These two classes provide bases for more specific content type

subclasses.

MIMENonMultipart: Base class for non-multipart/* content type subclass
specializations, e.g. image/gif.  This class overrides attach() which
raises an exception, since it makes no sense to attach a subpart to
e.g. an image/gif message.

MIMEMultipart: Base class for multipart/* content type subclass
specializations, e.g. multipart/mixed.  Does little more than provide
a useful constructor.

Lib/email/MIMENonMultipart.py

@@ -0,0 +1,20 @@
+# Copyright (C) 2002 Python Software Foundation
+# Author: barry@zope.com (Barry Warsaw)
+
+"""Base class for MIME type messages that are not multipart.
+"""
+
+from email import Errors
+from email import MIMEBase
+
+
+
+class MIMENonMultipart(MIMEBase.MIMEBase):
+    """Base class for MIME multipart/* type messages."""
+
+    def attach(self, payload):
+        # The public API prohibits attaching multiple subparts to MIMEBase
+        # derived subtypes since none of them are, by definition, of content
+        # type multipart/*
+        raise Errors.MultipartConversionError(
+            'Cannot attach additional subparts to non-multipart/*')