csv.Sniffer.has_header() leaks ValueError instead of csv.Error for an inconsistent guessed dialect

[tonghuaroot]

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

[encukou]

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.