#20476: add a message_factory policy attribute to email.

9 files changed, 128 insertions, 66 deletions

diff --git a/Doc/library/email.parser.rst b/Doc/library/email.parser.rst
index 4dbad49..2ac1f98 100644
--- a/Doc/library/email.parser.rst
+++ b/Doc/library/email.parser.rst
@@ -73,8 +73,9 @@ Here is the API for the :class:`BytesFeedParser`:
 .. class:: BytesFeedParser(_factory=None, *, policy=policy.compat32)
 
    Create a :class:`BytesFeedParser` instance.  Optional *_factory* is a
-   no-argument callable; if not specified determine the default based on the
-   *policy*.  Call *_factory* whenever a new message object is needed.
+   no-argument callable; if not specified use the
+   :attr:`~email.policy.Policy.message_factory` from the *policy*.  Call
+   *_factory* whenever a new message object is needed.
 
    If *policy* is specified use the rules it specifies to update the
    representation of the message.  If *policy* is not set, use the
@@ -91,6 +92,7 @@ Here is the API for the :class:`BytesFeedParser`:
    .. versionadded:: 3.2
 
    .. versionchanged:: 3.3 Added the *policy* keyword.
+   .. versionchanged:: 3.6 _factory defaults to the policy ``message_factory``.
 
 
    .. method:: feed(data)
@@ -146,6 +148,7 @@ message body, instead setting the payload to the raw body.
    .. versionchanged:: 3.3
       Removed the *strict* argument that was deprecated in 2.4.  Added the
       *policy* keyword.
+   .. versionchanged:: 3.6 _class defaults to the policy ``message_factory``.
 
 
    .. method:: parse(fp, headersonly=False)
@@ -194,6 +197,7 @@ message body, instead setting the payload to the raw body.
 
    .. versionchanged:: 3.3
       Removed the *strict* argument.  Added the *policy* keyword.
+   .. versionchanged:: 3.6 _class defaults to the policy ``message_factory``.
 
 
    .. method:: parse(fp, headersonly=False)
@@ -230,8 +234,7 @@ in the top-level :mod:`email` package namespace.
 .. currentmodule:: email
 
 
-.. function:: message_from_bytes(s, _class=None, *, \
-                                 policy=policy.compat32)
+.. function:: message_from_bytes(s, _class=None, *, policy=policy.compat32)
 
    Return a message object structure from a :term:`bytes-like object`.  This is
    equivalent to ``BytesParser().parsebytes(s)``.  Optional *_class* and
@@ -243,7 +246,7 @@ in the top-level :mod:`email` package namespace.
       Removed the *strict* argument.  Added the *policy* keyword.
 
 
-.. function:: message_from_binary_file(fp, _class=None, *, \
+.. function:: message_from_binary_file(fp, _class=None, *,
                                        policy=policy.compat32)
 
    Return a message object structure tree from an open binary :term:`file
@@ -256,8 +259,7 @@ in the top-level :mod:`email` package namespace.
       Removed the *strict* argument.  Added the *policy* keyword.
 
 
-.. function:: message_from_string(s, _class=None, *, \
-                                  policy=policy.compat32)
+.. function:: message_from_string(s, _class=None, *, policy=policy.compat32)
 
    Return a message object structure from a string.  This is equivalent to
    ``Parser().parsestr(s)``.  *_class* and *policy* are interpreted as
@@ -267,8 +269,7 @@ in the top-level :mod:`email` package namespace.
       Removed the *strict* argument.  Added the *policy* keyword.
 
 
-.. function:: message_from_file(fp, _class=None, *, \
-                                policy=policy.compat32)
+.. function:: message_from_file(fp, _class=None, *, policy=policy.compat32)
 
    Return a message object structure tree from an open :term:`file object`.
    This is equivalent to ``Parser().parse(fp)``.  *_class* and *policy* are
@@ -276,6 +277,7 @@ in the top-level :mod:`email` package namespace.
 
    .. versionchanged:: 3.3
       Removed the *strict* argument.  Added the *policy* keyword.
+   .. versionchanged:: 3.6 _class defaults to the policy ``message_factory``.
 
 
 Here's an example of how you might use :func:`message_from_bytes` at an
diff --git a/Doc/library/email.policy.rst b/Doc/library/email.policy.rst
index b499ed8..b345683 100644
--- a/Doc/library/email.policy.rst
+++ b/Doc/library/email.policy.rst
@@ -221,6 +221,14 @@ added matters.  To illustrate::
          The *mangle_from_* parameter.
 
 
+   .. 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`.
+
+      .. versionadded:: 3.6
+
    The following :class:`Policy` method is intended to be called by code using
    the email library to create policy instances with custom settings:
 
@@ -368,6 +376,9 @@ added matters.  To illustrate::
    on the type of the field.  The parsing and folding algorithm fully implement
    :rfc:`2047` and :rfc:`5322`.
 
