[Python-Dev] ANN: PEP 335: Overloadable Boolean Operators (original) (raw)

James Y Knight foom at fuhm.net
Wed Sep 15 08:33:50 CEST 2004

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.

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