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gh-109638: Fix for significant backtracking in csv.Sniffer#109639
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This reverts commit16d9612.
Lib/csv.py Outdated
| @@ -270,8 +270,9 @@ def _guess_quote_and_delimiter(self, data, delimiters): | |||
| # if we see an extra quote between delimiters, we've got a | |||
| # double quoted format | |||
| # in future Python versions this zero width look-ahead assert can be replaced with atomic groups | |||
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Please may you explain this comment?
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Sure, this zero-width lookahead assertion change in the Regex can be done with an atomic group which is cleaner and more concise.
# Current change(,|^)\W*"(?=(?P<zero>[^,|"\n]*))(?P=zero)"[^,|\n]*"\W*(,|$)# Atomic Group(,|^)\W*"(?>[^,|"\n]*)"[^,|\n]*"\W*(,|$)But atomic groups are only supported in Python 3.11 onwards so I avoided using them here.
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Thanks Sean. This PR (if merged) would be part of Python 3.13, so let's use the better atomic group method.
A
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Sure, I've switched us to the simpler atomic group setup. Performance is identical to the previous fix.
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This looks like a good quick fix for the problem.
Ultimately though, these regexs are hard to read and cause a few problems with lists and other items. I think we should be thinking about how to replace the sniffer to have a higher accuracy. (Seehttps://github.com/ws-garcia/CSVsniffer which shows that only 67.54% accuracy). I've posted to dpo on this topic here:https://discuss.python.org/t/rewrite-csv-sniffer
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#109638 it is possible to get significant backtracking in
csv.Sniffer()inside the doublequote checking regex. This change introduces a zero-length lookahead assertion to reduce the amount of backtracking.This yields a significant improvement in testing