RFE: Add signal.strsignal(): string describing a signal

[dolda2000]

BPO	22674
Nosy	@birkenfeld, @pitrou, @vstinner, @benjaminp, @ned-deily, @bitdancer, @vajrasky, @seirl
PRs	bpo-22674: signal: add strsignal() #6017 bpo-22674: fix test_strsignal on OSX #6085
Files	strsignal.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 = <Date 2018-03-12.19:03:28.289>
created_at = <Date 2014-10-19.19:12:54.060>
labels = ['3.8', 'type-feature', 'library']
title = 'RFE: Add signal.strsignal(): string describing a signal'
updated_at = <Date 2018-03-12.19:03:28.288>
user = 'https://bugs.python.org/Dolda2000'

bugs.python.org fields:

activity = <Date 2018-03-12.19:03:28.288>
actor = 'pitrou'
assignee = 'none'
closed = True
closed_date = <Date 2018-03-12.19:03:28.289>
closer = 'pitrou'
components = ['Library (Lib)']
creation = <Date 2014-10-19.19:12:54.060>
creator = 'Dolda2000'
dependencies = []
files = ['37018']
hgrepos = []
issue_num = 22674
keywords = ['patch']
message_count = 19.0
messages = ['229691', '229695', '229697', '229698', '229717', '229726', '229738', '230026', '304290', '304335', '313390', '313650', '313660', '313661', '313663', '313665', '313667', '313670', '313679']
nosy_count = 10.0
nosy_names = ['georg.brandl', 'pitrou', 'vstinner', 'benjamin.peterson', 'ned.deily', 'r.david.murray', 'cvrebert', 'Dolda2000', 'vajrasky', 'antoine.pietri']
pr_nums = ['6017', '6085']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue22674'
versions = ['Python 3.8']

[dolda2000]

Like it says on the tin, it would be nice if strsignal(3) were added to the signal module.

[bitdancer]

This seems like a reasonable request to me. Do you want to propose a patch?

[benjaminp]

A dictionary, signal number -> signal name might be more friendly.

[bitdancer]

Yeah, the thinnest possible exposure of the strsignal API wouldn't really be that sensible for Python. But making the OS information available *somehow* makes sense.

[birkenfeld]

Is it possible to determine the range of signal numbers? Otherwise it would be a guessing game where to stop querying when filling up the dictionary.

A problem is also that if the signal number is not valid, the return value of strsignal() is unspecified, *and* there is no way to check for this situation because "no errors are defined".

[vstinner]

I don't think that a strsignal() is required, signals now have a name attribute!

Python 3.5.0a0 (default:07ae7bc06af0, Oct 16 2014, 09:46:01) 
>>> import signal
>>> signal.SIGINT
<Signals.SIGINT: 2>
>>> signal.SIGINT.name
'SIGINT'

[birkenfeld]

Nice. However, strsignal() returns not just SIGINT, but "Interrupted" etc.

[vajrasky]

Here is the preliminary patch. It's a thin wrapper of strsignal.

Some issues and things:

1. About Benjamin Peterson's request, what is the name of the dictionary supposed to be? Is everyone okay with Benjamin's suggestion?

2. About George Brandl's question: "Is it possible to determine the range of signal numbers?" We have a heuristic algorithm:

#ifndef NSIG
# if defined(_NSIG)
#  define NSIG _NSIG            /* For BSD/SysV */
# elif defined(_SIGMAX)
#  define NSIG (_SIGMAX + 1)    /* For QNX */
# elif defined(SIGMAX)
#  define NSIG (SIGMAX + 1)     /* For djgpp */
# else
#  define NSIG 64               /* Use a reasonable default value */
# endif
#endif

if (sig_num < 1 || sig_num >= NSIG) {
        PyErr_SetString(PyExc_ValueError,
                        "signal number out of range");
        return NULL;
}

3. For the unknown signal, what is the description should be? "Unknown signal" like c function returns or None?

4. What is the name of the function that wrap strsignal should be? I use strsignal for now. I just don't think it is appropriate.

[seirl]

Hey everyone,

We would like to have that feature for a project where we have to use ctypes to achieve that. I don't know the answers to vajrasky's questions but I just wanted to chime in to say having this feature would be greatly appreciated. I can also work on the code if you need any help.

Thanks!

[vstinner]

3. For the unknown signal, what is the description should be? "Unknown signal" like c function returns or None?

Hum, Linux returns "Unknown signal 12345". I propose to use this behaviour on all platforms (which provide strsignal()).

[seirl]

I updated Vajrasky's patch to rebase it onto master, use the clinic argument parser and improve the docs. I made a PR on GitHub so the review can be easier than a patch.

I left a Co-Authored-By field so I'm not stealing the ownership of this patch :-P