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/MIMEMultipart.py

+# Copyright (C) 2002 Python Software Foundation
+# Author: barry@zope.com (Barry Warsaw)
+
+"""Base class for MIME multipart/* type messages.
+"""
+
+from email import MIMEBase
+
+
+
+class MIMEMultipart(MIMEBase.MIMEBase):
+    """Base class for MIME multipart/* type messages."""
+
+    def __init__(self, _subtype='mixed', boundary=None, *_subparts, **_params):
+        """Creates a multipart/* type message.
+
+        By default, creates a multipart/mixed message, with proper
+        Content-Type: and MIME-Version: headers.
+
+        _subtype is the subtype of the multipart content type, defaulting to
+        `mixed'.
+
+        boundary is the multipart boundary string.  By default it is
+        calculated as needed.
+
+        _subparts is a sequence of initial subparts for the multipart.  It
+        must be possible to convert this sequence to a list.  You can always
+        attach new subparts to the message by using the attach() method.
+
+        Additional parameters for the Content-Type: header are taken from the
+        keyword arguments (or passed into the _params argument).
+        """
+        MIMEBase.MIMEBase.__init__(self, 'multipart', _subtype, **params)
+        self.attach(*list(_subparts))

Lib/email/MIMENonMultipart.py

+# 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/*')