Get a handler (if present) from a namespace.

        Returns
        -------
        function or None
            The handler defined in the namespace.

def _get_handler(self, namespace, remove_handler=False):
        """Get a handler (if present) from a namespace.

        Returns
        -------
        function or None
            The handler defined in the namespace.

        """
        if hasattr(namespace, '_func'):
            _func = namespace._func
            if remove_handler:
                del namespace._func
            return _func