[Python-ideas] Idea: Deferred Default Arguments?

[Robert Vanden Eynde]

> Someone wrote :
> Thank you for your deferred default values idea, which we're now
working on together.
>
https://github.com/petered/peters_example_code/blob/master/peters_example_code/deferral.py

Allowing to write:

from deferral import deferrable_args, deferred
@deferrable_args
def f(x, y=2, z=3):
    return (x,y,z)

f(5, deferred, 7) == (5,2,7)

(I'd rename "deferrable_args" to simply "deferrable")

The api chosen in deferall.py is a deferall.deferred, one could also use
None or Ellipsis ? That looks nice :

from deferral import elideferrable
@elideferrable
def f(x, y=2, z=3):
    return (x,y,z)

f(5, ..., 7) == (5, 2, 7)

from deferral import nonedeferrable
@nonedeferrable
def f(x, y=2, z=3):
    return (x,y,z)

f(5, None, 7) == (5, 2, 7)

Le mar. 24 juil. 2018 à 14:26, Kyle Lahnakoski <klahnakoski at mozilla.com> a
écrit :

>
> I agree this is a problem, which I have seen solved by removing the
> method signature, which is unfortunate:
>
> > def flexible_method(**kwargs):
> >     # Read the code to find out the expected parameters
>
> I have an @override decorator to handle this type of pattern. It will
> perform the null-coalescing with properties found in a special "kwargs"
> parameter. "kwargs" is assigned a dict that has a copy of the method
> arguments. The value of a callee's argument is, in order,
>
> * a not None value provided by the caller or
> * a not None value found in the kwargs dict or
> * the default value provided by the method declaration or
> * None
>
> I was not clear on where you wanted to define your defaults.  Either
> like this:
>
> >     @override
> >     def subfunction_1(a=None, b=None, c=None, kwargs=None):
> >         return a+b*c
> >
> >     @override
> >     def subfunction_2(d=None, e=None, f=None, kwargs=None):
> >         return d*e+f
> >
> >     @orverride
> >     def main_function(a=2, b=3, c=4, d=5, e=6, f=7, kwargs=None):
> >         return subfunction_1(a, b, c) + subfunction_2(d, e, f)
> >         return subfunction_1(kwargs) + subfunction_2(kwargs)  # IF YOU
> WANT TO BE LAZY
>
> or like this:
>
> >     @override
> >     def subfunction_1(a=2, b=3, c=4, kwargs=None):
> >         return a+b*c
> >
> >     @override
> >     def subfunction_2(d=5, e=6, f=7, kwargs=None):
> >         return d*e+f
> >
> >     @orverride
> >     def main_function(a=None, b=None, c=None, d=None, e=None, f=None,
> kwargs=None):
> >         return subfunction_1(a, b, c) + subfunction_2(d, e, f)
> >         return subfunction_1(kwargs) + subfunction_2(kwargs)  # IF YOU
> WANT TO BE LAZY
>
> both are identical except for where you declare the default values.
>
>
> https://github.com/klahnakoski/mo-kwargs
>
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