python - 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? Right now, I am just iterating across the list and manually checking:

def only1(l)
    true_found = False
    for v in l:
        if v and not true_found:
            true_found=True
        elif v and true_found:
             return False #"Too Many Trues"
    return true_found

This seems inelegant and not very pythonic. Is there a cleverer way to do this?

Answer 1

One that doesn't require imports:

def single_true(iterable):
    i = iter(iterable)
    return any(i) and not any(i)

Alternatively, perhaps a more readable version:

def single_true(iterable):
    iterator = iter(iterable)

    # consume from "i" until first true or it's exhausted
    has_true = any(iterator) 

    # carry on consuming until another true value / exhausted
    has_another_true = any(iterator) 

    # True if exactly one true found
    return has_true and not has_another_true

This:

• Looks to make sure i has any true value
• Keeps looking from that point in the iterable to make sure there is no other true value