[Python-ideas] Keyword only argument on function call

David Mertz mertz at gnosis.cx
Sat Sep 8 10:05:40 EDT 2018

On Sat, Sep 8, 2018 at 9:34 AM Anders Hovmöller <boxed at killingar.net> wrote:

>     function(a=my_a, c=my_c, *, b, d)
>     function(*, b, c, d, a=my_a, c=my_c)

Yes, those look less bad.  They are also almost certainly should get this
message rather than working:

    TypeError: function() got multiple values for keyword argument 'c'

But they also force changing the order of keyword arguments in the call.
That doesn't do anything to the *behavior* of the call, but it often
affects readability.

For functions with lots of keyword arguments there is often a certain
convention about the order they are passed in that readers expect to see.
Those examples of opening and reading files that several people have given
are good examples of this.  I.e. most optional arguments are not used, but
when they are used they have certain relationships among them that lead
readers to expect them in a certain order.

Here's a counter-proposal that does not require any new syntax.  Is there
ANYTHING your new syntax would really get you that this solution does not
accomplish?! (other than save 4 characters; fewer if you came of with a one
character name for the helper)

>>> def function(a=11, b=22, c=33, d=44):
...     print(a, b, c, d)
>>> a, b, c = 1, 2, 3
>>> function(a=77, **use('b d'))
77 2 33 None

We could implement this helper function like this:

>>> def use(names):
...     kws = {}
...     for name in names.split():
...         try:
...             val = eval(name)
...         except:
...             val = None
...         kws[name] = val
...     return kws

Keeping medicines from the bloodstreams of the sick; food
from the bellies of the hungry; books from the hands of the
uneducated; technology from the underdeveloped; and putting
advocates of freedom in prisons.  Intellectual property is
to the 21st century what the slave trade was to the 16th.
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