[Python-Dev] Positional-only parameters in Python

[Larry Hastings]

On 01/17/2018 08:29 AM, Ethan Furman wrote:
> On 01/17/2018 08:14 AM, Serhiy Storchaka wrote:
>> 17.01.18 16:34, Victor Stinner пише:
>>> In Februrary 2017, I proposed on python-ideas to change the Python
>>> syntax to allow to declare positional-only parameters in Python:
>>>
>>> https://mail.python.org/pipermail/python-ideas/2017-February/044879.html 
>>>
>>> https://mail.python.org/pipermail/python-ideas/2017-March/044956.html
>>
>> The main problem -- designing a syntax that does not look ugly.
>
> The syntax question is already solved:
>
>    def some_func(a, b, /, this, that, *, the_other):
>        # some stuff
>
> Everything before the slash is positional-only, between the slash and 
> star is positional-or-keyword, and after the star is keyword-only.  
> This is what is in our generated help(), and there is a nice symmetry 
> between '/' and '*' being opposites, and positional/keyword being 
> opposites.  And slash is certainly no uglier than star.  ;)

To clarify: this is the syntax used by "Argument Clinic", both as its 
input language, and as part of its output, exposed via the 
__text_signature__ attribute on builtins.

Why did Argument Clinic choose that syntax?  It was suggested by one 
Guido van Rossum in March 2012:

    https://mail.python.org/pipermail/python-ideas/2012-March/014364.html
    https://mail.python.org/pipermail/python-ideas/2012-March/014378.html
    https://mail.python.org/pipermail/python-ideas/2012-March/014417.html


I'm not wading into the debate over what syntax Python should use if it 
adds positional-only parameters, except to say that I think "/" is 
reasonable.  If Python winds up using a different syntax, I'd look into 
modifying Argument Clinic so that it accepts both this hypothetical new 
syntax and the existing syntax using "/".



Would we be adding yet a third argument-parsing function, 
PyArg_ParseTupleAndKeywordsWithPositionalOnly()?  I would actually 
propose a different approach: modify Argument Clinic so it generates 
custom argument-parsing code for each function, adding a new call type 
(which I propose calling "METH_RAW" or "METH_STACK") where the stack is 
passed in directly.  I spent some time on this in the past, though I got 
distracted and now haven't touched it in years.



Cheers,


//arry/
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