Issue 9232: Allow trailing comma in any function argument list. (original) (raw)
Created on 2010-07-12 14:30 by mark.dickinson, last changed 2022-04-11 14:57 by admin. This issue is now closed.
Messages (30)
Author: Mark Dickinson (mark.dickinson) * 
Date: 2010-07-12 14:30
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
Author: Mark Dickinson (mark.dickinson) *
Date: 2010-07-12 18:37
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.
Author: Alyssa Coghlan (ncoghlan) *
Date: 2010-07-12 21:24
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?
Author: Mark Dickinson (mark.dickinson) *
Date: 2010-07-13 08:37
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?
Author: Mark Dickinson (mark.dickinson) *
Date: 2010-07-22 17:13
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.
Author: Martin v. Löwis (loewis) *
Date: 2010-12-12 07:55
Retargetting, as this falls under the moratorium, and also because 3.2b1 has been released.
Author: Martin v. Löwis (loewis) *
Date: 2010-12-12 20:52
In #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).
Author: Alexander Belopolsky (belopolsky) *
Date: 2010-12-13 19:59
In #10682, several committers indicated that they would prefer not to change this.
Issue #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 #10682 should have been closed as a duplicate of this issue and this issue should be marked as "after moratorium".
Author: Martin v. Löwis (loewis) *
Date: 2010-12-13 22:42
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.
Author: Alyssa Coghlan (ncoghlan) *
Date: 2010-12-13 23:56
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.
Author: Alyssa Coghlan (ncoghlan) *
Date: 2010-12-13 23:59
From 10682: the grammar is also inconsistent as to when trailing commas are allowed in function calls, not just definitions.
Author: Jan Kaliszewski (zuo)
Date: 2010-12-14 01:37
From 10682: The patch proposed in this (#9232) issue does not fix call syntax but def sytax only. I think it should fix call sytax as well (see code examples in #10682).
Author: Jan Kaliszewski (zuo)
Date: 2010-12-14 01:44
python-dev discussion continuation: http://mail.python.org/pipermail/python-dev/2010-December/106770.html
Author: Larry Hastings (larry) *
Date: 2014-08-01 06:36
Moratorium's long over. Will this patch rise from the dead?
Author: Mark Dickinson (mark.dickinson) *
Date: 2014-08-01 11:43
Will this patch rise from the dead?
It's really down to getting consensus that it's a good idea. That might require another python-dev discussion.
Author: Martin Panter (martin.panter) *
Date: 2015-03-24 00:50
See also Issue 22942 about existing problems with the language documentation.
I would like to see trailing commas supported consistently, especially in function calls. (I think the patch here only does function definitions?) I like to use them when writing arguments on multiple lines, and it is surprising that adding packed *positional arguments can trigger a syntax error.
Maybe this is stretching the scope a bit too far, but it would also be nice to allow more keyword arguments after the **keyword unpacking:
print(1, 2, end=".\n", *(3, 4)) # Supported, but confusing print(1, 2, *(3, 4), end=".\n") # Better; also suported print(1, 2, **dict(sep="-"), end=".\n") # Unsupported, but would be nice print(end=".\n", 1, 2) # Unsupported, for good reason
Maybe some of this is covered by Issue 2292 (generalizing * unpacking), but I haven’t been following that, so I’m not sure.
Author: Alexander Belopolsky (belopolsky) *
Date: 2015-03-24 01:07
It looks like if it was not for Raymond's mild dissent, [1], we would have a consensus last time this was raised on python-dev, [2-7].
[1] -? Raymond Hettinger https://mail.python.org/pipermail/python-dev/2010-December/106782.html [2] +0 Guido van Rossum https://mail.python.org/pipermail/python-dev/2010-December/106780.html [3] +0.5 Alexander Belopolsky https://mail.python.org/pipermail/python-dev/2010-December/106782.html [4] +1 Antoine Pitrou https://mail.python.org/pipermail/python-dev/2010-December/106783.html [5] +1 Glenn Linderman https://mail.python.org/pipermail/python-dev/2010-December/106784.html [6] +1 Cameron Simpson https://mail.python.org/pipermail/python-dev/2010-December/106788.html [7] +1 Terry Reedy https://mail.python.org/pipermail/python-dev/2010-December/106789.html
Author: Alexander Belopolsky (belopolsky) *
Date: 2015-03-24 01:12
.. and a couple more -1's on the tracker:
- Martin v. Löwis - Brett Cannon
It looks like a round on python-ideas is needed before this can move forward.