+   The default value for the :attr:`~email.policy.Policy.message_factory`
+   attribute is :class:`~email.message.EmailMessage`.
+
    In addition to the settable attributes listed above that apply to all
    policies, this policy adds the following additional attributes:
 
diff --git a/Doc/whatsnew/3.6.rst b/Doc/whatsnew/3.6.rst
index a5b467b..ec2f2e0 100644
--- a/Doc/whatsnew/3.6.rst
+++ b/Doc/whatsnew/3.6.rst
@@ -598,6 +598,13 @@ The :mod:`email.mime` classes now all accept an optional *policy* keyword.
 The :class:`~email.generator.DecodedGenerator` now supports the *policy*
 keyword.
 
+There is a new :mod:`~email.policy` attribute,
+:attr:`~email.policy.Policy.message_factory`, that controls what class is used
+by default when the parser creates new message objects.  For the
+:attr:`email.policy.compat32` policy this is :class:`~email.message.Message`,
+for the new policies it is :class:`~email.message.EmailMessage`.
+(Contributed by R. David Murray in :issue:`20476`.)
+
 
 encodings
 ---------
diff --git a/Lib/email/_policybase.py b/Lib/email/_policybase.py
index c0d98a4..d699484 100644
--- a/Lib/email/_policybase.py
+++ b/Lib/email/_policybase.py
@@ -154,6 +154,8 @@ class Policy(_PolicyBase, metaclass=abc.ABCMeta):
                            them. This is used when the message is being
                            serialized by a generator. Default: True.
 
+    message_factory     -- the class to use to create new message objects.
+
     """
 
     raise_on_defect = False
@@ -161,6 +163,8 @@ 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):
         """Based on policy, either raise defect or call register_defect.
diff --git a/Lib/email/feedparser.py b/Lib/email/feedparser.py
index 2fa77d7..3d74978 100644
--- a/Lib/email/feedparser.py
+++ b/Lib/email/feedparser.py
@@ -24,7 +24,6 @@ __all__ = ['FeedParser', 'BytesFeedParser']
 import re
 
 from email import errors
-from email import message
 from email._policybase import compat32
 from collections import deque
 from io import StringIO
@@ -148,13 +147,7 @@ class FeedParser:
         self.policy = policy
         self._old_style_factory = False
         if _factory is None:
-            # What this should be:
-            #self._factory = policy.default_message_factory
-            # but, because we are post 3.4 feature freeze, fix with temp hack:
-            if self.policy is compat32:
-                self._factory = message.Message
-            else:
-                self._factory = message.EmailMessage
+            self._factory = policy.message_factory
         else:
             self._factory = _factory
             try:
diff --git a/Lib/email/message.py b/Lib/email/message.py
index c07da43..f4380d9 100644
--- a/Lib/email/message.py
+++ b/Lib/email/message.py
@@ -4,18 +4,17 @@
 
 """Basic message object for the email package object model."""
 
-__all__ = ['Message']
+__all__ = ['Message', 'EmailMessage']
 
 import re
 import uu
 import quopri
-import warnings
 from io import BytesIO, StringIO
 
 # Intrapackage imports
 from email import utils
 from email import errors
-from email._policybase import compat32
+from email._policybase import Policy, compat32
 from email import charset as _charset
 from email._encoded_words import decode_b
 Charset = _charset.Charset
@@ -1163,3 +1162,6 @@ 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
diff --git a/Lib/email/policy.py b/Lib/email/policy.py
index 35d0e69..5131311ac 100644
--- a/Lib/email/policy.py
+++ b/Lib/email/policy.py
@@ -7,6 +7,7 @@ from email._policybase import Policy, Compat32, compat32, _extend_docstrings
 from email.utils import _has_surrogates
 from email.headerregistry import HeaderRegistry as HeaderRegistry
 from email.contentmanager import raw_data_manager
+from email.message import EmailMessage
 
 __all__ = [
     'Compat32',
@@ -82,6 +83,7 @@ class EmailPolicy(Policy):
 
     """
 
+    message_factory = EmailMessage
     utf8 = False
     refold_source = 'long'
     header_factory = HeaderRegistry()
diff --git a/Lib/test/test_email/test_parser.py b/Lib/test/test_email/test_parser.py
index 8ddc1763..06c8640 100644
--- a/Lib/test/test_email/test_parser.py
+++ b/Lib/test/test_email/test_parser.py
@@ -1,7 +1,7 @@
 import io
 import email
 import unittest
-from email.message import Message
+from email.message import Message, EmailMessage
 from email.policy import default
 from test.test_email import TestEmailBase
 
