Document that ctypes.xFUNCTYPE are decorators.

[lambertdw]

BPO	4260
Nosy	@theller, @birkenfeld, @amauryfa, @abalkin, @meadori, @berkerpeksag, @andresdelfino, @miss-islington
PRs	bpo-4260: Document that ctypes.xFUNCTYPE are decorators #7924 [3.7] bpo-4260: Document that ctypes.xFUNCTYPE are decorators (GH-7924) #8272 [3.6] bpo-4260: Document that ctypes.xFUNCTYPE are decorators (GH-7924) #8273

^{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 = <Date 2018-07-13.14:05:29.865>
created_at = <Date 2008-11-05.05:51:02.885>
labels = ['easy', '3.8', 'type-feature', '3.7', 'docs']
title = 'Document that ctypes.xFUNCTYPE are decorators.'
updated_at = <Date 2018-07-13.14:05:29.864>
user = 'https://bugs.python.org/LambertDW'

bugs.python.org fields:

activity = <Date 2018-07-13.14:05:29.864>
actor = 'berker.peksag'
assignee = 'docs@python'
closed = True
closed_date = <Date 2018-07-13.14:05:29.865>
closer = 'berker.peksag'
components = ['Documentation']
creation = <Date 2008-11-05.05:51:02.885>
creator = 'LambertDW'
dependencies = []
files = []
hgrepos = []
issue_num = 4260
keywords = ['patch', 'easy']
message_count = 8.0
messages = ['75515', '75535', '75577', '320464', '321601', '321610', '321612', '321614']
nosy_count = 10.0
nosy_names = ['theller', 'georg.brandl', 'amaury.forgeotdarc', 'belopolsky', 'kevinwatters', 'LambertDW', 'meador.inge', 'berker.peksag', 'adelfino', 'miss-islington']
pr_nums = ['7924', '8272', '8273']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue4260'
versions = ['Python 3.6', 'Python 3.7', 'Python 3.8']

[lambertdw]

http://docs.python.org/dev/3.0/library/ctypes.html#callback-functions

ctypes.xFUNCTYPE are another opportunity to advertise decorators.
Please consider inserting yet another qsort example written as a
decorator, perhaps as follows. Or---it could be that I'm slow and the
average pythonista will figure this out on first read.

@CFUNCTYPE(c_int, POINTER(c_int), POINTER(c_int))
def py_cmp_func(*args):
    (a,b,) = (t[0] for t in args)
    print("py_cmp_func", a, b)
    return a-b

qsort(ia,len(ia),sizeof(c_int),py_cmp_func)

[kevinwatters]

As far as I know, the above code will fail randomly in release mode if
you have multiple threads running Python, because when ctypes calls out
to CFUNCTYPE functions, it releases the GIL.

For decorating a python function to give to qsort, maybe you can use
PYFUNCTYPE?

[theller]

[David Lambert]

> @CFUNCTYPE(c_int, POINTER(c_int), POINTER(c_int))
> def py_cmp_func(*args):
> (a,b,) = (t[0] for t in args)
> print("py_cmp_func", a, b)
> return a-b
>
> qsort(ia,len(ia),sizeof(c_int),py_cmp_func)

[Kevin Watters]

> As far as I know, the above code will fail randomly in release mode if
> you have multiple threads running Python, because when ctypes calls out
> to CFUNCTYPE functions, it releases the GIL.
>
> For decorating a python function to give to qsort, maybe you can use
> PYFUNCTYPE?

Kevin is wrong - the code is perfect! Sure does calling a CFUNCTYPE instance
release the GIL, but ctypes aquires the GIL again before executing the python
code in the wrapped py_cmp_func above.

I guess that using PYFUNCTYPE instead would possibly crash because the call to
'qsort' releases the GIL, and the PYFUNCTYPE instance assumes the GIL but does not
acquire it.

But I have no time to work on the docs, sorry.

[andresdelfino]

I suspect we'll be working on this PR a little, but a least it's a start to get this moving.

[berkerpeksag]

New changeset 379e9d6 by Berker Peksag (Andrés Delfino) in branch 'master':
bpo-4260: Document that ctypes.xFUNCTYPE are decorators (GH-7924)
379e9d6

[miss-islington]

New changeset 08c1da7 by Miss Islington (bot) in branch '3.7':
bpo-4260: Document that ctypes.xFUNCTYPE are decorators (GH-7924)
08c1da7