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

csv.Sniffer.sniff could return a dialect whose delimiter equaled its quote character. Using that dialect (directly, or via csv.Sniffer.has_header, which builds a reader from the sniffed dialect) leaked a raw ValueError from the _csv extension instead of the module's own csv.Error. sniff now validates its guessed dialect before returning it and re-raises the _csv ValueError as csv.Error, so both sniff and has_header report the module exception type. The direct csv.reader and csv.writer paths and the Dialect._validate TypeError/ValueError behavior are unchanged.

Author: tonghuaroot

Lib/csv.py

@@ -264,6 +264,14 @@ class dialect(Dialect):
         dialect.quotechar = quotechar or '"'
         dialect.skipinitialspace = skipinitialspace
 
+        # A guess can be inconsistent (for example delimiter == quotechar).
+        # Building a reader validates it; re-raise the _csv ValueError as a
+        # csv.Error so callers see the module's own exception type.
+        try:
+            reader(StringIO(), dialect)
+        except ValueError as e:
+            raise Error(str(e)) from None
+
         return dialect
 
 

Lib/test/test_csv.py

@@ -1463,6 +1463,16 @@ def test_sniff(self):
         self.assertEqual(dialect.quotechar, "'")
         self.assertIs(dialect.skipinitialspace, False)
 
+    def test_sniff_delimiter_equals_quotechar(self):
+        # A sample that makes sniff() guess a dialect whose delimiter equals
+        # its quotechar is invalid; that must surface as csv.Error, not as a
+        # raw ValueError leaking from the _csv extension.
+        sniffer = csv.Sniffer()
+        for sample in ('"', '""', "''"):
+            with self.subTest(sample=sample):
+                self.assertRaises(csv.Error, sniffer.sniff, sample)
+                self.assertRaises(csv.Error, sniffer.has_header, sample)
+
     def test_delimiters(self):
         sniffer = csv.Sniffer()
         dialect = sniffer.sniff(self.sample3)

Misc/NEWS.d/next/Library/2026-07-09-16-41-20.gh-issue-153634.27Fs4T.rst

@@ -0,0 +1,4 @@
+:meth:`csv.Sniffer.sniff` now raises :exc:`csv.Error` for an inconsistent
+guessed dialect, for example one whose delimiter equals its quote character,
+instead of returning it. :meth:`csv.Sniffer.has_header` now raises
+:exc:`csv.Error` instead of leaking a :exc:`ValueError`.