'Returns an XBlock instance for the item for this UsageKey.
If any segment of the location is None except revision, raises
xmodule.modulestore.exceptions.InsufficientSpecificationError
If no object is found at that location, raises
xmodule.modulestore.exceptions.ItemNotFoundError
usage_key: a UsageKey that matches the module we are looking for.'

def get_item(self, usage_key, depth=0, **kwargs):

    try:
        return self.modules[usage_key.course_key][usage_key]
    except KeyError:
        raise ItemNotFoundError(usage_key)
