gh-145824: email.utils: replace AttributeError with TypeError in collapse_rfc2231_value for non-string non-3-tuple input

Doc/library/email.utils.rst

@@ -212,7 +212,7 @@ of the new API.
    :rfc:`2231` header is not known by Python; it defaults to ``'us-ascii'``.
 
    For convenience, if the *value* passed to :func:`collapse_rfc2231_value` is not
-   a tuple, it should be a string and it is returned unquoted.
+   a 3-tuple, it should be a string and it is returned unquoted.
 
 
 .. function:: decode_params(params)

Lib/email/utils.py

@@ -449,6 +449,10 @@ def decode_params(params):
 def collapse_rfc2231_value(value, errors='replace',
                            fallback_charset='us-ascii'):
     if not isinstance(value, tuple) or len(value) != 3:
+        if not isinstance(value, str):
+            raise TypeError(
+                f"expected str or 3-tuple, got {type(value).__name__}"
+            )
         return unquote(value)
     # While value comes to us as a unicode string, we need it to be a bytes
     # object.  We do not want bytes() normal utf-8 decoder, we want a straight

Lib/test/test_email/test_email.py

@@ -5789,6 +5789,20 @@ def test_should_not_hang_on_invalid_ew_messages(self):
             with self.subTest(m=m):
                 msg = email.message_from_string(m)
 
+    def test_collapse_rfc2231_value_non_3_tuple(self):
+        # collapse_rfc2231_value raises TypeError on values that are
+        # neither a string nor a 3-tuple.
+        from email.utils import collapse_rfc2231_value
+        for val in [(), ('a',), ('a', 'b'), ('a', 'b', 'c', 'd'), 42, None]:
+            with self.subTest(val=val):
+                with self.assertRaises(TypeError):
+                    collapse_rfc2231_value(val)
+        # A proper 3-tuple decodes correctly.
+        result = collapse_rfc2231_value(('us-ascii', 'en', 'hello'))
+        self.assertEqual(result, 'hello')
+        # A plain string passes through unquote.
+        self.assertEqual(collapse_rfc2231_value('"hello"'), 'hello')
+
 
 # Tests to ensure that signed parts of an email are completely preserved, as
 # required by RFC1847 section 2.1.  Note that these are incomplete, because the

Misc/NEWS.d/next/Library/2026-03-11-14-42-23.gh-issue-145824.HMjVqq.rst

@@ -0,0 +1,3 @@
+:func:`email.utils.collapse_rfc2231_value` now raises :exc:`TypeError`
+instead of :exc:`AttributeError` when called with a value that is neither a
+string nor a 3-tuple.