Reference

[Chris Angelico]

On Tue, Mar 4, 2014 at 10:02 AM, Roy Smith <roy at panix.com> wrote:
> In article <mailman.7669.1393885090.18130.python-list at python.org>,
>  Ben Finney <ben+python at benfinney.id.au> wrote:
>
>> That's right. Python provides this singleton and then recommends you
>> compare with ‘is’, precisely to protect against pathological cases like
>> a “return True when compared for equality with None” data type.
>
> Going off on a tangent, I've often wished Python provided more kinds of
> None-ness.  I'll often write:
>
> def f(arg=None):
>    whatever
>
> where it would be nice to differentiate between "this was called with no
> arguments" and "this was called with an argument of None".

That's why you have your own sentinel.

_NONE_PASSED=object()
def f(arg=_NONE_PASSED):
    if f is not _NONE_PASSED: pass

ChrisA