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

If there is only one True, then the length of the Trues should be one:

def only_1(l): return 1 == len(filter(None, l))