Convert ``string`` or ``bool`` to ``bool``.

def _parse_bool(value):
    """Convert ``string`` or ``bool`` to ``bool``."""
    if 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)