@classmethod
def parse_bool(cls, value, default=None):
    """Convert ``string`` or ``bool`` to ``bool``."""
    if value is None:
        return default
    elif isinstance(value, bool):
        return value
    elif isinstance(value, str):
        if value == 'True':
            return True
        elif value == 'False':
            return False
    raise Exception('Value %s is not boolean.' % value)
