@picnixz @AA-Turner I really appreciate your feedback! It's great. I will provide more benchmarks, including with enabled optimizations and ideally with pyperformance, rephrase NEWS, and add in a whatsnew.

Benchmarks without optimizations are not relevant so just run those with.

@picnixz @AA-Turner @ZeroIntensity

Thank you for all the comments:

• I created a much more rigorous benchmark than before, see the results: https://github.com/maurycy/cpython-test/tree/main/csv-sniffer-counter-set The benchmark is now on par with how people use the class.
• I've changed the approach, ditching confusing set operations. The speed up now comes mostly from not iterating over ascii over and over, and more efficient zero backfilling.
• There is nothing about csv.Sniffer() in pyperformance, so I was a bit stuck here.
• I updated the docs.

@picnixz

Thank you for your yesterday's review:

• I renamed the variables and simplified the zero-bucket, as per your suggestion,
• I added the tests for the tie break (insert v. append of the zero bucket) to confirm that the behavior has not changed,
• I updated the docs and the benchmark (it's now 1.6x faster!)

Thank you!

Love the enthusiasm, but please try to avoid continuously rebasing (pressing "update branch"). It wastes CI time and also puts a ding in all of our inboxes. Updating the branch should generally only be done to resolve merge conflicts.

FYI: I created a simple check ensuring that the output of csv.Sniffer()._guess_delimiter() did not change:

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

@ZeroIntensity @AA-Turner What do you think about this PR now?

Thanks!

⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot AMD64 Windows11 Bigmem 3.x (tier-1) has failed when building commit 6be6f8f.

What do you need to do:

1. Don't panic.
2. Check the buildbot page in the devguide if you don't know what the buildbots are or how they work.
3. Go to the page of the buildbot that failed (https://buildbot.python.org/#/builders/1079/builds/7354) and take a look at the build logs.
4. Check if the failure is related to this commit (6be6f8f) or if it is a false positive.
5. If the failure is related to this commit, please, reflect that on the issue and make a new Pull Request with a fix.

You can take a look at the buildbot page here:

https://buildbot.python.org/#/builders/1079/builds/7354

Failed tests:

• test_zipimport

Failed subtests:

• testZip64LargeFile - test.test_zipimport.UncompressedZipImportTestCase.testZip64LargeFile
• testZip64LargeFile - test.test_zipimport.ZStdCompressedZipImportTestCase.testZip64LargeFile
• testZip64LargeFile - test.test_zipimport.DeflateCompressedZipImportTestCase.testZip64LargeFile

Summary of the results of the build (if available):

==

Traceback (most recent call last):
  File "R:\buildarea\3.x.ambv-bb-win11.bigmem\build\Lib\test\test_zipimport.py", line 934, in testZip64LargeFile
    with open(os_helper.TESTFN, "wb") as f:
         ~~~~^^^^^^^^^^^^^^^^^^^^^^^^
OSError: [Errno 28] No space left on device


Traceback (test.test_zipimport.DeflateCompressedZipImportTestCase.testTraceback) ... ok


Traceback (test.test_zipimport.UncompressedZipImportTestCase.testTraceback) ... ok


Traceback (most recent call last):
  File "R:\buildarea\3.x.ambv-bb-win11.bigmem\build\Lib\test\test_zipimport.py", line 997, in testZip64LargeFile
    with open(TEMP_ZIP, "wb") as f:
         ~~~~^^^^^^^^^^^^^^^^
OSError: [Errno 28] No space left on device


Traceback (most recent call last):
  File "R:\buildarea\3.x.ambv-bb-win11.bigmem\build\Lib\test\test_zipimport.py", line 1000, in testZip64LargeFile
    f.seek(offset, os.SEEK_SET)
    ~~~~~~^^^^^^^^^^^^^^^^^^^^^
OSError: [Errno 28] No space left on device


Traceback (test.test_zipimport.ZStdCompressedZipImportTestCase.testTraceback) ... ok

Buildbot failure unrelated: "OSError: [Errno 28] No space left on device".

I'll inform the owner.