'Returns the mock XBlock (MockXBlock) associated with the
given block_key.
Raises ItemNotFoundError if the item is not found.'

def get_item(self, block_key, depth=None, lazy=False):

    self.get_items_call_count += 1
    item = self.blocks.get(block_key)
    if (not item):
        raise ItemNotFoundError
    return item
