From 84fb618ceee5c912ce563034186987c0cfc673e7 Mon Sep 17 00:00:00 2001
From: Mark Sapiro <mark@msapiro.net>
Date: Sun, 19 Jan 2020 18:43:07 -0800
Subject: [PATCH 1/3] bpo-27321 Fix email.generator.py to not attempt to
 replace a non-existent header.

---
 Lib/email/generator.py                        |  5 ++-
 Lib/test/test_email/test_email.py             | 33 +++++++++++++++++++
 .../2020-01-19-18-40-26.bpo-27321.8e6SpM.rst  |  2 ++
 3 files changed, 39 insertions(+), 1 deletion(-)
 create mode 100644 Misc/NEWS.d/next/Library/2020-01-19-18-40-26.bpo-27321.8e6SpM.rst

diff --git a/Lib/email/generator.py b/Lib/email/generator.py
index ae670c2353c858a..9e084d98d34e093 100644
--- a/Lib/email/generator.py
+++ b/Lib/email/generator.py
@@ -186,7 +186,10 @@ def _write(self, msg):
         # If we munged the cte, copy the message again and re-fix the CTE.
         if munge_cte:
             msg = deepcopy(msg)
-            msg.replace_header('content-transfer-encoding', munge_cte[0])
+            if msg.get('content-transfer-encoding') is not None:
+                msg.replace_header('content-transfer-encoding', munge_cte[0])
+            else:
+                msg['Content-Transfer-Encoding'] = munge_cte[0]
             msg.replace_header('content-type', munge_cte[1])
         # Write the headers.  First we see if the message object wants to
         # handle that itself.  If not, we'll do it generically.
diff --git a/Lib/test/test_email/test_email.py b/Lib/test/test_email/test_email.py
index 8ec39190ea8da4c..1474e99e33a097a 100644
--- a/Lib/test/test_email/test_email.py
+++ b/Lib/test/test_email/test_email.py
@@ -311,6 +311,39 @@ def test_as_string_policy(self):
         g.flatten(msg)
         self.assertEqual(fullrepr, s.getvalue())
 
+    def test_nonascii_as_string_without_cte(self):
+        m = textwrap.dedent("""\
+            Content-type: text/plain; charset="iso-8859-1"
+
+            Test if non-ascii messages with no Content-Transfer-Encoding set
+            can be as_string'd:
+            Föö bär
+            """)
+        source = m.encode('iso-8859-1')
+        expected = textwrap.dedent("""\
+            Content-type: text/plain; charset="iso-8859-1"
+            Content-Transfer-Encoding: quoted-printable
+
+            Test if non-ascii messages with no Content-Transfer-Encoding set
+            can be as_string'd:
+            F=F6=F6 b=E4r
+            """)
+        msg = email.message_from_bytes(source)
+        self.assertEqual(msg.as_string(), expected)
+
+    def test_nonascii_as_string_without_content_type_and_cte(self):
+        m = textwrap.dedent("""\
+            MIME-Version: 1.0
+
+            Test if non-ascii messages with no Content-Type nor
+            Content-Transfer-Encoding set can be as_string'd:
+            Föö bär
+            """)
+        source = m.encode('iso-8859-1')
+        expected = source.decode('ascii', 'replace')
+        msg = email.message_from_bytes(source)
+        self.assertEqual(msg.as_string(), expected)
+
     def test_as_bytes(self):
         msg = self._msgobj('msg_01.txt')
         with openfile('msg_01.txt') as fp:
diff --git a/Misc/NEWS.d/next/Library/2020-01-19-18-40-26.bpo-27321.8e6SpM.rst b/Misc/NEWS.d/next/Library/2020-01-19-18-40-26.bpo-27321.8e6SpM.rst
new file mode 100644
index 000000000000000..28acf7f6ef919c4
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2020-01-19-18-40-26.bpo-27321.8e6SpM.rst
@@ -0,0 +1,2 @@
+Fixed KeyError exception when flattening an email to a string attempts to
+replace a non-existent Content-Transfer-Encoding header.

From cf284919184b3cd7a18d4bf5a44f1e604f17f1ac Mon Sep 17 00:00:00 2001
From: Mark Sapiro <mark@msapiro.net>
Date: Sun, 19 Jan 2020 20:12:34 -0800
Subject: [PATCH 2/3] Added a MIME-Version: header to the test.

---
 Lib/test/test_email/test_email.py | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/Lib/test/test_email/test_email.py b/Lib/test/test_email/test_email.py
index 1474e99e33a097a..9ea85eee8946087 100644
--- a/Lib/test/test_email/test_email.py
+++ b/Lib/test/test_email/test_email.py
@@ -313,6 +313,7 @@ def test_as_string_policy(self):
 
     def test_nonascii_as_string_without_cte(self):
         m = textwrap.dedent("""\
+            MIME-Version: 1.0
             Content-type: text/plain; charset="iso-8859-1"
 
             Test if non-ascii messages with no Content-Transfer-Encoding set
@@ -321,6 +322,7 @@ def test_nonascii_as_string_without_cte(self):
             """)
         source = m.encode('iso-8859-1')
         expected = textwrap.dedent("""\
+            MIME-Version: 1.0
             Content-type: text/plain; charset="iso-8859-1"
             Content-Transfer-Encoding: quoted-printable
 

From b3f3157c18b524c2e104550eec5998eea56aae56 Mon Sep 17 00:00:00 2001
From: Barry Warsaw <barry@python.org>
Date: Mon, 19 Oct 2020 15:06:02 -0700
Subject: [PATCH 3/3] Avoid a "double negative" conditional

---
 Lib/email/generator.py | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/Lib/email/generator.py b/Lib/email/generator.py
index 9e084d98d34e093..c9b121624e08d5e 100644
--- a/Lib/email/generator.py
+++ b/Lib/email/generator.py
@@ -186,10 +186,11 @@ def _write(self, msg):
         # If we munged the cte, copy the message again and re-fix the CTE.
         if munge_cte:
             msg = deepcopy(msg)
-            if msg.get('content-transfer-encoding') is not None:
-                msg.replace_header('content-transfer-encoding', munge_cte[0])
-            else:
+            # Preserve the header order if the CTE header already exists.
+            if msg.get('content-transfer-encoding') is None:
                 msg['Content-Transfer-Encoding'] = munge_cte[0]
+            else:
+                msg.replace_header('content-transfer-encoding', munge_cte[0])
             msg.replace_header('content-type', munge_cte[1])
         # Write the headers.  First we see if the message object wants to
         # handle that itself.  If not, we'll do it generically.