Update our current default with the new_default.

        Args:
            new_default: New default to set.
            respect_none: Flag to determine if ``None`` is a valid value.

def update_default(self, new_default, respect_none=False):
        """Update our current default with the new_default.

        Args:
            new_default: New default to set.
            respect_none: Flag to determine if ``None`` is a valid value.

        """
        if new_default is not None:
            self.default = new_default
        elif new_default is None and respect_none:
            self.default = None