[Python-ideas] Syntax for allowing extra keys when unpacking a dict as keyword arguments

[Eric V. Smith]

Viktor is looking for something at the call site, not the function 
definition site (which he might not control).

I wrote calllib (horrible name, I know). It can do this, although I just 
noticed it needs updating for keyword-only params.

But, here it is without keyword-only params:

 >>> from calllib import apply
 >>> def func(a):
...   print(f'a={a!r}')
...
 >>> func(**{'a': 1, 'b': 2})
Traceback (most recent call last):
   File "<stdin>", line 1, in <module>
TypeError: func() got an unexpected keyword argument 'b'
 >>> apply(func, {'a': 1, 'b': 2})
a=1

I don't claim it's an efficient solution, since it inspects the callable 
every time to extract its params. But it will work for this use case. Or 
at least it will when I update it for keyword-only params. It works 
today without keyword-only params.

https://pypi.org/project/calllib/

Eric

On 4/12/2019 12:17 PM, Lucas Bourneuf wrote:
> Hello !
> 
> I made *fief*, a small python package allowing just that using a decorator:
> 
>      from fief import filter_effective_parameters as fief
> 
>      @fief
>      def func(a, b):
>          # some implementation
> 
>      # and then, use it as you want to:
>      func(**MY_BIG_CONFIG_DICT_WITH_MANY_WEIRD_KEYS)
> 
> The code is quite simple. You may want to use it, with modifications (i didn't touch the code for months, maybe years ; it could probably be improved now).
> Link: https://github.com/aluriak/fief
> 
> The code:
> 
> def filter_effective_parameters(func):
>      """Decorator that filter out parameters in kwargs that are not related to
>      any formal parameter of the given function.
>      """
>      @wraps(func)
>      def wrapper(*args, **kwargs):
>          formal_parameters = frozenset(signature(func).parameters.keys())
>          return func(*args, **{
>              arg: value
>              for arg, value in kwargs.items()
>              if arg in formal_parameters
>          })
>      return wrapper
> 
> Best regards,
> --lucas
> 
> 
> 
> ----- Mail original -----
>> De: "Viktor Roytman" <viktor.roytman at gmail.com>
>> À: "python-ideas" <python-ideas at googlegroups.com>
>> Envoyé: Vendredi 12 Avril 2019 18:01:43
>> Objet: Re: [Python-ideas] Syntax for allowing extra keys when unpacking a dict as keyword arguments
> 
>> I could see this being an option, but to someone unfamiliar with it, it
>> might seem strange that * unpacks iterables, ** unpacks dicts, and *** is a
>> special thing only for keyword arguments that mostly behaves like **.
>>
>> On Friday, April 12, 2019 at 11:26:37 AM UTC-4, Bruce Leban wrote:
>>>
>>>
>>> On Fri, Apr 12, 2019, 8:12 AM Viktor Roytman <viktor... at gmail.com
>>> <javascript:> wrote:
>>>
>>>>
>>>>      >>> func(**{'a': 1, 'b': 2})
>>>>      Traceback (most recent call last):
>>>>        File "<stdin>", line 1, in <module>
>>>>      TypeError: func() got an unexpected keyword argument 'b'
>>>>
>>>
>>> Perhaps func(***kws)?
>>>
>>> I think this is a real problem given the frequent convention that you can
>>> freely add fields to json objects with the additional fields to be ignored.
>>>
>>
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