[Python-ideas] Idea: Deferred Default Arguments?

[Jonathan Fine]

Hi Peter

Interesting problem. We can already get something like your proposed
solution by using None instead of deferred.

You have to start with
    def subfunction_1(a=None, b=None, c=None):
        if a is None: a = 2
        # similarly for b and c.
        return a+b*c

You will loose the default values being shown when you do help(subfunction).

There's another thread on PEP 505, active right now, that would allow
you to write something like
    a = a OR 2
to provide the default values.
https://mail.python.org/pipermail/python-ideas/2018-July/052071.html

I hope this helps. Please let us know if that might work for you.

-- 
Jonathan

On Fri, Jul 20, 2018 at 10:03 AM, Peter O'Connor
<peter.ed.oconnor at gmail.com> wrote:
> Often when programming I run into a situation where it would be nice to have
> "deferred defaults".  Here is an example of what I mean:
>
>     def subfunction_1(a=2, b=3, c=4):
>         return a+b*c
>
>     def subfunction_2(d=5, e=6, f=7):
>         return d*e+f
>
>     def main_function(a=2, b=3, c=4, d=5, e=6, f=7):
>         return subfunction_1(a=a, b=b, c=c) + subfunction_2(d=d, e=e, f=f)
>
> Here you can see that I had to redefine the defaults in the main_function.
> In larger codebases, I find bugs often arise because defaults are defined in
> multiple places, and somebody changes them in a lower-level function but
> fails to realize that they are still defined differently in a higher
> function.
>
> The only way I currently see to achieve this is not very nice at all, and
> completely obfuscates the signature of the function:
>
>     def main_function(**kwargs):
>         return subfunction_1(**{k: v for k, v in kwargs.items() if k in
> ['a', 'b', 'c']})
>                + subfunction_2(**{k: v for k, v in kwargs.items() if k in
> ['d', 'e', 'f']})
>
> What I was thinking was a "deferred" builtin that would just allow a lower
> function to define the value (and raise an exception if anyone tried to use
> it before it was defined)
>
>     def main_function(a=deferred, b=deferred, c=deferred, d=deferred,
> e=deferred, f=deferred):
>         return subfunction_1(a=a, b=b, c=c) + subfunction_2(d=d, e=e, f=f)
>
> I assume this has been discussed before somewhere, but couldn't find
> anything on it, so please feel free to point me towards any previous
> discussion on the topic.
>
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