[Python-Dev] A grammatical oddity: trailing commas in argument lists -- continuation

Jan Kaliszewski zuo at chopin.edu.pl
Mon Dec 13 12:44:49 CET 2010

Dear Python Developers,

It is s my first post to python-dev, so let me introduce myself briefly:
Jan Kaliszewski, programmer and composer, sometimes also NGO activist.

Coming to the matter... The discussion started with remark by Mark
Dickinson about such a syntax oddity:

> def f(a, b,): ...     is fine, but
> def f(*, a, b,): ...  is a SyntaxError

Then some other similar oddities were pointed at (*args/**kwargs-related
ones as well as calls like f(*, a=3,) causing SyntaxError too).

* http://mail.python.org/pipermail/python-dev/2010-July/101636.html
* http://bugs.python.org/issue9232
* http://bugs.python.org/issue10682

But yesterday both mentioned issues has been closed as rejected -- with
suggestion that it would probably require a PEP to modify Python in this
aspect (as there is no clear consensus). So I'd opt for re-opening the
discussion -- I suppose that more people could be interested in solving
the issue (at least after the end of PEP 3003 moratorium period).

I think that seeing that:

    def f(a, b): ...
    def f(a, b,): ...
    def f(a, *, b): ...
    def f(a, *args, b): ...
    x(1, 2, 3, 4, z=5)
    x(1, 2, 3, 4, z=5,)
    x(1, *(2,3,4), z=5)

...are ok, then --

    def f(a, *, b,): ...
    def f(a, *args, b,): ...
    x(1, *(2,3,4), z=5,): ...

...should be ok as well, and consequently --

    def f(a, *args,): ...
    def f(a, **kwargs,): ...
    x(1, *(2,3,4),)
    x(1, **dict(z=6),)

...should also be ok.

Please also note that Py3k's function annotations make one-def-argument-
-per-line formattig style the most suitable in some cases, e.g.:

    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_inquisition:"Nobody expects this!",
    ) -> "Spam, spam, spam, spam, spam, spam, spam, spam, spam, spam":


Jan Kaliszewski

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