how to let argument be optional falling back to certain integer

[Chris Angelico]

On Sun, Jun 21, 2020 at 2:02 AM Boris Dorestand <bdorestand at example.com> wrote:
>
> I just wrote
>
> def f(y, N, k = None):
>   k = k or (N - 1)
>   return k
>
> I was surprised to find out that 0 == False, so f(7, 31, 0) produces 31.
>
> I'd like 0 to be a valid choice for k.
>
> How do you guys let k be an optional argument such that it defaults to
> N - 1?
>

The easiest way is to explicitly check for None.

if k is None: k = N - 1

Zero being false shouldn't be a surprise. If None can count as false,
then so should other "emptiness" values. (Remember, the canonical
falseness value is False, not None.)

ChrisA