Merge #14291: if a header has non-ascii unicode, default to CTE using utf-8

In Python2, if a unicode string was assigned as the value of a header,
email would automatically CTE encode it using the UTF8 charset.
This capability was lost in the Python3 translation, and this patch
restores it.

Patch by Ali Ikinci, assisted by R. David Murray.

I also added a fix for the mailbox test that was depending (with a comment
that it was a bad idea to so depend) on non-ASCII causing message_from_string
to raise an error.  It now uses support.patch to induce an error during
message serialization.

5 files changed, 33 insertions, 7 deletions

diff --git a/Lib/email/header.py b/Lib/email/header.py
index 2e687b7..3250d36 100644
--- a/Lib/email/header.py
+++ b/Lib/email/header.py
@@ -283,7 +283,12 @@ class Header:
         # character set, otherwise an early error is thrown.
         output_charset = charset.output_codec or 'us-ascii'
         if output_charset != _charset.UNKNOWN8BIT:
-            s.encode(output_charset, errors)
+            try:
+                s.encode(output_charset, errors)
+            except UnicodeEncodeError:
+                if output_charset!='us-ascii':
+                    raise
+                charset = UTF8
         self._chunks.append((s, charset))
 
     def encode(self, splitchars=';, \t', maxlinelen=None, linesep='\n'):
diff --git a/Lib/test/test_email/test_email.py b/Lib/test/test_email/test_email.py
index 121c939..a4d39ab 100644
--- a/Lib/test/test_email/test_email.py
+++ b/Lib/test/test_email/test_email.py
@@ -604,6 +604,19 @@ class TestMessageAPI(TestEmailBase):
         msg['Dummy'] = 'dummy\nX-Injected-Header: test'
         self.assertRaises(errors.HeaderParseError, msg.as_string)
 
+    def test_unicode_header_defaults_to_utf8_encoding(self):
+        # Issue 14291
+        m = MIMEText('abc\n')
+        m['Subject'] = 'É test'
+        self.assertEqual(str(m),textwrap.dedent("""\
+            Content-Type: text/plain; charset="us-ascii"
+            MIME-Version: 1.0
+            Content-Transfer-Encoding: 7bit
+            Subject: =?utf-8?q?=C3=89_test?=
+
+            abc
+            """))
+
 # Test the email.encoders module
 class TestEncoders(unittest.TestCase):
 
@@ -1045,9 +1058,13 @@ Subject: =?iso-8859-1?q?Britische_Regierung_gibt_gr=FCnes_Licht_f=FCr_Offshore-W
                          'f\xfcr Offshore-Windkraftprojekte '
                          '<a-very-long-address@example.com>')
         msg['Reply-To'] = header_string
-        self.assertRaises(UnicodeEncodeError, msg.as_string)
+        eq(msg.as_string(maxheaderlen=78), """\
+Reply-To: =?utf-8?q?Britische_Regierung_gibt_gr=C3=BCnes_Licht_f=C3=BCr_Offs?=
+ =?utf-8?q?hore-Windkraftprojekte_=3Ca-very-long-address=40example=2Ecom=3E?=
+
+""")
         msg = Message()
-        msg['Reply-To'] = Header(header_string, 'utf-8',
+        msg['Reply-To'] = Header(header_string,
                                  header_name='Reply-To')
         eq(msg.as_string(maxheaderlen=78), """\
 Reply-To: =?utf-8?q?Britische_Regierung_gibt_gr=C3=BCnes_Licht_f=C3=BCr_Offs?=
diff --git a/Lib/test/test_mailbox.py b/Lib/test/test_mailbox.py
index 212ceb9..54963a9 100644
--- a/Lib/test/test_mailbox.py
+++ b/Lib/test/test_mailbox.py
@@ -111,10 +111,10 @@ class TestMailbox(TestBase):
         self.assertMailboxEmpty()
 
     def test_add_that_raises_leaves_mailbox_empty(self):
-        # XXX This test will start failing when Message learns to handle
-        # non-ASCII string headers, and a different internal failure will
-        # need to be found or manufactured.
-        with self.assertRaises(ValueError):
+        def raiser(*args, **kw):
+            raise Exception("a fake error")
+        support.patch(self, email.generator.BytesGenerator, 'flatten', raiser)
+        with self.assertRaises(Exception):
             self._box.add(email.message_from_string("From: Alphöso"))
         self.assertEqual(len(self._box), 0)
         self._box.close()
diff --git a/Misc/ACKS b/Misc/ACKS
index a11d4eb..eeb61e0 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -470,6 +470,7 @@ Gerhard Häring
 Fredrik Håård
 Catalin Iacob
 Mihai Ibanescu
+Ali Ikinci
 Lars Immisch
 Bobby Impollonia
 Meador Inge
diff --git a/Misc/NEWS b/Misc/NEWS
index 9b4d2c2..0d6cdbb 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -24,6 +24,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #14291: Email now defaults to utf-8 for non-ASCII unicode headers
+  instead of raising an error.  This fixes a regression relative to 2.7.
+
 - Issue #989712: Support using Tk without a mainloop.
 
 - Issue #5219: Prevent event handler cascade in IDLE.