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Mon Jan 26 20:04:25 CET 2015 · https://mail.python.org/pipermail/python-dev/2015-January/137911.html

On 01/26/2015 10:55 AM, Ethan Furman wrote:
> On 01/26/2015 10:40 AM, Paul Moore wrote:
> 
>> There *are* some nastily non-intuitive corner cases (for example, if
>> from_env={'a':12} and from_config={'a':13}, I don't have any sort of
>> intuition as to what a would be in f(**from_env, **from_config). I'd
>> go with 12 because the PEP links multiple **-unpackings with
>> collections.ChainMap, but I wouldn't dare rely on that guess).
> 
> In the your example
> 
>   from_env = {'a': 12}
>   from_config = {'a': 13}
> 
>   f(**from_env, **from_config)
> 
> I would think 'a' should be 13, as from_config is processed /after/ from_env.
> 
> So which is it?

Going to the PEP:

  kwargs = dict(kw_arguments)
  kwargs.update(more_arguments)
  function(**kwargs)

  or, if you know to do so:

  from collections import ChainMap
  function(**ChainMap(more_arguments, arguments))

I see the arguments to ChainMap are reversed (more_arguments is first), so in Paul's example 'a' would be 13.

--
~Ethan~

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[Python-Dev] Any grammar experts?