[Python-Dev] A grammatical oddity: trailing commas in argument lists -- continuation (original) (raw)

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).

References:

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":
    ...

Regards,

Jan Kaliszewski

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