How safe is this way of getting a default that allows None?

[Dan Stromberg]

How safe is this?  I like the idea.

#!/usr/bin/python

UNSPECIFIED = object()

def fn(x, y=UNSPECIFIED):
   if y is UNSPECIFIED:
      print x, 'default'
   else:
      print x, y

fn(0, 1)
fn(0, None)
fn(0)
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