Author: Martin Panter (martin.panter) *
Date: 2015-04-01 12:44
Actual post by Raymond: <https://mail.python.org/pipermail/python-dev/2010-December/106790.html>
Just noticed there are some arguments for trailing commas in the FAQ: <https://docs.python.org/dev/faq/design.html#why-does-python-allow-commas-at-the-end-of-lists-and-tuples>
Author: Grégory Starck (gstarck) *
Date: 2015-07-13 18:44
Have also been confronted to this bug (imo) and this happen from time to time to me :
I often like to ends my (big) functions defs and calls (those that span over multiple lines thus..) with that extra comma,
so that when/if I add another argument (on a new line) later then there will be only a + line in my VCS).
There seems to have a consensus to apply the patch (unless it's not finished?)..
regards,
Author: Adam Bartoš (Drekin) *
Date: 2015-07-21 13:13
Reposting from from my newest duplicate of this issue (Issue 24677), which is now closed:
I think that a trailing comma in function definition should be allowed also after *.
Current situation with definitions: def f(args, ): pass # SyntaxError def f(, ): pass # SyntaxError def f(, a, ): pass # SyntaxError def f(, a=2, ): pass # SyntaxError
def f(a, ): pass # Ok def f(, ): pass # SyntaxError – this should probably stay error
Corresponding calls: f(*args, ) # Ok f(*args, a, ) # Ok f(*args, a=2, ) # Ok
f(a, ) # Ok f(, ) # SyntaxError – this is why the corresponding def behavior should stay
My use case: def f(*, long = 1, list = 2, of = 3, kwonly = 4, parameters = 5, ): ...
Author: Robert Collins (rbcollins) *
Date: 2015-07-22 20:22
FWIW I would like to see this, but I think it does need a PEP given the contention so far. For that, we need a BDFL delegate AIUI.
Author: Adam Bartoš (Drekin) *
Date: 2015-08-11 10:21
Some remarks:
• A trailing comma after a non-empty argument list is allowed in every call form, including class statement and optional call in decorator syntax. In the grammar, this correponds to arglist.
• In function definition, trailing comma is allowed only if there is no star before:
def f(a, b, c,): # allowed
def f(a=1, b=2, c=3,): # allowed
def f(*args,): # disallowed
def f(*kwargs,): # disallowed
def f(, a, b, c,): # disallowed
The last example is what bothers me. The presence of the star should not affect whether trailing comma is allowed or not. If f(a, b, c,) is allowed as a call, it should be allowed in a definition, and if def f(a, b, c,) is allowed, f(*, a, b, c,) should be allowed as well.
In the grammar this corresponds to typedargslist for functions and varargslist for lambdas.
• A traling comma is allowed in tuples, lists, dicts, sets, the corresponding comprehensions, augmented assignments, and subscripts. It is also allowed in from module import names in the names part, but only if there are surrounding parentheses. Also a trailing semicolon is allowed for multiple statements in one line.
• A traling comma is not allowed in with statement, import modules, assert statement (there is just optional second argument), global and nonlocal statements. In all these cases surrounding parentheses are not allowed.
Author: Guido van Rossum (gvanrossum) *
Date: 2015-08-11 15:22
I'm +1 on adding this. I don't believe it requires a PEP. A trailing comma in definitions is already supported in some places, so I don't buy the argument that it catches errors. During the moratorium we were perhaps too strict.
Author: Roundup Robot (python-dev) 
Date: 2015-08-11 20:00
New changeset 419ceb531bab by Robert Collins in branch 'default': Issue #9232: Support trailing commas in function declarations. https://hg.python.org/cpython/rev/419ceb531bab
Author: Robert Collins (rbcollins) *
Date: 2015-08-11 20:01
The patch had some conflicts in the reference docs, I think I resolved it correctly: if someone wanted to cross check my work that would be great. However I was feeling (perhaps wrongly :)) confident so I have committed it as-is.
Author: Adam Bartoš (Drekin) *
Date: 2015-08-11 20:18
Do we want to allow a trailing comma after *args or **kwargs in a function definition? Unlike in a call, **kwargs is always the last thing in the list and nothing can be added after that. Just asking.
