gh-112014: correct buffer protocol support in ctypes (use native formats)

[skirpichev]

This allows better interoperation with the memoryview, e.g. support for ctypes arrays. Just like NumPy arrays, they also don't specify endianness:

>>> import numpy as np
>>> memoryview(np.ndarray(3, dtype=np.float16)).format
'e'

But keep specified byteorder for byte-swapped types.

• Issue: Allow memoryview objects to support ctypes Arrays #112014

[vstinner]

I think that I can understand the rationale for this change, but it seems like @encukou has a different opinion on how the issue should be addressed: #112014 (comment).

[vstinner]

From this NEWS entry, it's uneasy to understand the rationale for the change. According to #112014 (comment), I understand that before, it was not possible to modify a ctypes type using memoryview, since the buffer format was rejected by memoryview when attempting to modify the view.

[skirpichev]

it's uneasy to understand the rationale for the change

It's a bug :-) Memoryview support for ctypes buffers is a side effect of the bugfix.

[skirpichev]

I think that I can understand the rationale for this change, but it seems like @encukou has a different opinion on how the issue should be addressed: #112014 (comment).

Well, obviously it's a breaking change. We should decide whether the current behavior is a bug or not. My arguments are in the issue thread: #112014 (comment)

[encukou]

As is, currently, this PR is wrong.
I have a 64-bit machine where long has 8-bytes:

>>> import ctypes
>>> ctypes.sizeof(ctypes.c_long)
8

Per struct docs, an array of native longs should use the format l, but with this PR, it uses q:

>>> memoryview((ctypes.c_long*8)()).format
'q'
>>> 

(Without this PR, it correctly uses <q, which means “8 bytes, little endian”.)

[bedevere-app]

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[skirpichev]

You are right, probably I should use _ctypes_alloc_format_string_for_type() helper for swapped types. And just copy format code per default.

Unfortunately, ctypes and struct use "slightly different format codes" (c).