On Thu, Nov 6, 2008 at 3:31 PM, George Sakkis <george.sakkis@gmail.com> wrote:
> Several times I find myself using the following idiom:
>
> _Missing = object() # sentinel
>
> def foo(x, y=_Missing):
> if y is _Missing:
> do_this
> else:
> do_that

> It would be nice if a new object or keyword, say __missing__, was
> introduced as a canonical way to address this common scenario.

> Thoughts ?

This has been proposed and shot down before:

http://mail.python.org/pipermail/python-dev/2007-May/073439.html

The older proposal didn't give __missing__ the special run-time and
compile-time error behaviors,

I agree that it doesn't make sense without special compile/runtime behavior; that would be like adding a second None.

but this actually just makes things
worse. Why complicate the language merely to save a single idiomatic
line of code dealing with a relatively infrequent use-case?

I can't tell how much it would complicate the implementation of the language but the semantics are pretty straightforward. But you have a point there, None is more often than not an adequare sentinel, so if implementing __missing__ requires significant work it's probably not worth the effort.

George