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

Terry Reedy tjreedy at udel.edu
Fri Jul 9 23:59:58 CEST 2010

On 7/9/2010 4:26 PM, Mark Dickinson wrote:

On Fri, Jul 9, 2010 at 8:37 PM, Dino Viehland<dinov at microsoft.com> wrote:

Terry wrote:

This violates the important principle that allowed def and call arg sequences should match to the extent sensible and possible. In this sense, the SyntaxError is a bug. So I would fix this now for 3.2 and notify the other implementors.

+1 on fixing it - trailing commas are awesome. I'm always annoyed in C# where I frequently can't use them. This seems like a bug fix level change that should be easy for the other implementations to fix. Thanks for all the feedback. If the grammar is changed to allow "def f(*, a,): pass", that still leaves some more open questions: which of the following should be valid? (1) def f(*args,): pass (2) def f(**kwargs,): pass (3) def f(*,): pass

I would follow the same principle of making def and call match. Currently

def f(*args,): pass

SyntaxError: invalid syntax

def f(*args): pass

f(*(1,2,3),) SyntaxError: invalid syntax

Giving the call doc "A trailing comma may be present after the positional and keyword arguments but does not affect the semantics.", I was not really expecting the second syntax error. Same behavior for f(**k,).

So if you allow the definition, allow the call too.

Just for the sake of simplicity it would seem to make sense allow all these, even if there's no obvious immediate use; for me, it keeps the mental footprint of the language small---I don't have to remember when the comma is or isn't allowed.

Agreed.

-- Terry Jan Reedy

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