gh-151454: Accept Header values in email.message.Message.add_header

Message.items() can yield (name, Header) pairs under the compat32 policy
(for a header carrying 8-bit data), and Message[name] = value accepts such
Header values, but add_header() raised "TypeError: sequence item 0: expected
str instance, Header found" because it assembled the value with
SEMISPACE.join().

When no extra parameters are given, add_header() now mirrors __setitem__ and
stores the value (Header or str) unmodified, fixing the reported header-copy
loop.  None is still coerced to '' to preserve add_header()'s historical
behavior.  The parameterized branch is unchanged: it still builds the header
as a string and so continues to require a str value.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: harjothkhara

Doc/library/email.compat32-message.rst

@@ -419,6 +419,15 @@ Here are the methods of the :class:`Message` class:
 
          Content-Disposition: attachment; filename*="iso-8859-1''Fu%DFballer.ppt"
 
+      When called without any *_params*, this method stores *_value* the same
+      way :meth:`__setitem__` does, so *_value* may also be an
+      :class:`~email.header.Header` instance, such as those returned by
+      :meth:`items` under a ``compat32`` policy.
+
+      .. versionchanged:: next
+         When no parameters are given, an :class:`~email.header.Header`
+         *_value* is now accepted, consistent with :meth:`__setitem__`.
+
 
    .. method:: replace_header(_name, _value)
 

Lib/email/message.py

@@ -576,6 +576,13 @@ def add_header(self, _name, _value, **_params):
         msg.add_header('content-disposition', 'attachment',
                        filename='Fußballer.ppt'))
         """
+        if not _params:
+            # With no parameters, mirror __setitem__ so add_header() accepts
+            # the same values, e.g. the Header instances that items() returns
+            # under the compat32 policy.  None is coerced to '' to preserve
+            # add_header()'s historical behavior (__setitem__ would store None).
+            self[_name] = '' if _value is None else _value
+            return
         parts = []
         for k, v in _params.items():
             if v is None:

Lib/test/test_email/test_email.py

@@ -839,6 +839,23 @@ def test_add_header_with_no_value(self):
         msg.add_header('X-Status', None)
         self.assertEqual('', msg['X-Status'])
 
+    def test_add_header_copies_Header_returned_by_items(self):
+        # gh-151454: items() returns (name, Header) under the compat32 policy
+        # for a header carrying 8-bit data.  Copying such headers with the
+        # bug report's loop (newmsg.add_header(h, v)) used to raise
+        # "TypeError: sequence item 0: expected str instance, Header found".
+        old = email.message_from_bytes(b'X-Ham-Report: spam \xff report\n\n')
+        name, value = old.items()[0]
+        self.assertIsInstance(value, Header)
+        msg = Message()
+        msg.add_header(name, value)
+        # The Header is stored unmodified, exactly as __setitem__ does (rather
+        # than stringified, which would lose the 8-bit bytes to U+FFFD).
+        self.assertIs(next(msg.raw_items())[1], value)
+        ref = Message()
+        ref[name] = value
+        self.assertEqual(msg.as_bytes(), ref.as_bytes())
+
     # Issue 5871: reject an attempt to embed a header inside a header value
     # (header injection attack).
     def test_embedded_header_via_Header_rejected(self):

Misc/NEWS.d/next/Library/2026-06-13-15-00-38.gh-issue-151454.BQQ7up.rst

@@ -0,0 +1,5 @@
+When called without extra parameters, :meth:`email.message.Message.add_header`
+now stores the value the same way ``msg[name] = value`` does, so it accepts the
+:class:`~email.header.Header` instances that :meth:`~email.message.Message.items`
+returns under the ``compat32`` policy.  Previously it raised ``TypeError:
+sequence item 0: expected str instance, Header found``.