[Python-Dev] Arbitrary non-identifier string keys when using **kwargs

[Chris Barker]

On Sun, Oct 7, 2018 at 11:42 AM João Santos <jmcs at jsantos.eu> wrote:

> *locals *and *globals* are documented as dictionaries (for example exec's
> documentation states that " If only *globals* is provided, it must be a
> dictionary")
>

well, that is specifically about exec() -- it may or may not apply to
everywhere nameapaces are used in the interpreter...


> but __dict__ is described as " [a] dictionary or other mapping object".
>

exactly.

-CHB

On Sun, 7 Oct 2018 at 19:38, Chris Barker via Python-Dev <
> python-dev at python.org> wrote:
>
>> On Fri, Oct 5, 2018 at 3:01 PM Brett Cannon <brett at python.org> wrote:
>>
>>> I'm also fine with saying that keys in **kwargs that are not proper
>>> identifiers is an implementation detail.
>>>
>>
>> It's not just **kwargs -- you can also use arbitrary names with setattr()
>> / getattr() :
>>
>> In [6]: setattr(foo, "4 not an identifier", "this works")
>>
>> In [7]: getattr(foo, "4 not an identifier")
>> Out[7]: 'this works'
>>
>> Which brings up a question I've had for years -- is the fact that cPython
>> uses a regular old dict for namespaces (and **kwargs) part of the language
>> spec, or an implementation detail?
>>
>> I would say that for the get/setattr() example, it is kinda handy when
>> you want to use a class instance to model some external data structure that
>> may have different identifier rules. Though I tend to think that's mingling
>> data and code too much.
>>
>> -CHB
>>
>>
>>>
>>> On Thu, 4 Oct 2018 at 02:20, Serhiy Storchaka <storchaka at gmail.com>
>>> wrote:
>>>
>>>> 04.10.18 11:56, Steven D'Aprano пише:
>>>> > While keyword arguments have to be identifiers, using **kwargs allows
>>>> > arbitrary strings which aren't identifiers:
>>>> >
>>>> > py> def spam(**kwargs):
>>>> > ....     print(kwargs)
>>>> > ....
>>>> > py> spam(**{"something arbitrary": 1, '\n': 2})
>>>> > {'something arbitrary': 1, '\n': 2}
>>>> >
>>>> >
>>>> > There is some discussion on Python-Ideas on whether or not that
>>>> > behaviour ought to be considered a language feature, an accident of
>>>> > implementation, or a bug.
>>>> >
>>>> > Can we get some guidence on this please?
>>>>
>>>> This is an implementation detail. Currently CPython doesn't ensure that
>>>> keyword argument names are identifiers for performance reasons. But
>>>> this
>>>> can be changed in future versions or in other implementations.
>>>>
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>>
>>
>> --
>>
>> Christopher Barker, Ph.D.
>> Oceanographer
>>
>> Emergency Response Division
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>

-- 

Christopher Barker, Ph.D.
Oceanographer

Emergency Response Division
NOAA/NOS/OR&R            (206) 526-6959   voice
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