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On Thu, Oct 4, 2018 at 10:58 AM Steven D'Aprano <steve@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}

>>>