On Sun, Feb 10, 2019 at 2:18 PM Avi Gross <avigr...@verizon.net> wrote: > I am not sure how python implements some of the functionality it does as > compared to other languages with similar features. But I note that there are > rules, presumably some for efficiency, such as requiring all keyword > arguments to be placed after the last positional argument. I mean I can call > func(a,b,c,d=1,e=2) but not func(a,b,d=1, c, e=2). > > So if you allowed a keyword to optionally be used for any positional > argument other than the last, c, would it not require a change in this rule? > I mean func(a="invalid",b,c,d=1,e=2) would violate the rule and so would > making b a keyword version. In my example, and for this reason only, maybe c > could work.
My suggestion was not to allow keyword arguments to arbitrarily replace any positional parameter, or to otherwise change argument-passing in any way. The suggestion was to drop positional-only arguments from functions implemented in C instead of just documenting them better. In the example above, if you want to pass a by keyword, you would have to pass b and c by keyword as well. That would still be true for functions implemented in C if a, b, and c are no longer positional-only. > The original motivation for keyword arguments included the concept of > specifying a default if not used. Positional arguments have no default. > Similarly, they are required if explicitly named in the function definition. > So we are intermingling multiple concepts in the previous design. Positional arguments are allowed to have defaults, and keyword-only arguments are allowed to not have defaults. These are all valid syntax: # Allows a and b to be passed either positionally or by keyword def foo(a=1, b=2): pass # b is keyword-only def foo(a=1, *, b=2): pass # Allows a and b to be passed either positionally or by keyword def foo(a, b): pass # b is keyword-only and has no default def foo(a, *, b): pass Positional-ONLY arguments are not directly supported by the language, but they exist in the C API and can also have defaults. For example, dict.get takes two positional-only arguments. The second argument is optional, and its default is None. My point is that the axis of positional-only versus positional-or-keyword versus keyword-only, and the axis of required versus optional are entirely separate. > I won't go on except to say it would break lots of existing code and > potentially complicate new code. Can you give an example of something that would be broken by updating C API functions to name positional-only arguments instead of just updating them to document that they're positional-only? -- https://mail.python.org/mailman/listinfo/python-list
