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