customizing and/or/not

[Hans Nowak]

Simon Burton wrote:
> Just reading about the new Sets... and this came up.
> Why can't we customize the and/or/not operators?
> I see this as making one's own boolean type.
> Is it possible?

Probably not.  The 'and' and 'or' operators are special, in that they do 
short-circuit evaluation (as opposed to the binary operators & and |).  Even if 
it would be possible to define a magic method __land__ or __lor__, they are not 
guaranteed to be called.  Consider

   a and b

...if a is true, then b is never evaluated.  A hypothetical call a.__land__(b) 
would be impossible without evaluating b first.  Ditto for 'or'.

'not' is a different bowl of soup, it *can* be customized using the __nonzero__ 
magic method:

 >>> class Foo:
	def __init__(self, value):
		self.value = value
	def __nonzero__(self):
		return not self.value
	
 >>> f = Foo(42)
 >>> not f, not not f
(1, 0)
 >>> f = Foo([])
 >>> not f, not not f
(0, 1)

HTH,