Cap text diff computation for huge strings in non-verbose mode - #14749
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Cap text diff computation for huge strings in non-verbose mode#14749Zac-HD wants to merge 1 commit into
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At the default verbosity the displayed assertion explanation is truncated to a handful of lines, but ``_diff_text`` always ran ``difflib.ndiff`` over the full strings first. For multi-million-line strings this could take hours (difflib's runtime blows up on large inputs), only for almost all of the output to be discarded. Now, when ``verbose < 1``, the diff is computed over at most the first 100 lines of each side (after the existing identical leading/trailing character skipping), with a message pointing at ``-v`` to get the full diff. This mirrors the existing behaviour of skipping identical leading/trailing characters unless ``-v`` is given, and the precedent of ``_compare_eq_iterable`` not computing a full diff at low verbosity. The message starts with "Skipping" so that ``_notin_text``'s existing filtering drops it there, like the other skip hints. Fixes pytest-dev#12406 Co-Authored-By: Claude Fable 5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01HVRWXxfTLrU4B95jeoeuHN
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More conservative approach than #14543 that keep the exact output even when it would take a lot of time to generate ? (Might not fix #8998) |
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Also, if you're interested in making assertion message faster there's also #14523 that is very impactful on everything but the ndiff part when the verbosity is low and the assert message will be truncated :) |
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Closing this in favor of #14543; my patch would not handle the case of very long individual lines, and I'm happy to defer to work already in flight. |
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At the default verbosity the displayed assertion explanation is truncated to a handful of lines, but _diff_text always ran difflib.ndiff over the full strings first. For multi-million-line strings this could take hours (difflib's runtime blows up on large inputs), only for almost all of the output to be discarded.
Now, when verbose < 1, the diff is computed over at most the first 100 lines of each side (after the existing identical leading/trailing character skipping), with a message pointing at -v to get the full diff. This mirrors the existing behaviour of skipping identical leading/trailing characters unless -v is given, and the precedent of _compare_eq_iterable not computing a full diff at low verbosity.
The message starts with "Skipping" so that _notin_text's existing filtering drops it there, like the other skip hints.
Fixes #12406