Python 3 grammar, function parameters

[Chris Rebert]

On Mon, Jul 12, 2010 at 3:32 PM, Junkman <j at junkwallah.org> wrote:
> Greetings to Python users,
>
> I'm trying to parse Python code using the grammar supplied with the
> documentation set, and have a question on the grammar for function
> parameters:
>
> funcdef: 'def' NAME parameters ['->' test] ':' suite
> parameters: '(' [typedargslist] ')'
> typedargslist: ((tfpdef ['=' test] ',')*
>                ('*' [tfpdef] (',' tfpdef ['=' test])* [',' '**' tfpdef]
> | '**' tfpdef)
>                | tfpdef ['=' test] (',' tfpdef ['=' test])* [','])
> tfpdef: NAME [':' test]
>
> >From what I understand, a naked asterisk - i.e. it is not a prefix to an
> identifier - is not a valid parameter, but the grammar  explicitly
> allows it by making the  identifier that immediately follows the
> asterisk optional.
>
> Are there cases where naked asterisk is allowed as a function
> parameter?

Yes, for keyword-only arguments, a new feature in Python 3.x. See PEP
3102 (http://www.python.org/dev/peps/pep-3102/ ).
A lone asterisk signals that the function does not take extra
positional arguments. All keyword-only arguments must be declared
after a lone or normal *-argument.

Example:
def compare(a, b, *, key=None):

compare() does not accept extra positional arguments and has 1
keyword-only argument (namely, `key`).

Cheers,
Chris
--
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