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
And I often catch myself trying to write def foo(x, y=object()): if y is ... # oops, I need to define the y *outside* the function. But once I've defined it outside the function, it gets tempting to reuse it for something else, such as another method in the same class with a similar need. And eventually, it may end up getting promoted to a real object, like None... I'll freely grant that this is a small use case, but ... it is one I would appreciate seeing handled better.
Specifically, the only valid usages of __missing__ would be: 1. As a default argument in a callable. 2. In identity tests: <var> is __missing__
I assume you could also write 2a. var is not __missing__
If this was to be accepted, a further generalization could be to allow the `var is __missing__` expression even if `var` is not a formal parameter. This would be equivalent to:
try: var except NameError: expr = True else: expr = False
I wouldn't do this, at least at first. I see now that it is effectively a way of checking for run-time parameters added to builtins, which can be used to control imports, etc. But that wasn't obvious as first. -jJ