: memoryview gives incorrect PEP3118 format strings for both packed and unpacked structs

[Alexander Belopolsky]

Change by Alexander Belopolsky :


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assignee:  -> belopolsky

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[issue32780] ctypes: memoryview gives incorrect PEP3118 format strings for both packed and unpacked structs

[Alexander Belopolsky]

Alexander Belopolsky  added the comment:

Unfortunately, the PEP authors did very little in terms of implementing the PEP 
and neither CPython nor numpy has a fully compliant implementation.

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nosy: +aerojockey, teoliphant

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[issue32780] ctypes: memoryview gives incorrect PEP3118 format strings for both packed and unpacked structs

[Eric Wieser]

Eric Wieser  added the comment:

Pinging, as recommended by https://devguide.python.org/pullrequest/#reviewing. 
PEP3118 as a protocol is far less useful if the canonical implementation is 
non-compliant.

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[issue32780] ctypes: memoryview gives incorrect PEP3118 format strings for both packed and unpacked structs

[Terry J. Reedy]

Change by Terry J. Reedy :


--
nosy: +amaury.forgeotdarc, belopolsky, meador.inge
versions:  -Python 3.4, Python 3.5

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[issue32780] ctypes: memoryview gives incorrect PEP3118 format strings for both packed and unpacked structs

[Eric Wieser]

Change by Eric Wieser :


--
type:  -> behavior

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[issue32780] ctypes: memoryview gives incorrect PEP3118 format strings for both packed and unpacked structs

[Eric Wieser]

Change by Eric Wieser :


--
keywords: +patch
pull_requests: +5383
stage:  -> patch review

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[issue32780] ctypes: memoryview gives incorrect PEP3118 format strings for both packed and unpacked structs

[Eric Wieser]

New submission from Eric Wieser :

Discovered [here](https://github.com/numpy/numpy/issues/10528)

Consider the following structure, and a memoryview created from it:

class foo(ctypes.Structure):
_fields_ = [('one', ctypes.c_uint8),
('two', ctypes.c_uint32)]
f = foo()
mf = memoryview(f)

We'd expect this to insert padding, and it does:

>>> mf.itemsize
8

But that padding doesn't show up in the format string:

>>> mf.format
'T{ No padding is added when using non-native size and alignment, e.g. with ‘<’, 
> ‘>’

But ctypes doesn't even get it right for packed structs:


class foop(ctypes.Structure):
_fields_ = [('one', ctypes.c_uint8),
('two', ctypes.c_uint32)]
_pack_ = 1
f = foo()
mf = memoryview(f)

The size is what we'd expect:

>>> mf.itemsize
5

But the format is garbage:

>>> mf.format
'B'  # sizeof(byte) == 5!?

--
components: ctypes
messages: 311705
nosy: Eric.Wieser
priority: normal
severity: normal
status: open
title: ctypes: memoryview gives incorrect PEP3118 format strings for both 
packed and unpacked structs
versions: Python 2.7, Python 3.4, Python 3.5, Python 3.6, Python 3.7, Python 3.8

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