On Tue, Nov 9, 2021 at 6:31 AM Steven D'Aprano <st...@pearwood.info> wrote:
> By the way, this discussion is probably better suited to the > Python-Ideas mailing list. But since we're here... > > On Tue, Nov 09, 2021 at 11:37:40AM +0100, Sebastian Rittau wrote: > > > >>To me, the "natural" solution looks like this: > > >> > > >>def foo(x=None, y): ... > [...] > > Chris Angelico asked: > > >What would this mean, though: > > > > > >foo(2) > > > > > >Is that legal? > > > > No. This would be equal to foo(x=2) (same as now), meaning the required > > argument "y" is missing. > > That's an odd interpretation. What you described earlier is very similar > to the calling convention of range, which conceptually looks like this: > > range([start=0,] end, [step=1]) > > With your example of "foo(x=None, y)" I would expect foo(2) to mean that > x gets the default and y gets the passed in argument 2, similar to the > way that range(2) works. > But we all understand range() because we have been using it for years and there's a mathematical understanding of what it represents. If you were taught "range(2, 5) returns the numbers 2 through 5, exclusive" and then were asked, "what does range(2) do?", I'm not sure what a beginner would assume, but thinking it goes from 5 to infinity wouldn't be an unreasonable leap to make. An API like range() is also not pervasive, so it can be supported without any special parameter support and not feel icky; `def range(x, y=None, z=1, /)` covers its unique situation. But for me, the key issue is simply remembering what one argument means compares to two. foo(1) versus foo(1, 2) is not self-documenting whether that `1` in the first call would get bound to the same thing as the `1` in the second call. I would need to have quite the understanding of the API to know that without having to look something up in the docs. But the current semantics don't have this issue, and `1` means the same thing in both scenarios (unless you're doing something really weird like range(), which is something I wouldn't want to encourage).
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