Thanks, Raymond. That file says (in the -u section) "If a range is empty, its beginning line number shall be the number of the line just before the range, or 0 if the empty range starts the file." The last clause says to me that gnu diff is right and that the 1,0 range of difflib.unified_diff is a bug.

I think we should add this link to the difflib doc (I am still thinking about place and wording).

News entry might be
Issue # 11747: Correct difflib.unified_diff empty file range from 1,0 to 0,0 in conformance with Single Unix Specification for diff output formats. Patch by ysj.ray.