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

[Edward Loper]

Fredrik Lundh wrote:
>> And again, why would you *make* me, the user-programmer, type
>>
>> make_person(name=namex, age=agex, phone=phonex, location = locationx)
>> #instead of
>> make_person(namex,agex,phonex,locationx)
>> ?
> 
> because a good API designer needs to consider more than just the current
> release.

I believe that it's quite possible that you're right, but I think more 
concrete answers would be helpful.  I.e., how does having parameters 
that are syntactically keyword-only (as opposed to being simply 
documented as keyword-only) help you develop an API over time?

I gave some thought to it, and can come up with a few answers.  In all 
cases, assume a user BadUser, who decided to use positional arguments 
for arguments that you documented as keyword-only.

- You would like to deprecate, and eventually remove, an argument to
   a function.  For people who read your documentation, and use
   keyword args for the keyword-only arguments, their code will break
   in a clean, easy-to-understand way.  But BadUser's code may break in
   strange hard-to-understand ways, since their positional arguments will
   get mapped to the wrong function arguments.

- You would like to add a new parameter to a function, and would like
   to make that new parameter available for positional argument use.
   So you'd like to add it before all the keyword arguments.  But this
   will break BadUser's code.

- You have a function that takes one argument and a bunch of keyword
   options, and would like to change it to accept *varargs instead of
   just one argument.  But this will break BadUser's code.

I think that this kind of *concrete* use-case provides a better 
justification for the feature than just saying "it will help API 
design."  As someone who has had a fair amount of experience designing & 
maintaining APIs over time, perhaps you'd care to contribute some more 
use cases where you think having syntactically keyword-only arguments 
would help?

-Edward