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

Mark Dickinson dickinsm at gmail.com
Fri Jul 9 22:26:21 CEST 2010

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

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. Note that (1) and (2) aren't valid (and never have been, as far as I know) in Python 2.x, though. (Neither is (3), of course, since keyword-only arguments are 3.x only.)

(3) is a bit subtle: "def f(*): pass" is actually allowed by the grammar, but produces a SyntaxError later on, when the parse tree is converted to AST:

>>> def f(*): pass
...
  File "<stdin>", line 1
SyntaxError: named arguments must follow bare *

So it probably doesn't matter much whether (3) is permitted by the grammar or not.

-- Mark

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