Why python doesn't use syntax like function(, , x) for default parameters?

[Steven D'Aprano]

On Fri, 10 Mar 2006 12:06:57 +0300, Dmitry Anikin wrote:

> I mean, it's very convenient when default parameters
> can be in any position, like
> def a_func(x = 2, y = 1, z):
>     ...
> (that defaults must go last is really a C++ quirk which
> is needed for overload resolution, isn't it?)

I'm confused... it looks to me like the defaults are *first*, not last, in
the above definition. We write from left to write in English, so normal
convention in English is that first -> last, not last <- first.


> and when calling, just omit parameter when you want to
> use defaults:
> a_func(, , 3)

Can you do this? another_func(,,,4,,,,)

That looks like bug-city to me. (Did I mean 4 to go in the fourth position
or fifth?)

In any case, given the way Python defaults work, the commas are
superfluous. There is no ambiguity:

def f(w, x, y=1, z=1):
    pass

Positional arguments without defaults *must* go first, and must be
supplied:

f(5, 6) => w=5, x=6, y=1, z=1

Commas would be pointless: f(5, 6, , ) isn't needed, because the first
and second arguments are already assigned to the w and x arguments before
the first empty comma is spotted.

You can't do this: f(, , 5, 6) => w,x = default, y=5, z=6

because w and x don't have defaults.


> There are often situations when a function has independent
> parameters, all having reasonable defaults, and I want to
> provide just several of them. In fact, I can do it using
> keyword parameters, but it's rather long and you have to
> remember/lookup names of parameters.

And you think it is easier to remember the position of parameters rather
than their names?


-- 
Steven.