Guido van Rossum wrote:
> On Fri, Apr 16, 2010 at 4:38 PM, Steve Holden <st...@holdenweb.com> wrote:
>> I'm sure we wouldn't want to go so far as to inhibit this. (Py 3.1)
>>
>>>>> def f(**kwargs):
>> ... kwargs[1] = "dummy"
>> ... print(kwargs)
>> ...
>>>>> f(this="Guido", that="Raymond", the_other="Steve")
>> {'this': 'Guido', 1: 'dummy', 'the_other': 'Steve', 'that': 'Raymond'}
>> Or would we? If it's OK to mutate kwargs inside the function to contain
>> a non-string key, why isn't it OK to pass a non-string key in?
>
> Because Python promises that the object the callee sees as 'kwargs' is
> "just a dict". But the requirement for what the caller passes in is
> different: it must pass in a dict whose keys represent argument names.
>
> If you want an API where you can pass in an arbitrary dict to be
> received unchanged, don't use **kw. Remember that the caller can
> independently decide whether or not to use the **kw notation -- there
> is no implied correspondence between the caller's use of **kw and the
> callee's use of it. Note this example:
>
> def f(a, b, **k):
> print(a, b, k)
>
> d = {'a': 1, 'b': 2, 'c': 3}
> f(**d)
>
> This will print
>
> 1 2 {'c': 3}
>
> Note that the k received by f is not the same as the d passed in! (And
> yet d of course is not modified by the operation.)
>
Good point, and one I hadn't thought of. I was blithely assuming that
the dict seen by the called function was always what was passed as the
dict argument.
>> I understand that it couldn't be generated using keyword argument
>> syntax, but I don't see why we discriminate against f(**dict(...)) to
>> limit it to what could be generated using keyword argument syntax. Is
>> this such a big deal?
>
> Is portability of code to Jython, IronPython, PyPy a big deal?
> According to a slide you recently posted to the PSF board list, it is.
>
And I haven't changed my mind since. Thanks for the comprehensive response.
regards
Steve
--
Steve Holden +1 571 484 6266 +1 800 494 3119
See PyCon Talks from Atlanta 2010 http://pycon.blip.tv/
Holden Web LLC http://www.holdenweb.com/
UPCOMING EVENTS: http://holdenweb.eventbrite.com/
_______________________________________________
Python-Dev mailing list
Python-Dev@python.org
http://mail.python.org/mailman/listinfo/python-dev
Unsubscribe:
http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
