[Python-Dev] Order of positional and keyword arguments (original) (raw)

Serhiy Storchaka storchaka at gmail.com
Thu Apr 26 15:25:09 EDT 2018

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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