amphibient amphibient - 1 month ago 5
Python Question

Is only one element in boolean list true?

What would be the most minimal way of determining whether only one element in a boolean list is

True
in
Python
?

I was thinking of converting each boolean to 0 (false) or 1 (true) and adding them all up and checking if the sum is 1. That is pretty minimalist but I was wondering if there is a (bitwise) operation that will return true if only one element is true and all the other ones false, which would save me the bool-->int conversion (however simple it is). I am mostly just curious on whether such a bitwise operation exists.

Answer

Python bool subclass from int so you don't need to do any conversion:

>>> sum([True, False, False])
1
>>> sum([True, True, True])
3

This solution doesn't short-circuit however ... there are some cases where you might want to be able to bail out earlier:

result = 0
for item in boolean_iterable:
    result += item
    if result > 1:
        break  # Short-circuit early

However, unless your boolean iterables are really large, and you expect to short-circuit frequently, I would expect this to perform worse than the sum in the average case (which can push the loop to more optimized code).

Also if you're looking for clever ways to do this with bitwise arithmetic, you can use xor in a reduce operation:

>>> from functools import reduce
>>> import operator
>>> reduce(operator.xor, [True, False, False], False)
True
>>> reduce(operator.xor, [True, False, True], False)
False
>>> reduce(operator.xor, [], False)
False
>>> reduce(operator.xor, [True], False)
True

But I wouldn't advise using this version :-)