import sys

_true_set = {'yes', 'true', 't', 'y', '1'}
_false_set = {'no', 'false', 'f', 'n', '0'}


def str2bool(value, raise_exc=False):
    if isinstance(value, str) or sys.version_info[0] < 3 and isinstance(value, basestring):
        value = value.lower()
        if value in _true_set:
            return True
        if value in _false_set:
            return False

    if raise_exc:
        raise ValueError('Expected "%s"' % '", "'.join(_true_set | _false_set))
    return None


def str2bool_exc(value):
    return str2bool(value, raise_exc=True)