#11558: Better message if attach called on non-multipart.

Original patch by Varun Sharma.

Lib/email/message.py

@@ -203,7 +203,11 @@ class Message:
         if self._payload is None:
             self._payload = [payload]
         else:
-            self._payload.append(payload)
+            try:
+                self._payload.append(payload)
+            except AttributeError:
+                raise TypeError("Attach is not valid on a message with a"
+                                " non-multipart payload")
 
     def get_payload(self, i=None, decode=False):
         """Return a reference to the payload.

Lib/test/test_email/test_email.py

@@ -124,6 +124,14 @@ class TestMessageAPI(TestEmailBase):
         msg.set_payload([])
         self.assertEqual(msg.get_payload(), [])
 
+    def test_attach_when_payload_is_string(self):
+        msg = Message()
+        msg['Content-Type'] = 'multipart/mixed'
+        msg.set_payload('string payload')
+        sub_msg = MIMEMessage(Message())
+        self.assertRaisesRegex(TypeError, "[Aa]ttach.*non-multipart",
+                               msg.attach, sub_msg)
+
     def test_get_charsets(self):
         eq = self.assertEqual
 

Misc/ACKS

@@ -1188,6 +1188,7 @@ Daniel Shahaf
 Ha Shao
 Mark Shannon
 Richard Shapiro
+Varun Sharma
 Vlad Shcherbina
 Justin Sheehy
 Charlie Shepherd

Misc/NEWS

@@ -20,6 +20,10 @@ Core and Builtins
 Library
 -------
 
+- Issue #11558: ``email.message.Message.attach`` now returns a more
+  useful error message if ``attach`` is called on a message for which
+  ``is_multipart`` is False.
+
 - Issue #20283: RE pattern methods now accept the string keyword parameters
   as documented.  The pattern and source keyword parameters are left as
   deprecated aliases.