[Python-Dev] Order of positional and keyword arguments

[Paul Moore]

I see no practical benefit to making such a restriction, and there's a
risk of breaking existing code.

While it's not something I've ever used myself in real code,

    def wrapper(*args, **kw):
        return wrapped_fn(some_arg=1, *args, **kw)

seems like a perfectly reasonable way to write a wrapper to me. I'm
against breaking this for no better reason than "to simplify the
grammar" and to exclude cases that some people might find confusing (I
actually found your example perfectly comprehensible, if convoluted).

-1 from me.

Paul


On 26 April 2018 at 20:25, Serhiy Storchaka <storchaka at gmail.com> wrote:
> There is an inconsistence in passing arguments to functions.
>
> Explicit positional arguments should precede keyword arguments (both
> explicit and variable), but variable positional arguments can follow
> explicit keyword arguments and precede variable keyword arguments.
>
> For example, for function
>
>     def f(a=None, b=None):
>         return a, b
>
> the following is valid syntax:
>
>     f(1, b=2)
>     f(1, **{'b': 2})
>     f(*[1], b=2)
>     f(*[1], **{'b': 2})
>     f(b=2, *[1])
>
> but the following is an error:
>
>     f(b=2, 1)
>     f(**{'b': 2}, 1)
>     f(**{'b': 2}, *[1])
>
> f(b=2, *[1]) is surprised in two ways:
>
> 1. Argument values are passed not in order. The first value is assigned to
> the second parameter, and the second value is assigned to the first
> parameter.
>
> 2. Argument values are evaluated not from left to right. This contradicts
> the general rule that expressions are evaluated from left to right (with few
> known exceptions).
>
> I never seen the form `f(b=2, *[1])` in practice (though the language
> reference contains an explicit example for it), and it looks weird to me. I
> don't see reasons of writing `f(b=2, *[1])` instead of more natural `f(*[1],
> b=2)`. I propose to disallow it.
>
> This will also make the grammar simpler. Current grammar:
>
>    argument_list: `positional_arguments` ["," `starred_and_keywords`]
>                 :   ["," `keywords_arguments`]
>                 : | `starred_and_keywords` ["," `keywords_arguments`]
>                 : | `keywords_arguments`
>    positional_arguments: ["*"] `expression` ("," ["*"] `expression`)*
>    starred_and_keywords: ("*" `expression` | `keyword_item`)
>                 : ("," "*" `expression` | "," `keyword_item`)*
>    keywords_arguments: (`keyword_item` | "**" `expression`)
>                 : ("," `keyword_item` | "," "**" `expression`)*
>    keyword_item: `identifier` "=" `expression`
>
> Proposed grammar:
>
>    argument_list: `positional_arguments` ["," `keywords_arguments`]
>                 : | `keywords_arguments`
>    positional_arguments: ["*"] `expression` ("," ["*"] `expression`)*
>    keywords_arguments: `keyword_argument` ("," `keyword_argument`)*
>    keyword_argument: `identifier` "=" `expression` | "**" `expression`
>
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