[Python-Dev] PEP 3102: Keyword-only arguments

["Martin v. Löwis"]

Terry Reedy wrote:
>> Are you asking why that feature (keyword-only arguments) is desirable?
>> That's the whole point of the PEP. Or are you asking why the user
>> shouldn't be allowed to pass keyword-only arguments by omitting the
>> keyword? Because they wouldn't be keyword-only arguments then, anymore.
> 
> There are two subproposals: first, keyword-only args after a variable 
> number of positional args, which requires allowing keyword parameter 
> specifications after the *args parameter, and second, keyword-only args 
> after a fixed number number of positional args, implemented with a naked 
> '*'.  To the first, I said "The rationale for this is pretty obvious.".  To 
> the second, I asked, and still ask, "Why?".

One reason I see is to have keyword-only functions, i.e. with no
positional arguments at all:

def make_person(*, name, age, phone, location):
    pass

which also works for methods:

    def make_person(self, *, name, age, phone, location):
        pass

In these cases, you don't *want* name, age to be passed in a positional
way. How else would you formulate that if this syntax wasn't available?
(I know it is possible to formulate it elsehow, I'm asking what notation
you would use)

> Again: if a function has a fixed number n of params, why say that the first 
> k can be passed by position, while the remaining n-k *must* be passed by 
> name?

I see an important use case for k=0 functions, and k=1 methods (where
the only positional argument is self).

Regards,
Martin