Turn SIG_DFL and SIG_IGN into functions

[serhiy-storchaka]

BPO	23325
Nosy	@warsaw, @brettcannon, @vstinner, @giampaolo, @tiran, @serhiy-storchaka, @zhangyangyu, @miss-islington
PRs	bpo-23325: Turn signal.SIG_DFL and signal.SIG_IGN into functions. #8920 bpo-23325: Fix SIG_IGN and SIG_DFL int comparison in signal module #31759 [3.10] bpo-23325: Fix SIG_IGN and SIG_DFL int comparison in signal module (GH-31759) #31768
Files	signal_std_handlers.patch

^{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 2015-01-26.20:38:30.138>
labels = ['type-feature', 'library', '3.9', '3.10', '3.11']
title = 'Turn SIG_DFL and SIG_IGN into functions'
updated_at = <Date 2022-03-31.17:54:28.870>
user = 'https://github.com/serhiy-storchaka'

bugs.python.org fields:

activity = <Date 2022-03-31.17:54:28.870>
actor = 'brett.cannon'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2015-01-26.20:38:30.138>
creator = 'serhiy.storchaka'
dependencies = []
files = ['37868']
hgrepos = []
issue_num = 23325
keywords = ['patch']
message_count = 14.0
messages = ['234775', '238327', '238495', '238538', '324075', '324084', '324096', '411037', '411045', '411104', '414747', '414749', '414765', '414767']
nosy_count = 9.0
nosy_names = ['barry', 'brett.cannon', 'vstinner', 'giampaolo.rodola', 'christian.heimes', 'eli.bendersky', 'serhiy.storchaka', 'xiang.zhang', 'miss-islington']
pr_nums = ['8920', '31759', '31768']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue23325'
versions = ['Python 3.9', 'Python 3.10', 'Python 3.11']

[serhiy-storchaka]

In C the SIG_DFL and SIG_IGN macros expand into integral expressions that are not equal to an address of any function. In Python they are int objects with platform depended values. Second argument of the signal() function should be SIG_DFL, SIG_IGN, or a function. The getsignal() function returns SIG_DFL, SIG_IGN, None, or a function. They are tested for identity in signal() and getsignal() returns identical values. So actually SIG_DFL and SIG_IGN are just singletons.

I propose to turn SIG_DFL and SIG_IGN into functions. Benefits:

1. They will have names and self-descriptive representation.
2. They will have docstrings.
3. The signature of signal() will be simpler. The type of second argument would be function.
4. Their pickling will be portable.

This patch depends on the backout of turning these constants to enums (bpo-21076).

[ethanfurman]

A private method is being added to Enum to better support Enum replacement of constants, part of which includes changing __reduce_ex__ to return the string of the name.

These changes answer points 1 and 4.

Point 2 would be nice, but seems somewhat less important if Enums are being used.

Point 3 -- I don't see how 'Signal(xxx, yyy)' is more complicated than 'Signal(xxx, zzz)'?

[serhiy-storchaka]

Currently the action parameter of signal.signal() can be SIG_DFL, SIG_IGN, or a callable Python object. Would be just a callable Python object. This is simpler.

The main benefit is that they will be better look in pydoc output.

[ethanfurman]

Thanks for the explanation, point taken.

However, it seems to me that changing the type of the constants from 'int' to 'function' is backwards incompatible, will break code, and is probably not worth it (and would require a deprecation period if it was worth it).

I still think the better answer is to delve into Modules/signalmodule.c and fix the comparisons.

[zhangyangyu]

I'm afraid this could break old codes so not sure it's worth or not. I've seen code like SIGTERM = 15; signal.signal(SIGTERM, handler). I won't be surprised to see a handcraft SIG_DFL/IGN.

[serhiy-storchaka]

Numerical values of signal numbers are standardized. signal.signal(15, handler) is valid. SIGTERM is just a handy name for the constant 15.

But SIG_DFL and SIG_IGN are not integers in C (they are special pointers), and it is not declared anywhere that they should be integers in Python. The code that converts them to Python integer is implementation depending (PyLong_FromVoidPtr). Handcrafted SIG_DFL and SIG_IGN are broken by design, because the signal module uses identity comparison for SIG_DFL and SIG_IGN.

[zhangyangyu]

I miss that. So this change seems to be user transparent and make more sense then.

[tiran]

Serhiy, could you please rebase your PR to tip of main branch? I'd like to try it out.