[Python-Dev] Arbitrary non-identifier string keys when using **kwargs (original) (raw)

João Santos jmcs at jsantos.eu
Sun Oct 7 14:42:02 EDT 2018

*locals *and globals are documented as dictionaries (for example exec's documentation states that " If only globals is provided, it must be a dictionary") but dict is described as " [a] dictionary or other mapping object".

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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