#20476: Deal with the message_factory circular import differently.

It turns out we can't depend on email.message getting imported every place
message_factory is needed, so to avoid a circular import we need to special
case Policy.message_factory=None in the parser instead of using monkey
patching.  I had a feeling that was a bad idea when I did it.

Doc/library/email.policy.rst

@@ -224,8 +224,8 @@ added matters.  To illustrate::
    .. attribute:: message_factory
 
       A factory function for constructing a new empty message object.  Used
-      by the parser when building messages.  Defaults to
-      :class:`~email.message.Message`.
+      by the parser when building messages.  Defaults to ``None``, in
+      which case :class:`~email.message.Message` is used.
 
       .. versionadded:: 3.6
 

Lib/email/_policybase.py

@@ -155,6 +155,7 @@ class Policy(_PolicyBase, metaclass=abc.ABCMeta):
                            serialized by a generator. Default: True.
 
     message_factory     -- the class to use to create new message objects.
+                           If the value is None, the default is Message.
 
     """
 
@@ -163,7 +164,6 @@ class Policy(_PolicyBase, metaclass=abc.ABCMeta):
     cte_type = '8bit'
     max_line_length = 78
     mangle_from_ = False
-    # XXX To avoid circular imports, this is set in email.message.
     message_factory = None
 
     def handle_defect(self, obj, defect):

Lib/email/feedparser.py

@@ -147,6 +147,10 @@ class FeedParser:
         self.policy = policy
         self._old_style_factory = False
         if _factory is None:
+            if policy.message_factory is None:
+                from email.message import Message
+                self._factory = Message
+            else:
                 self._factory = policy.message_factory
         else:
             self._factory = _factory

Lib/email/message.py

@@ -1162,6 +1162,3 @@ class EmailMessage(MIMEPart):
         super().set_content(*args, **kw)
         if 'MIME-Version' not in self:
             self['MIME-Version'] = '1.0'
-
-# Set message_factory on Policy here to avoid a circular import.
-Policy.message_factory = Message

Lib/test/test_email/test_policy.py

@@ -24,7 +24,7 @@ class PolicyAPITests(unittest.TestCase):
         'cte_type':                 '8bit',
         'raise_on_defect':          False,
         'mangle_from_':             True,
-        'message_factory':          email.message.Message,
+        'message_factory':          None,
         }
     # These default values are the ones set on email.policy.default.
     # If any of these defaults change, the docs must be updated.