gh-134151 Fix TypeError in email.utils.decode_params() when sorting RFC 2231 continuations

Lib/email/utils.py

@@ -417,8 +417,13 @@ def decode_params(params):
         for name, continuations in rfc2231_params.items():
             value = []
             extended = False
-            # Sort by number
-            continuations.sort()
+            # Sort by number, treating None as 0 if there is no 0, ignore it if there is already a 0.
+            has_zero = any(x[0] == 0 for x in continuations)
+            if has_zero:
+                continuations = [x for x in continuations if x[0] is not None]
+            else:
+                continuations = [(0 if x[0] is None else x[0], x[1], x[2]) for x in continuations]
+            continuations.sort(key=lambda x: x[0])
             # And now append all values in numerical order, converting
             # %-encodings for the encoded segments.  If any of the
             # continuation names ends in a *, then the entire string, after

Lib/test/test_email/test_email.py

@@ -389,6 +389,24 @@ def test_bad_param(self):
         msg = email.message_from_string("Content-Type: blarg; baz; boo\n")
         self.assertEqual(msg.get_param('baz'), '')
 
+    def test_continuation_sorting_part_order(self):
+        msg = email.message_from_string(
+            "Content-Disposition: attachment; "
+            "filename*=\"ignored\"; "
+            "filename*0*=\"utf-8''foo%20\"; "
+            "filename*1*=\"bar.txt\"\n"
+        )
+        filename = msg.get_filename()
+        self.assertEqual(filename, 'foo bar.txt')
+
+    def test_sorting_no_continuations(self):
+        msg = email.message_from_string(
+            "Content-Disposition: attachment; "
+            "filename*=\"bar.txt\"; "
+        )
+        filename = msg.get_filename()
+        self.assertEqual(filename, 'bar.txt')
+
     def test_missing_filename(self):
         msg = email.message_from_string("From: foo\n")
         self.assertEqual(msg.get_filename(), None)

Misc/NEWS.d/next/Library/2025-05-25-23-23-05.gh-issue-134151.13Wwsb.rst

@@ -0,0 +1,2 @@
+:mod:`email`: Fix TypeError in email.utils.decode_params() when sorting RFC
+2231 continuations that contain an unnumbered section.