Reference

[Roy Smith]

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".  Sure, I can 
work around that with things like **kwargs, and then test

"args" in kwargs

vs.

kwargs["args"] is None

but that always feels clumsy.  It also makes the function declaration 
less sell-describing.  "Hmmm, let's see what help() says.  Oh, gee, I 
can pass it some stuff".