gh-129374: adds quoting detection to csv sniffer

Lib/csv.py

@@ -265,7 +265,7 @@ class dialect(Dialect):
         # _csv.reader won't accept a quotechar of ''
         dialect.quotechar = quotechar or '"'
         dialect.skipinitialspace = skipinitialspace
-
+        dialect.quoting = self._guess_quoting(sample, dialect)
         return dialect
 
 
@@ -448,6 +448,66 @@ def _guess_delimiter(self, data, delimiters):
                             data[0].count("%c " % delim))
         return (delim, skipinitialspace)
 
+    def _guess_quoting(self, data, dialect):
+        """
+        Looks for the quoting rules that are used in the data setting the most conservative quoting rule.
+        """
+        lines = data.split(dialect.lineterminator)
+        if len(lines) == 1:
+            lines = data.split("\n")
+        policies = []
+        for line in lines[:5]:
+            elems = line.split(dialect.delimiter)
+            num_quoted_elems = 0
+            num_float_elems = 0
+            num_none_elems = 0
+            for elem in elems:
+                if not elem:
+                    continue
+                if elem[0] == dialect.quotechar:
+                    num_quoted_elems += 1
+                elem = elem.strip(dialect.quotechar)
+                if dialect.doublequote:
+                    if elem[0] == dialect.quotechar:
+                        elem = elem.strip(dialect.quotechar)
+                    elif elem[0] == '"':
+                        elem = elem.strip('"')
+                    else:
+                        elem = elem.strip("'")
+                try:
+                    float(elem)
+                    num_float_elems += 1
+                    is_float = True
+                except ValueError:
+                    is_float = False
+                if elem == "None":
+                    num_none_elems += 1
+            max_quote_policy = QUOTE_NONE
+            if num_quoted_elems == len(elems):
+                max_quote_policy = QUOTE_ALL
+            elif num_quoted_elems == len(elems) - num_none_elems:
+                max_quote_policy = QUOTE_NOTNULL
+            elif num_quoted_elems == len(elems) - num_float_elems:
+                max_quote_policy = QUOTE_NONNUMERIC
+            elif num_quoted_elems == len(elems) - num_none_elems - num_float_elems:
+                max_quote_policy = QUOTE_STRINGS
+            elif num_quoted_elems > 0:
+                max_quote_policy = QUOTE_MINIMAL
+            policies.append(max_quote_policy)
+        return self._determine_quote_priority(policies)
+
+    def _determine_quote_priority(self, quote_policies):
+        priority_order = {
+            QUOTE_ALL: 50,
+            QUOTE_NOTNULL: 40,
+            QUOTE_NONNUMERIC: 30,
+            QUOTE_STRINGS: 20,
+            QUOTE_MINIMAL: 10,
+            QUOTE_NONE: 0
+        }
+        reverse_priority_order = {v: k for k, v in priority_order.items()}
+        max_priority = map(lambda e:priority_order.get(e, 0), quote_policies)
+        return reverse_priority_order[max(max_priority)]
 
     def has_header(self, sample):
         # Creates a dictionary of types of data in each column. If any

Lib/test/test_csv.py

@@ -1342,6 +1342,14 @@ def test_has_header_strings(self):
         self.assertFalse(sniffer.has_header(self.sample10))
         self.assertFalse(sniffer.has_header(self.sample11))
 
+    def test_issue129374_writing_quoted_strings(self):
+        s = '42,"hello","world",-1\n'
+        d = csv.Sniffer().sniff(s)
+        x = list(csv.reader([s], d))
+        f = StringIO()
+        csv.writer(f,d).writerows(x)
+        self.assertEqual(f.getvalue(), '42.0,"hello","world",-1.0\r\n')
+
     def test_has_header(self):
         sniffer = csv.Sniffer()
         self.assertIs(sniffer.has_header(self.sample1), False)
@@ -1415,6 +1423,14 @@ def test_doublequote(self):
         dialect = sniffer.sniff(self.sample9)
         self.assertTrue(dialect.doublequote)
 
+    def test_quote_detection(self):
+        sniffer = csv.Sniffer()
+        dialect = sniffer.sniff(self.sample1)
+        self.assertEqual(dialect.quoting, csv.QUOTE_NONE)
+        dialect = sniffer.sniff(self.sample2)
+        self.assertEqual(dialect.quoting, csv.QUOTE_ALL)
+
+
 class NUL:
     def write(s, *args):
         pass

Misc/NEWS.d/next/Library/2025-05-21-12-57-29.gh-issue-129374.nVYHmg.rst

@@ -0,0 +1 @@
+csv sniffer gains support for detecting quoting policy