Merge #11401 fix from 3.1.

Lib/email/header.py

@@ -314,7 +314,7 @@ class Header:
                                     self._continuation_ws, splitchars)
         for string, charset in self._chunks:
             lines = string.splitlines()
-            formatter.feed(lines[0], charset)
+            formatter.feed(lines[0] if lines else '', charset)
             for line in lines[1:]:
                 formatter.newline()
                 if charset.header_encoding is not None:

Lib/email/test/test_email.py

@@ -3698,6 +3698,13 @@ A very long line that must get split to something other than at the
         h = Header('文', charset='shift_jis')
         self.assertEqual(h.encode(), '=?iso-2022-jp?b?GyRCSjgbKEI=?=')
 
+    def test_flatten_header_with_no_value(self):
+        # Issue 11401 (regression from email 4.x)  Note that the space after
+        # the header doesn't reflect the input, but this is also the way
+        # email 4.x behaved.  At some point it would be nice to fix that.
+        msg = email.message_from_string("EmptyHeader:")
+        self.assertEqual(str(msg), "EmptyHeader: \n\n")
+
 
 
 # Test RFC 2231 header parameters (en/de)coding

Misc/NEWS

@@ -40,6 +40,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #11401: fix handling of headers with no value; this fixes a regression
+  relative to Python2 and the result is now the same as it was in Python2.
+
 - Issue #9298: base64 bodies weren't being folded to line lengths less than 78,
   which was a regression relative to Python2.  Unlike Python2, the last line
   of the folded body now ends with a carriage return.