Allow trailing comma in any function argument list.

[mdickinson]

BPO	9232
Nosy	@gvanrossum, @loewis, @rhettinger, @mdickinson, @ncoghlan, @abalkin, @larryhastings, @ericvsmith, @rbtcollins, @Trundle, @gst, @vadmium, @phmc
Files	trailing_commas.patch trailing_commas2.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 2015-08-11.20:01:47.555>
created_at = <Date 2010-07-12.14:30:16.029>
labels = ['interpreter-core', 'type-feature']
title = 'Allow trailing comma in any function argument list.'
updated_at = <Date 2015-10-04.03:03:41.342>
user = 'https://github.com/mdickinson'

bugs.python.org fields:

activity = <Date 2015-10-04.03:03:41.342>
actor = 'python-dev'
assignee = 'none'
closed = True
closed_date = <Date 2015-08-11.20:01:47.555>
closer = 'rbcollins'
components = ['Interpreter Core']
creation = <Date 2010-07-12.14:30:16.029>
creator = 'mark.dickinson'
dependencies = []
files = ['17969', '18129']
hgrepos = []
issue_num = 9232
keywords = ['after moratorium']
message_count = 30.0
messages = ['110089', '110122', '110143', '110161', '111194', '123829', '123851', '123896', '123906', '123909', '123910', '123915', '123917', '224470', '224488', '239081', '239084', '239085', '239797', '246697', '247032', '247150', '248399', '248411', '248425', '248426', '248427', '248430', '248449', '252252']
nosy_count = 16.0
nosy_names = ['gvanrossum', 'loewis', 'rhettinger', 'mark.dickinson', 'ncoghlan', 'belopolsky', 'larry', 'eric.smith', 'rbcollins', 'zuo', 'Trundle', 'python-dev', 'gstarck', 'martin.panter', 'Drekin', 'pconnell']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue9232'
versions = ['Python 3.6']

[mdickinson]

Python's current grammar allows a trailing comma after the argument list in:

def f(a, b,):
    pass

but not in

def f(*, a, b,):
    pass

I propose allowing trailing commas in both situations.

See python-dev discussion starting at

http://mail.python.org/pipermail/python-dev/2010-July/101636.html

[mdickinson]

Here's a patch. I've checked with PEP-306, but besides changing Grammar, test_grammar.py and the parser module (which there's a separate issue open for), I don't think any other changes are required.

[ncoghlan]

I take it the AST generation just throws the extra comma away? You're sure this doesn't upset any of the node counts in that stage of the compiler?

[mdickinson]

No, I'm not sure. :) I'll double check.

So I'm looking at ast_for_arguments and handle_keywordonly_args in ast.c. As far as I can tell, that's the only relevant bit; is there anywhere else I should be checking?

[mdickinson]

There was one place that needed to be changed in ast.c: namely, the check to make sure that there are keyword-only arguments following a bare star.

Here's a new patch, that fixes that issue, updates the grammar in the ast.c comment to match that in Grammar/Grammar, and also updates the production list given in the docs for function definitions.

[loewis]

Retargetting, as this falls under the moratorium, and also because 3.2b1 has been released.

[loewis]

In bpo-10682, several committers indicated that they would prefer not to change this. So I'm closing this as rejected. Per convention, it would probably require a PEP to modify Python in this aspect (as there is no clear consensus).

[abalkin]

In bpo-10682, several committers indicated that they would prefer
not to change this.

Issue bpo-10682 has been open for less than 24 hours before it was rejected. In contrast, this issue was open after an almost week-long discussion on python-dev where the proposal was well received.

I think bpo-10682 should have been closed as a duplicate of this issue and this issue should be marked as "after moratorium".

[loewis]

I stand by my evaluation: there is clearly no consensus about this change, so it certainly requires more discussion, potentially leading to proponents being asked to write a PEP.

[ncoghlan]

An open issue more accurately reflects the lack of consensus than a closed one, though. We just won't commit it until there *is* consensus that it is a better option than the status quo.