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Re: Python keyword args can be any string

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Steven D'Aprano <steve@pearwood.info> writes:

> A work colleague wanted to pass an argument starting with "-" to a
> function.
>
> Apparently he didn't have a specific argument in mind. He just wanted
> to test the function to breaking point by passing invalid argument
> names.

That seems a reasonable test.

    >>> kwargs = {
    ...         'spam': 13,
    ...         'eggs': 17,
    ...         '-beans': 11,
    ...         }
    >>> foo(**kwargs)    # Should this fail?

It exposes a design smell; by capturing ‘**kwargs’, a function has
deiberately not specified which keyword arguments it will accept.

    >>> def foo_captures_kwargs(*args, **kwargs):
    ...     for (name, value) in kwargs.items():
    ...         print("Got argument {name}={value!r}".format(
    ...                 name=name, value=value))
    ... 
    >>> foo_captures_kwargs(**kwargs)
    Got argument eggs=17
    Got argument spam=13
    Got argument -beans=11

Still not a bug in Python IMO. It may be a bug in the program; the
design of the function doesn't provide any way to know.

Perhaps the design can be improved by not using ‘**kwargs’ at all, and
instead using a specific set of keyword-only arguments.

    >>> def foo_names_every_argument(*, spam="Lorem", eggs="ipsum"):
    ...     print("Got argument {name}={value!r}".format(
    ...             name='spam', value=spam))
    ...     print("Got argument {name}={value!r}".format(
    ...             name='eggs', value=eggs))
    ... 
    >>> foo_names_every_argument(**kwargs)
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    TypeError: foo_names_every_argument() got an unexpected keyword argument '-beans'

This is IMO another good reason to migrate ASAP to Python 3; better
design is easier that before.

-- 
 \          “Judge: A law student who marks his own papers.” —Henry L. |
  `\                                                           Mencken |
_o__)                                                                  |
Ben Finney

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Thread

Python keyword args can be any string Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2016-02-18 16:42 +1100
  Re: Python keyword args can be any string Chris Angelico <rosuav@gmail.com> - 2016-02-18 16:57 +1100
  Re: Python keyword args can be any string Ben Finney <ben+python@benfinney.id.au> - 2016-02-18 16:59 +1100
  Re: Python keyword args can be any string Terry Reedy <tjreedy@udel.edu> - 2016-02-18 01:31 -0500
  Re: Python keyword args can be any string Mark Lawrence <breamoreboy@yahoo.co.uk> - 2016-02-18 07:55 +0000
    Re: Python keyword args can be any string Steven D'Aprano <steve@pearwood.info> - 2016-02-19 11:03 +1100
      Re: Python keyword args can be any string Ben Finney <ben+python@benfinney.id.au> - 2016-02-19 11:58 +1100
  Re: Python keyword args can be any string Rustom Mody <rustompmody@gmail.com> - 2016-02-19 03:41 -0800

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