Newbie: Truth values (three-valued logic)

[Olaf Delgado]

Hi everybody!

I am just starting to use python seriously. Currently, I am porting a
small class library from C++ to python, which is kind of fun. Everything's
much more simple and elegant now.

There's one thing I could do in C++, though, which doesn't seem to work in
python: some methods need to return truth values, which can be 1 for
'true', 0 for 'false' and x for 'frankly I don't know'. Now this x,
when used in an if-clause, should evaluate to false, which is easy:

 class uncertain:
   def __nonzero__(self):
     return 0
   def __repr__(self):
     return 'dunno'

So the following works fine:

>>> maybe = uncertain()
>>> maybe
dunno
>>> if maybe: print 't' # prints nothing
...
>>>

Unfortunately:

>>> if not maybe: print 'f'
...
f
>>>

This behaviour is completely consistent in a two-valued logic, but not
what I actually want, because maybe should neither be true nor false, nor
should its negation. As far as I know, the logical negation operator can
not be overloaded, so I have no chance to change this, right? No big deal,
really, but I'd like to know if I missed something here. So, is there some
magical trickery to achieve the behaviour I want?

certainly-uncertain'ly yours,  Olaf

-- 
   ////         Olaf Delgado Friedrichs, Uni Bielefeld, Mathematik
   Olaf         Postfach 100131                  (room:  V4-109)
   `='          D-33501 Bielefeld, Germany       (phone: +49 521 106-4765)
                           http://www.mathematik.uni-bielefeld.de/~delgado