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Fri Oct 12 03:08:32 EDT 2018 · https://mail.python.org/pipermail/python-dev/2018-October/155481.html

On Thu, Oct 4, 2018 at 10:58 AM Steven D'Aprano <steve at pearwood.info> wrote:

> 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.
>
>
I would expect this to be costly/annoying for implementations to enforce,
doing it at call time is probably too late to be efficient, it would need
help from dicts themselves or even strings.

A hack that currently works because of this is with dict itself:

>>> d = {'a-1': 1, 'a-2': 2, 'a-3': 3}
>>> d1 = dict(d, **{'a-2': -2, 'a-1': -1})
>>> d1 is d
False
>>> d1
{'a-1': -1, 'a-2': -2, 'a-3': 3}
>>>
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[Python-Dev] Arbitrary non-identifier string keys when using **kwargs