def one(iter):
    """ Returns True if exactly one member of iter has a truthy value, else False.

    Args:
        iter (iterable): an iterable containing values that can be evaluated as bools.

    Returns:
        (bool): True if exactly one member is truthy, else False.

    >>> one({"a", None, True})
    False

    >>> one({None, None, None})
    False

    >>> one({True, False, False})
    True

    >>> one({False, False, True})
    True
    """
    return len([s for s in iter if s]) == 1