Notset: A Do-Not-Care value for Python

> Suppose you have a function which takes a person and allows you to update the person's name, age, both, or neither

The problem is not the lack of some fundamental feature, it is one of obviousness in interface design. A trixy interface as given by the example leads exactly to the kind of problems the library hopes to eliminate. Instead how about:

    def update(person, attrs):
        pass

    def update_with_email(person, attrs):
        update(person, attrs)
        send_email(person)

For example, how does one add a 'use_html=True' parameter to update_with_email()? Perhaps by adding a 'use_html' kwarg that hopefully doesn't conflict with Person's attribute namespace, or perhaps by adding '_use_html', hoping to skirt the problem by introduction of ugliness. For a 'clean' backwards-compatible solution, we're forced into something like:

    def _real_update_with_email(person, use_html, kwargs):
        update(person, **kwargs)
        send_email(person, use_html=use_html)

    def update_with_email(person, **kwargs):
        _real_update_with_email(person, False, kwargs)

    def update_with_html_email(person, **kwargs):
        _real_update_with_email(person, True, kwargs)

    # TODO: something seems terribly wrong here, I can't quite put my finger on it.
    update(person, **{'previous_' + attr: value})

I realize abuse of \* \* is very much a religious issue, and at first sight, one of the superficial attractions to Python (at least for me, way back in time), however with experience it seems to regularly introduce more problems than it solves outside a few niche uses. The idea of adding an 'undefined' value has been discussed going back years (try grepping python-ideas and python-dev) and it's never made it in for good reason.

There is one place where an 'undefined' might seem useful at first, for example in the implementation of `dict.pop()` where a missing second argument signals the need to raise KeyError. The problem is that no published, public value including 'undefined' can be used as placeholder without introducing another ugly rule to the language: the ability to use `pop(..., default)` with any default value except 'undefined'! (net simplicity gain: zero)

The only way this might work is if NotSet (or whatever it's called) is a keyword and it is only allowed in (a) function signatures and (b) comparison with `is` and `is not`; everything else throws a syntax error.

    NotSet = type("NotSet", (object, ), dict(__repr__=lambda self: "NotSet"))

Why would anyone consider this notable, useful or even important enough to warrant an upvote?