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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