How can I check that a list has one and only one truthy value?

Question

In python, I have a list that should have one and only one truthy value (that is, bool(value) is True). Is there a clever way to check for this

Answer 1

It depends if you are just looking for the value True or are also looking for other values that would evaluate to True logically (like 11 or "hello"). If the former:

def only1(l):
    return l.count(True) == 1

If the latter:

def only1(l):
    return sum(bool(e) for e in l) == 1

since this would do both the counting and the conversion in a single iteration without having to build a new list.