[Python-Dev] PEP 3102: Keyword-only arguments (original) (raw)

"Martin v. Löwis" martin at v.loewis.de
Mon May 1 07:46:55 CEST 2006

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

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