[Python-ideas] __missing__ object/keyword

[George Sakkis]

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

The reason for using a "private" sentinel is that any python object,
including None, might be a valid argument. Another option is using
*args or **kwds instead of y but that obfuscates unnecessarily the
function signature.

It would be nice if a new object or keyword, say __missing__, was
introduced as a canonical way to address this common scenario.
Specifically, the only valid usages of __missing__ would be:
1. As a default argument in a callable.
2. In identity tests: <var> is __missing__
Anything else would raise either a SyntaxError (e.g. `x =
__missing__`) or a RuntimeError/TypeError (e.g. `x = y` if y is
__missing__). Only the interpreter could assign __missing__ to a name
when binding objects to formal parameters.

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

Thoughts ?

George