Description

maurycy

opened on Aug 11, 2025

Issue body actions

Bug report

Bug description:

I know that there were discussions about rewriting CSV Sniffer, and they seem to be on hold, to avoid breaking any code relying on its behavior.

I believe it's possible, though, to still make it significantly faster.

The current code iterates over 127 ASCII characters and counts their occurences on each line, even if they're not present:

cpython/Lib/csv.py

Line 382 in b36d23f

for char in ascii:

It's highly inefficient. We can count only present characters, and backfill zeros.

CPython versions tested on:

3.15

Operating systems tested on:

macOS

Linked PRs

• gh-137627: Make csv.Sniffer.sniff() delimiter detection 1.6x faster #137628

Activity

1. maurycy

   added

   type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error

   on Aug 11, 2025
2. picnixz

   added

   stdlibStandard Library Python modules in the Lib/ directoryStandard Library Python modules in the Lib/ directory

   on Aug 11, 2025
3. serhiy-storchaka

   added this to CSV issues 📊on Aug 13, 2025
4. hugovk

   added a commit that references this issue on Oct 23, 2025

   gh-137627: Make `csv.Sniffer.sniff()` delimiter detection 1.6x faster (…

   6be6f8f
5. maurycy commented on Oct 23, 2025

   

   maurycy

   on Oct 23, 2025

   ContributorAuthor

   More actions

   #137628 is merged, so closing this as completed.
6. maurycy

   closed this as completedon Oct 23, 2025
7. github-project-automation

   moved this to Done in CSV issues 📊on Oct 23, 2025
8. StanFromIreland

   added a commit that references this issue on Dec 6, 2025

   pythongh-137627: Make `csv.Sniffer.sniff()` delimiter detection 1.6x …

   662abf9

Issue actions