Author: Larry Hastings (larry) *
Date: 2015-08-11 21:44
With PEP 448, we can now have
fronkulate(**kwargs, **kwargs2)Author: Guido van Rossum (gvanrossum) *
Date: 2015-08-12 06:38
To be explicit, yes, I want to allow trailing comma even after *args or **kwds. And that's what the patch does.
Author: Roundup Robot (python-dev)
Date: 2015-10-04 03:03
New changeset 6db349fac3ec by Terry Jan Reedy in branch 'default': Issue #9232: Escape rst markup char in NEWS entry to avoid Sphinx warning. https://hg.python.org/cpython/rev/6db349fac3ec
History
Date
User
Action
Args
2022-04-11 14:57:03
admin
set
github: 53478
2015-10-04 03:03:41
python-dev
set
messages: +
2015-08-12 06:38:23
gvanrossum
set
messages: +
2015-08-11 21:44:03
larry
set
messages: +
2015-08-11 20🔞19
Drekin
set
messages: +
2015-08-11 20:01:47
rbcollins
set
status: open -> closed
resolution: fixed
messages: +
stage: commit review -> resolved
2015-08-11 20:00:34
python-dev
set
nosy: + python-dev
messages: +
2015-08-11 15:22:43
gvanrossum
set
nosy: + gvanrossum
messages: +
2015-08-11 10:21:42
Drekin
set
messages: +
2015-07-22 20:22:03
rbcollins
set
nosy: + rbcollins
messages: +
versions: + Python 3.6, - Python 3.5
2015-07-21 13:13:56
Drekin
set
nosy: + Drekin
messages: +
2015-07-21 12:53:37
martin.panter
link
2015-07-13 18:44:31
gstarck
set
nosy: + gstarck
messages: +
2015-07-09 21:01:09
r.david.murray
link
2015-04-01 12:44:13
martin.panter
set
messages: +
2015-03-24 01:12:02
belopolsky
set
messages: +
2015-03-24 01:07:00
belopolsky
set
nosy: + rhettinger
messages: +
2015-03-24 00:50:48
martin.panter
set
messages: +
2014-08-01 11:43:27
mark.dickinson
set
messages: +
2014-08-01 06:36:26
larry
set
nosy: + larry
messages: +
2014-07-09 18:21:24
pconnell
set
nosy: + pconnell
2014-02-26 10:32:18
martin.panter
set
nosy: + martin.panter
2014-01-31 22:05:49
yselivanov
set
versions: + Python 3.5, - Python 3.3
2012-10-25 07:51:34
chris.jerdonek
link
2012-03-10 18:34:32
mark.dickinson
set
assignee: mark.dickinson ->
2010-12-14 01:49:59
Trundle
set
nosy: + Trundle
2010-12-14 01:44:21
zuo
set
messages: +
2010-12-14 01:37:18
zuo
set
messages: +
2010-12-14 01:10:11
eric.smith
set
nosy: + eric.smith
2010-12-13 23:59:20
ncoghlan
set
messages: +
2010-12-13 23:57:44
ncoghlan
link
2010-12-13 23:56:33
ncoghlan
set
status: closed -> open
keywords: + after moratorium, - patch
resolution: rejected -> (no value)
messages: +
2010-12-13 22:42:41
loewis
set
messages: +
2010-12-13 19:59:21
belopolsky
set
nosy: + belopolsky
messages: +
2010-12-13 11:07:46
zuo
set
nosy: + zuo
2010-12-12 20:52:56
loewis
set
status: open -> closed
resolution: rejected
messages: +
2010-12-12 10:40:16
ncoghlan
set
stage: needs patch -> commit review
2010-12-12 07:55:32
loewis
set
nosy: + loewis
messages: +
versions: + Python 3.3, - Python 3.2
2010-07-22 17:16:10
mark.dickinson
set
files: - trailing_commas2.patch
2010-07-22 17:16:05
mark.dickinson
set
files: + trailing_commas2.patch
2010-07-22 17:13:47
mark.dickinson
set
files: + trailing_commas2.patch
messages: +
2010-07-20 12:32:08
mark.dickinson
set
assignee: mark.dickinson
2010-07-13 08:37:15
mark.dickinson
set
messages: +
2010-07-12 21:24:31
ncoghlan
set
nosy: + ncoghlan
messages: +
2010-07-12 18:37:30
mark.dickinson
set
files: + trailing_commas.patch
keywords: + patch
messages: +
2010-07-12 14:30:16
mark.dickinson
create