Issue 3473: In function call, keyword arguments could follow *args

functions with keyword-only arguments have this form:
    def f(x, *args, y):
        pass
parameters can appear after the *arg, they are required to be passed by
keyword.

It would be more consistent to allow this function call:
    f(X, *ARGS, y=Y)
This is invalid syntax, *ARGS is required to be at the end of the
arguments, together with an eventual **KWARGS. This restriction should
be lifted.

See the use case in
http://mail.python.org/pipermail/python-3000/2008-July/014437.html

Should this apply to 2.6 as well? See r65321, I find the last line
easier to read when arguments are in this order.

    def grouper(n, iterable, fillvalue=None):
        args = [iter(iterable)] * n
        return izip_longest(*args, fillvalue=fillvalue)

On the cons side, keyword-only arguments don't exist in 2.6, so the
consistency with function definition syntax does not apply.

+1 for applying to 2.6.  

izip_longest() is a perfect example of where it's important.

Patches for both versions are attached.

The patches look good to me.

I'll have a look at this in the next day or two.

I will not have internet access for the next week. 
Raymond, would you take care of this issue?

Another use case: upon reading A.Baxter's Porting to 3 talk, I realized,
slightly generalizing from his example, that
print(s.join(map(str,it))) == print(*it,sep=s) -- or would, except that
it currently has to be written non-intuitively as print(sep=s,*it),
which I might not have tried except for knowing about this issue.

Given that many have problems with .join and that most uses are to
produce immediate output not otherwise processed, I think having the
replacement work in the way many would expect would be a win.
So I hope this makes the next beta.

Ping!

Barry, is it still time for this to be included in 2.6b3?
Guido already approved the idea:
http://mail.python.org/pipermail/python-3000/2008-July/014506.html

Guido's approved it, so please go ahead and add it before beta 3.

Applied for 2.6 in r65872.

Done for py3k in r65877.

Now test_compiler is breaking for us because the compiler package can't
handle the change.

The compiler package was fixed some time ago with r65891

The documentation change in this patch introduced a bug in the Call grammar:

| "*" `expression` ["," "*" `expression`] ["," "**" `expression`]

instead of

| "*" `expression` ["," `keyword_arguments`] ["," "**" `expression`]

giving the impression that `*expression` is allowed twice.

New changeset 3ae399c6ecf6 by Benjamin Peterson in branch '2.7':
correct call grammar error (#3473)
http://hg.python.org/cpython/rev/3ae399c6ecf6