Not _quite_ equivalent to what simont had originally:

>>> class foo:
...   def __cmp__(self, other):
...     if other is True: return 0
...     if other is False: return 0
...     return 1
... 
>>> f = foo()
>>> g = foo()
>>> f == g
False
>>> d = {}
>>> d[5] = f
>>> d.get(6, not f) == f
True
>>> 6 in d and d[6] == f
False

Note cmp is -1,0,1 for less than/equal/greater than.

If you abuse semantics like that you deserve to lose :-)

Yes, I think that clever as ptc24's counterexample is (and doubly so because I tried to construct a similar thing and didn't quite get it to work), it falls into the same category as cartesiandaemon's question about NaN: if you define a class which has deliberately weird behaviour when compared, then you've violated the warranty of any of these methods once you try to use an instance of that class as a key or value in the dictionary or as a key or value to test for. It's only fair game to use it as the dummy non-value.