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

[eric-wieser]

BPO	32780
Nosy	@terryjreedy, @vsajip, @amauryfa, @abalkin, @aerojockey, @skrah, @meadori, @mattip, @eric-wieser
PRs	gh-76961: Fix the PEP3118 format string for ctypes.Structure #5561

^{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/abalkin'
closed_at = None
created_at = <Date 2018-02-06.05:27:59.358>
labels = ['3.8', 'ctypes', 'type-bug', '3.7']
title = 'ctypes: memoryview gives incorrect PEP3118 format strings for both packed and unpacked structs'
updated_at = <Date 2019-12-23.17:24:49.037>
user = 'https://github.com/eric-wieser'

bugs.python.org fields:

activity = <Date 2019-12-23.17:24:49.037>
actor = 'ned.deily'
assignee = 'belopolsky'
closed = False
closed_date = None
closer = None
components = ['ctypes']
creation = <Date 2018-02-06.05:27:59.358>
creator = 'Eric.Wieser'
dependencies = []
files = []
hgrepos = []
issue_num = 32780
keywords = ['patch']
message_count = 8.0
messages = ['311705', '321279', '321280', '321730', '322225', '340234', '340603', '358806']
nosy_count = 10.0
nosy_names = ['terry.reedy', 'teoliphant', 'vinay.sajip', 'amaury.forgeotdarc', 'belopolsky', 'aerojockey', 'skrah', 'meador.inge', 'mattip', 'Eric.Wieser']
pr_nums = ['5561']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue32780'
versions = ['Python 2.7', 'Python 3.7', 'Python 3.8']

Linked PRs

• gh-76961: Possible fix for buildbot failures in test_pep3118 #101587

[eric-wieser]

Discovered here

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{<B:one:<I:two:}'

That format string describes the packed version of the struct, with the two field starting at offset 1, based on the struct documentation on how < should be interpreted:

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!?

[eric-wieser]

Pinging, as recommended by https://devguide.python.org/pullrequest/#reviewing. PEP-3118 as a protocol is far less useful if the canonical implementation is non-compliant.

[abalkin]

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

[aerojockey]

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<size>" 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.)

[eric-wieser]

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

Seems reasonable, although:

• I think it should be "<size>B" or "(<size>)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.

[terryjreedy]

Fixing one case is better than fixing no cases.

[skrah]

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.

[mattip]

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