'Create a new usage id from an XML dom node.
Args:
node (lxml.etree.Element): The DOM node to interpret.
parent_id: The usage ID of the parent block
id_generator (IdGenerator): The :class:`.IdGenerator` to use
for creating ids
Returns:
UsageKey: the usage key for the new xblock'

def _usage_id_from_node(self, node, parent_id, id_generator=None):

    return self.xblock_from_node(node, parent_id, id_generator).scope_ids.usage_id
