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difflib.HtmlDiff produces different output from difflib.ndiff #71896
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SilentGhost commented
on Aug 8, 2016 SilentGhostmannequinMannequinAuthormsg264842 in bpo-26945 reports an odd results of HtmlDiff.make_file, digging into it I've noticed couple of things: different output to one generated directly from difflib.ndiff and underlying issue further up the stack that generates same faulty output with a slightly modified files.
The patch I'm attaching fixes the issue with different outputs, from make_file and ndiff, by not removing EOL characters, I had to re-generate expected test output file.
SilentGhost commented
on Aug 8, 2016 SilentGhostmannequinMannequinAuthorThe degenerate behaviour appears ultimately due to the autojunk heuristic. Given that autojunk defaults to True and there isn't any way to change via most user-facing functions, I don't think there is an easy resolution that can be found here. I'm going to withdraw the earlier patch, since it feels like a half-measure which enable correct behaviour by accident.
As described in the #118150 issue and handled there, yes the
autojunkbehavior is surprising at times.
But there is also another issue here (reproduced using 3.13, 3.14 and 3.15):import difflib A = ["same\n", "diffme\n"] B = ["same", "diffme\n"] # line 0 differs only by trailing newline print(difflib.SequenceMatcher(None, A, B).get_opcodes()) # [('replace', 0, 1, 0, 1), ('equal', 1, 2, 1, 2)] print(list(difflib.ndiff(A, B))) # ['- same\n', '? -\n', '+ same', ' diffme\n'] print(list(difflib.unified_diff(A, B, lineterm=''))) # [..., '-same\n', '+same', ' diffme\n'] print(list(difflib.context_diff(A, B, lineterm=''))) # [..., '! same\n', ..., '! same', ' diffme\n'] print(difflib.HtmlDiff().make_file(A, B)) # no line is marked as changed/added/removed (classes diff_add/diff_chg/diff_sub) A_stripped = [elem.rstrip() for elem in A] B_stripped = [elem.rstrip() for elem in B] print(difflib.SequenceMatcher(None, A_stripped, B_stripped).get_opcodes()) # [('equal', 0, 2, 0, 2)] print(list(difflib.ndiff(A_stripped, B_stripped))) # [' same', ' diffme'] print(list(difflib.unified_diff(A_stripped, B_stripped, lineterm=''))) # [] print(list(difflib.context_diff(A_stripped, B_stripped, lineterm=''))) # [] print(difflib.HtmlDiff().make_file(A_stripped, B_stripped)) # still no line is marked as changed/added/removed
The
HtmlDiffis the only one that does end-of-line stripping from the wholedifflib. To me, this is a bug because it hides actual differences.
It is caused by the_tab_newline_replacemethod being called before_mdiff.Petr Viktorin just told me we should NOT fix the issue (which dates back tot he original implementation in 2004, 22 years ago, cf e064b41). Instead, I will add a warning about it, and an automated test to prevent unexpected regression (if someone would fix the bug).
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difflib.HtmlDiff(GH-153930) #153965difflib.HtmlDiff(GH-153930) #153966difflib.HtmlDiff(GH-153930) #153968