[issue10682] With '*args' or even bare '*' in def/call argument list, trailing comma causes SyntaxError

[Mike Lissner]

Mike Lissner added the comment:

This is an old issue, but where I run into it frequently is when I use the 
format function and string interpolation. For example, this throws a 
SyntaxError:

"The name of the person is {name_first} {name_last}".format(
**my_obj.__dict__,
)

Because strings tend to be fairly long, it's pretty common that the arguments 
to format end up on their own line. 

I was always taught to use trailing commas in Python, and I'm fanatical about 
ensuring they're there. It's a smart part of the language that saves you from 
many bugs and much typing when copy/pasting/tweaking. 

This is the first time I've ever run into an implementation bug in CPython, and 
at least from the post on StackOverflow, this looks like the parser isn't 
obeying the grammar: 
https://stackoverflow.com/questions/16950394/python-why-is-this-invalid-syntax

--
nosy: +Mike.Lissner

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[issue10682] With '*args' or even bare '*' in def/call argument list, trailing comma causes SyntaxError

[Grégory Starck]

Grégory Starck added the comment:

>  unneeded consistency

"unneeded" consistency !:?

I'd say that either there is a "full or complete" consistency on the mentionned 
point/element of syntax, or there is none .. but an unneeded one.. or an 
half-one, isn't "consistent consistency" by then ;)

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[issue10682] With '*args' or even bare '*' in def/call argument list, trailing comma causes SyntaxError

[Nick Coghlan]

Changes by Nick Coghlan :


--
resolution: rejected -> duplicate
superseder:  -> Allow trailing comma in any function argument list.

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[issue10682] With '*args' or even bare '*' in def/call argument list, trailing comma causes SyntaxError

[Martin v . Löwis]

Martin v. Löwis  added the comment:

Ok, so closing as rejected.

--
resolution:  -> rejected
status: open -> closed

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[issue10682] With '*args' or even bare '*' in def/call argument list, trailing comma causes SyntaxError

[Brett Cannon]

Brett Cannon  added the comment:

I'm with Raymond; this is unneeded consistency. I honestly would rather see 
what little support there is for a trailing comma to go away, but w/o looking 
at the grammar I am willing to bet that would be a pain to get right and not be 
worth the effort.

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nosy: +brett.cannon

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[issue10682] With '*args' or even bare '*' in def/call argument list, trailing comma causes SyntaxError

[Raymond Hettinger]

Raymond Hettinger  added the comment:

The current behavior for function definitions is beneficial because a trailing 
comma in the argument list is likely to signal a real error (omitted variable).

In contrast, the trailing comma for lists is useful because entries on separate 
lines in a repeating pattern that makes it easy to add or remove entries.

The use cases for both function definitions and lists are best served by the 
current behavior, more so that a notion driven by "foolish consistency".

Recommend rejecting and closing.

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nosy: +rhettinger

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[issue10682] With '*args' or even bare '*' in def/call argument list, trailing comma causes SyntaxError

[R. David Murray]

Changes by R. David Murray :


--
nosy: +mark.dickinson

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[issue10682] With '*args' or even bare '*' in def/call argument list, trailing comma causes SyntaxError

[Martin v . Löwis]

Martin v. Löwis  added the comment:

It's clearly not a bug: the documented grammar matches the implemented grammar. 
Asking for more abstract consistency is a feature request. This also clearly 
falls under the PEP 3003 moratorium.

--
nosy: +loewis
type: behavior -> feature request
versions:  -Python 2.7, Python 3.1, Python 3.2

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[issue10682] With '*args' or even bare '*' in def/call argument list, trailing comma causes SyntaxError

[Jan Kaliszewski]

Jan Kaliszewski  added the comment:

Oops, I see the problem was partly addressed @ issue9232.

But:

* the proposed patch doesn't fix calls (but defs only),

* shouldn't be the issue considered as a bug -- and fixed also in 2.7 and 3.1?

--

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[issue10682] With '*args' or even bare '*' in def/call argument list, trailing comma causes SyntaxError

[Jan Kaliszewski]

Jan Kaliszewski  added the comment:

s/**{5: 6}/**{'5': 6}/g (but it's a detail)

--

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[issue10682] With '*args' or even bare '*' in def/call argument list, trailing comma causes SyntaxError

[Jan Kaliszewski]

New submission from Jan Kaliszewski :

Let examples speak:

def x(a, z): pass  # this is ok
def x(a, z,): pass # this is ok
def x(a, *, z): pass   # this is ok in Py3k
def x(a, *, z,): pass  # but this causes SyntaxError (!)

def x(a, *args): pass  # this is ok
def x(a, *args,): pass # but this causes SyntaxError
def x(a, **kwargs): pass   # this is ok
def x(a, **kwargs,): pass  # but this causes SyntaxError
def x(a, *args, z): pass   # this is ok in Py3k
def x(a, *args, z,): pass  # but this causes SyntaxError (!)

And similarly -- calls:

def x(*args, **kwargs): pass
x(1, *(2,3,4))# this is ok
x(1, *(2,3,4),)   # but this causes SyntaxError
x(1, **{5: 6})# this is ok
x(1, **{5: 6},)   # but this causes SyntaxError
x(1, 2, 3, 4, z=5)# this is ok
x(1, *(2,3,4), z=5)   # this is ok in Py2.6+ and Py3k
x(1, *(2,3,4), z=5,)  # but this causes SyntaxError (!)

Similar problem was discussed years ago as a docs bug -- see: issue798652, but 
-- as inconsistent with intuition and a general Python rule that trailing 
commas are ok in argument lists and sequence literals (except empty ones) -- it 
seems to be rather a language syntax definition issue.

Now, after introducing new syntax possibilities in Py2.6 and especially Py3k 
(see the examples above), the issue is even much more painful.

Also, please consider that Py3k's function annotations encourage to format 
argument list in one-argument-per-line-manner:

def my_func(
spam:"Very tasty and nutritious piece of food",
ham:"For experts only",
*more_spam:"Not less tasty and not less nutritious!",
spammish:"Nobody expects this!"='Inquisition',
):
...

--
components: Interpreter Core
messages: 123823
nosy: zuo
priority: normal
severity: normal
status: open
title: With '*args' or even bare '*' in def/call argument list, trailing comma 
causes SyntaxError
type: behavior
versions: Python 2.7, Python 3.1, Python 3.2, Python 3.3

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