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

This seems to work and should be able to handle any iterable, not justlists. It short-circuits whenever possible to maximize efficiency. Works in both Python 2 and 3.

def only1(iterable):
    for i, x in enumerate(iterable):  # check each item in iterable
        if x: break                   # truthy value found
    else:
        return False                  # no truthy value found
    for x in iterable[i+1:]:          # one was found, see if there are any more
        if x: return False            #   found another...
    return True                       # only a single truthy value found

testcases = [  # [[iterable, expected result], ... ]
    [[                          ], False],
    [[False, False, False, False], False],
    [[True,  False, False, False], True],
    [[False, True,  False, False], True],
    [[False, False, False, True],  True],
    [[True,  False, True,  False], False],
    [[True,  True,  True,  True],  False],
]

for i, testcase in enumerate(testcases):
    correct = only1(testcase[0]) == testcase[1]
    print('only1(testcase[{}]): {}{}'.format(i, only1(testcase[0]),
                                             '' if correct else
                                             ', error given '+str(testcase[0])))

Output:

only1(testcase[0]): False
only1(testcase[1]): False
only1(testcase[2]): True
only1(testcase[3]): True
only1(testcase[4]): True
only1(testcase[5]): False
only1(testcase[6]): False