[Python-Dev] ANN: PEP 335: Overloadable Boolean Operators

[James Y Knight]

On Sep 15, 2004, at 12:34 AM, Greg Ewing wrote:
> There's more to it than short-circuiting. Consider
>
>   a = array([42, ""])
>   b = array([(), "spam"])
>
> One might reasonably expect the result of 'a or b' to
> be
>
>   array([42, "spam"])
>
> which is considerably different from a bitwise operation.

One might, but *I* would reasonably expect it to give me array a, by 
extrapolation from every other data type in python.

Consider also this:
   x and 4 or 5
which is of course a common idiom to workaround the lack of an 
if-then-else expression.

So, try with x = array([42, 0])

Currently, doing this with numarray raises an exception "An array 
doesn't make sense as a truth value.  Use sometrue(a) or alltrue(a).". 
Odd, since nearly all python objects can somehow be turned into a truth 
value, but ok. [Forbidding __nonzero__ prevents horrible mistakes from 
occurring because of the misuse of the comparison operators as 
element-wise comparison.  "if array([1,2,3]) == array([3,2,1]): print 
'Bad'" of course oughtn't print 'Bad'.]

However, with this change, it may instead return:
  array([4, 5])
and that's nothing like what was meant.

The idiom would change to:
   bool(x) and 4 or 5
I suppose...

James

PS: Perl6 has distinct element-wise operators ("hyper" operators). I 
find that less distasteful than misusing regular operators as 
element-wise operators, when they really have vastly different 
semantics.