Description

tonghuaroot

opened on Jul 13, 2026

Issue body actions

Bug description:

csv.Sniffer.sniff() can guess a dialect whose delimiter equals its quote character. Using such a dialect leaks a raw ValueError from the _csv extension instead of the module's csv.Error; csv.Sniffer.has_header(), which builds a reader from the sniffed dialect, is the common trigger.

import csv
csv.Sniffer().has_header("''")   # ValueError, not csv.Error

CPython versions tested on:

3.13, 3.14, 3.15

Operating systems tested on:

macOS

Linked PRs

• gh-153634: Raise csv.Error, not ValueError, from csv.Sniffer for an inconsistent guessed dialect #153635

Activity

1. aisk

   added

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

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

   on Jul 13, 2026
2. aisk

   added this to CSV issues 📊on Jul 13, 2026
3. serhiy-storchaka

   added

   3.13only security fixesonly security fixes

   3.14bugs and security fixesbugs and security fixes

   3.15bugs and security fixesbugs and security fixes

   on Jul 21, 2026
4. encukou commented on Aug 3, 2026

   

   encukou

   on Aug 3, 2026

   Member

   More actions

   In the main branch, the sniffer was rewritten with this issue fixed.

   I don't think fixing this in maintenance branches is worth it -- if someone wrote a workaround or otherwise depends on the existing behaviour, their code can break. Better to break them just once, in 3.16.

   I'll close; if anyone disagrees we can reopen.

   Reacted by tonghuaroot (童话)
5. encukou

   closed this as not plannedon Aug 3, 2026
6. github-project-automation

   moved this to Done in CSV issues 📊on Aug 3, 2026

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