concrete list of files--that sequence of filenames itself could be lazy iterable per the api given besides the chaining with itertoolswe should mention collec tions chainmap(in the same breath dictionaries (or generally any collections abc mappingare iterable (over their keysjust as we might want to chain multiple sequence-like iterableswe sometimes want to chain together multiple mappings without needing to create single larger concrete one chainmap(is handyand does not alter the underlying mappings used to construct it moduleitertools