'Returns the TransformerData for the given
transformer for the block identified by the given usage_key.
Raises KeyError if not found.
Arguments:
usage_key (UsageKey) - Usage key of the block whose
transformer data is requested.
transformer (BlockStructureTransformer) - The transformer
whose dictionary data is requested.'

def get_transformer_block_data(self, usage_key, transformer):

    return self._block_data_map[usage_key].transformer_data[transformer]
