[Python-3000] PEP 3102

[Dj Gilcrease]

On Fri, Feb 15, 2008 at 8:46 AM, Joel Bender <jjb5 at cornell.edu> wrote:
>  How about a BNFish notation?  Use brackets around optional parameters.
>
>      def test([arg1, [arg2,]] arg3):
>          ...

If I understand right, positional only arguments would not be optional
in any way shape or form, thats what default arguments are for.
Positional only args would be arguments that you cannot specify by
keyword, so having a positional only arg that has a default value,
thus making it optional just seems broken to me.

The notation you recommended just confuses the hell outa me when
reading it, it is neither clean nor elegant for the stated intent,
which is a method of defining positional only arguments.

eg.
def test([arg1=1, arg2=2,] arg3): return (arg1, arg2, arg3)
    assert test(9) == (1, 2, 9)
    assert test(7, 8, 9) == (7, 8, 9)
    test(8, 9) == Exception
    test(arg3=9) == ???

This example clearly shows what confuses me about this notation. arg1
and arg2 are supposed to be positional only, but your first example
the first argument you set end up in the 3rd position for no reason
other then you defined default arguments before the positional
arguments, which should and does raise "SyntaxError: non-default
argument follows default argument"


The intent is to make it so (Assuming arg1 and arg2 are positional
only) you cannot do test(arg3=9, arg1=7, arg2=8), thus you would be
required to do test(7, 8, 9) {since doing test(7, 8, arg3=9) would
raise an error of positional arg being set by position and keyword at
the same time}