[issue9232] Allow trailing comma in any function argument list.

[Roundup Robot]

Roundup Robot added the comment:

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

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[issue9232] Allow trailing comma in any function argument list.

[Guido van Rossum]

Guido van Rossum added the comment:

To be explicit, yes, I want to allow trailing comma even after *args or **kwds. 
And that's what the patch does.

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[issue9232] Allow trailing comma in any function argument list.

[Larry Hastings]

Larry Hastings added the comment:

With PEP 448, we can now have

fronkulate(**kwargs, **kwargs2)

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[issue9232] Allow trailing comma in any function argument list.

[Adam Bartoš]

Adam Bartoš added the comment:

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.

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[issue9232] Allow trailing comma in any function argument list.

[Guido van Rossum]

Guido van Rossum added the comment:

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.

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[issue9232] Allow trailing comma in any function argument list.

[Robert Collins]

Robert Collins added the comment:

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.

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[issue9232] Allow trailing comma in any function argument list.

[Adam Bartoš]

Adam Bartoš added the comment:

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.

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[issue9232] Allow trailing comma in any function argument list.

[Roundup Robot]

Roundup Robot added the comment:

New changeset 419ceb531bab by Robert Collins in branch 'default':
Issue #9232: Support trailing commas in function declarations.
https://hg.python.org/cpython/rev/419ceb531bab

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[issue9232] Allow trailing comma in any function argument list.

[Robert Collins]

Robert Collins added the comment:

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.

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[issue9232] Allow trailing comma in any function argument list.

[Adam Bartoš]

Adam Bartoš added the comment:

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, 
):
...

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[issue9232] Allow trailing comma in any function argument list.

[Grégory Starck]

Grégory Starck added the comment:

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,

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[issue9232] Allow trailing comma in any function argument list.

[Martin Panter]

Martin Panter added the comment:

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

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[issue9232] Allow trailing comma in any function argument list.

[Alexander Belopolsky]

Alexander Belopolsky added the comment:

.. and a couple more -1's on the tracker:

msg123851 - Martin v. Löwis
msg123848 - Brett Cannon

It looks like a round on python-ideas is needed before this can move forward.

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[issue9232] Allow trailing comma in any function argument list.

[Alexander Belopolsky]

Alexander Belopolsky added the comment:

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

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[issue9232] Allow trailing comma in any function argument list.

[Martin Panter]

Martin Panter added the comment:

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.

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[issue9232] Allow trailing comma in any function argument list.

[Larry Hastings]

Larry Hastings added the comment:

Moratorium's long over.  Will this patch rise from the dead?

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[issue9232] Allow trailing comma in any function argument list.

[Mark Dickinson]

Mark Dickinson added the comment:

 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.

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[issue9232] Allow trailing comma in any function argument list.

[Phil Connell]

Changes by Phil Connell pconn...@gmail.com:


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[issue9232] Allow trailing comma in any function argument list.

[Martin Panter]

Changes by Martin Panter vadmium...@gmail.com:


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[issue9232] Allow trailing comma in any function argument list.

[Yury Selivanov]

Changes by Yury Selivanov yselivanov...@gmail.com:


--
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[issue9232] Allow trailing comma in any function argument list.

[Mark Dickinson]

Changes by Mark Dickinson dicki...@gmail.com:


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[issue9232] Allow trailing comma in any function argument list.

[Jan Kaliszewski]

Changes by Jan Kaliszewski z...@chopin.edu.pl:


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[issue9232] Allow trailing comma in any function argument list.

[Alexander Belopolsky]

Alexander Belopolsky belopol...@users.sourceforge.net added the comment:

 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.

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[issue9232] Allow trailing comma in any function argument list.

[Martin v . Löwis]

Martin v. Löwis mar...@v.loewis.de added the comment:

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.

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[issue9232] Allow trailing comma in any function argument list.

[Nick Coghlan]

Nick Coghlan ncogh...@gmail.com added the comment:

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.

--
keywords: +after moratorium -patch
resolution: rejected - 
status: closed - open

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[issue9232] Allow trailing comma in any function argument list.

[Nick Coghlan]

Nick Coghlan ncogh...@gmail.com added the comment:

From 10682: the grammar is also inconsistent as to when trailing commas are 
allowed in function calls, not just definitions.

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[issue9232] Allow trailing comma in any function argument list.

[Eric Smith]

Changes by Eric Smith e...@trueblade.com:


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[issue9232] Allow trailing comma in any function argument list.

[Jan Kaliszewski]

Jan Kaliszewski z...@chopin.edu.pl added the comment:

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).

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[issue9232] Allow trailing comma in any function argument list.

[Jan Kaliszewski]

Jan Kaliszewski z...@chopin.edu.pl added the comment:

python-dev discussion continuation: 
http://mail.python.org/pipermail/python-dev/2010-December/106770.html

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[issue9232] Allow trailing comma in any function argument list.

[Andreas Stührk]

Changes by Andreas Stührk andy-pyt...@hammerhartes.de:


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[issue9232] Allow trailing comma in any function argument list.

[Nick Coghlan]

Changes by Nick Coghlan ncogh...@gmail.com:


--
stage: needs patch - commit review

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[issue9232] Allow trailing comma in any function argument list.

[Martin v . Löwis]

Martin v. Löwis mar...@v.loewis.de added the comment:

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).

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[issue9232] Allow trailing comma in any function argument list.

[Martin v . Löwis]

Martin v. Löwis mar...@v.loewis.de added the comment:

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

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[issue9232] Allow trailing comma in any function argument list.

[Mark Dickinson]

Mark Dickinson dicki...@gmail.com added the comment:

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.

--
Added file: http://bugs.python.org/file18127/trailing_commas2.patch

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[issue9232] Allow trailing comma in any function argument list.

[Mark Dickinson]

Changes by Mark Dickinson dicki...@gmail.com:


Added file: http://bugs.python.org/file18129/trailing_commas2.patch

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[issue9232] Allow trailing comma in any function argument list.

[Mark Dickinson]

Changes by Mark Dickinson dicki...@gmail.com:


Removed file: http://bugs.python.org/file18127/trailing_commas2.patch

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[issue9232] Allow trailing comma in any function argument list.

[Mark Dickinson]

Changes by Mark Dickinson dicki...@gmail.com:


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[issue9232] Allow trailing comma in any function argument list.

[Mark Dickinson]

Mark Dickinson dicki...@gmail.com added the comment:

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?

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[issue9232] Allow trailing comma in any function argument list.

[Mark Dickinson]

New submission from Mark Dickinson dicki...@gmail.com:

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

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priority: normal
severity: normal
stage: needs patch
status: open
title: Allow trailing comma in any function argument list.
type: feature request
versions: Python 3.2

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[issue9232] Allow trailing comma in any function argument list.

[Mark Dickinson]

Mark Dickinson dicki...@gmail.com added the comment:

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.

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Added file: http://bugs.python.org/file17969/trailing_commas.patch

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[issue9232] Allow trailing comma in any function argument list.

[Nick Coghlan]

Nick Coghlan ncogh...@gmail.com added the comment:

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?

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