[Python-ideas] parameter omit

[Jim Jewett]

On 5/11/07, Aaron Brady <castironpi at comcast.net> wrote:

> And while I'm at it and thinking clearly, with cool weather and open doors,
> the basic problem here, underlying and distilled, is that None is serving
> double-duty.

Yes.  When None appears in a parameter list (or an argument list, at
the calling site), it might really be a sentinel for something else,
and that is a wart.

> You could always require all parameter defaults to equal `None'.  But since
> they're not, take this.

> DefaultArg = object()

Yes; this is used when None actually is a valid and meaningful value.

I don't see a good way to *always* use this idiom though, because

(1)  If DefaultArg is a pre-existing object, then there will be times
when it is valid and meaningful data -- such as when introspecting.

(2)  Creating it afresh is a bit of boilerplate that we wouldn't want
to require when it isn't really needed.

    sentinel=object()
    def f(val=sentinel):
        if val is sentinel:

And note that the creation can't be inline, such as

     def f(val=object()):

or there wouldn't be any good way to test for it.  Adding a keyword to do that

    def f(val=object()):
        if defaulted val:

would probably be a good idea.  (I don't think it would get used often
enough to justify the costs of a change, but there would be a clear
benefit to include in the cost-benefit calculations.)

-jJ