bpo-27321 Fix email.generator.py to not replace a non-existent header. (GH-18074)
This PR replaces #1977. The reason for the replacement is two-fold.
The fix itself is different is that if the CTE header doesn't exist in the original message, it is inserted. This is important because the new CTE could be quoted-printable whereas the original is implicit 8bit.
Also the tests are different. The test_nonascii_as_string_without_cte test in #1977 doesn't actually test the issue in that it passes without the fix. The test_nonascii_as_string_without_content_type_and_cte test is improved here, and even though it doesn't fail without the fix, it is included for completeness.
Automerge-Triggered-By: @warsaw
diff --git a/Lib/email/generator.py b/Lib/email/generator.py
index ae670c2..c9b1216 100644
--- a/Lib/email/generator.py
+++ b/Lib/email/generator.py
@@ -186,7 +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)
- msg.replace_header('content-transfer-encoding', munge_cte[0])
+ # 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.