[Python-ideas] Optional parameters without default value

[Paul Moore]

On 2 March 2017 at 13:11, Serhiy Storchaka <storchaka at gmail.com> wrote:
> On 02.03.17 14:20, Paul Moore wrote:
>>
>> So I guess I'm +0.5 on the proposed "positional only parameters"
>> syntax, and -1 on any form of new language-defined sentinel value.
>
>
> My proposition is not about "positional-only parameters".

Bah, sorry. I'm getting muddled between two different threads. I'm not
having a good day, it seems :-(

On the proposed feature, I don't like any of the proposed syntaxes
(I'd rate "default=" with no value as the least bad, but I don't like
it much; "default?" as opposed to "?default" is a possible option).
I'm not convinced that the version using the new syntax is any easier
to read or understand - the sentinel pattern is pretty well-understood
by now, and a built-in replacement would need to result in noticeably
simpler code (which this proposal doesn't seem to).

Agreed that the help() output is ugly. It would of course be possible
to give the sentinel a nicer repr, if you wanted:

>>> class Sentinel(object):
...     def __repr__(self): return "<Raise an exception>"
...
>>> _sentinel = Sentinel()
>>> def get(store, key, default=_sentinel):
...   pass
...
>>> help(get)
Help on function get in module __main__:

get(store, key, default=<Raise an exception>)

Whether it's worth doing this depends on the application, of course
(just like it's possible to hide the name of the sentinel if it
matters sufficiently).

Paul