@@ -39,38 +39,71 @@ class TestParserBase:
         # The unicode line splitter splits on unicode linebreaks, which are
         # more numerous than allowed by the email RFCs; make sure we are only
         # splitting on those two.
-        msg = self.parser(
-            "Next-Line: not\x85broken\r\n"
-            "Null: not\x00broken\r\n"
-            "Vertical-Tab: not\vbroken\r\n"
-            "Form-Feed: not\fbroken\r\n"
-            "File-Separator: not\x1Cbroken\r\n"
-            "Group-Separator: not\x1Dbroken\r\n"
-            "Record-Separator: not\x1Ebroken\r\n"
-            "Line-Separator: not\u2028broken\r\n"
-            "Paragraph-Separator: not\u2029broken\r\n"
-            "\r\n",
-            policy=default,
-        )
-        self.assertEqual(msg.items(), [
-            ("Next-Line", "not\x85broken"),
-            ("Null", "not\x00broken"),
-            ("Vertical-Tab", "not\vbroken"),
-            ("Form-Feed", "not\fbroken"),
-            ("File-Separator", "not\x1Cbroken"),
-            ("Group-Separator", "not\x1Dbroken"),
-            ("Record-Separator", "not\x1Ebroken"),
-            ("Line-Separator", "not\u2028broken"),
-            ("Paragraph-Separator", "not\u2029broken"),
-        ])
-        self.assertEqual(msg.get_payload(), "")
+        for parser in self.parsers:
+            with self.subTest(parser=parser.__name__):
+                msg = parser(
+                    "Next-Line: not\x85broken\r\n"
+                    "Null: not\x00broken\r\n"
+                    "Vertical-Tab: not\vbroken\r\n"
+                    "Form-Feed: not\fbroken\r\n"
+                    "File-Separator: not\x1Cbroken\r\n"
+                    "Group-Separator: not\x1Dbroken\r\n"
+                    "Record-Separator: not\x1Ebroken\r\n"
+                    "Line-Separator: not\u2028broken\r\n"
+                    "Paragraph-Separator: not\u2029broken\r\n"
+                    "\r\n",
+                    policy=default,
+                )
+                self.assertEqual(msg.items(), [
+                    ("Next-Line", "not\x85broken"),
+                    ("Null", "not\x00broken"),
+                    ("Vertical-Tab", "not\vbroken"),
+                    ("Form-Feed", "not\fbroken"),
+                    ("File-Separator", "not\x1Cbroken"),
+                    ("Group-Separator", "not\x1Dbroken"),
+                    ("Record-Separator", "not\x1Ebroken"),
+                    ("Line-Separator", "not\u2028broken"),
+                    ("Paragraph-Separator", "not\u2029broken"),
+                ])
+                self.assertEqual(msg.get_payload(), "")
+
+    class MyMessage(EmailMessage):
+        pass
+
+    def test_custom_message_factory_on_policy(self):
+        for parser in self.parsers:
+            with self.subTest(parser=parser.__name__):
+                MyPolicy = default.clone(message_factory=self.MyMessage)
+                msg = parser("To: foo\n\ntest", policy=MyPolicy)
+                self.assertIsInstance(msg, self.MyMessage)
+
+    def test_factory_arg_overrides_policy(self):
+        for parser in self.parsers:
+            with self.subTest(parser=parser.__name__):
+                MyPolicy = default.clone(message_factory=self.MyMessage)
+                msg = parser("To: foo\n\ntest", Message, policy=MyPolicy)
+                self.assertNotIsInstance(msg, self.MyMessage)
+                self.assertIsInstance(msg, Message)
+
+# Play some games to get nice output in subTest.  This code could be clearer
+# if staticmethod supported __name__.
+
+def message_from_file(s, *args, **kw):
+    f = io.StringIO(s)
+    return email.message_from_file(f, *args, **kw)
 
 class TestParser(TestParserBase, TestEmailBase):
-    parser = staticmethod(email.message_from_string)
+    parsers = (email.message_from_string, message_from_file)
+
+def message_from_bytes(s, *args, **kw):
+    return email.message_from_bytes(s.encode(), *args, **kw)
+
+def message_from_binary_file(s, *args, **kw):
+    f = io.BytesIO(s.encode())
+    return email.message_from_binary_file(f, *args, **kw)
 
 class TestBytesParser(TestParserBase, TestEmailBase):
-    def parser(self, s, *args, **kw):
-        return email.message_from_bytes(s.encode(), *args, **kw)
+    parsers = (message_from_bytes, message_from_binary_file)
 
 
 if __name__ == '__main__':
diff --git a/Lib/test/test_email/test_policy.py b/Lib/test/test_email/test_policy.py
index 70ac4db..1d95d03 100644
--- a/Lib/test/test_email/test_policy.py
+++ b/Lib/test/test_email/test_policy.py
@@ -5,6 +5,7 @@ import unittest
 import email.policy
 import email.parser
 import email.generator
+import email.message
 from email import headerregistry
 
 def make_defaults(base_defaults, differences):
@@ -23,6 +24,7 @@ class PolicyAPITests(unittest.TestCase):
         'cte_type':                 '8bit',
         'raise_on_defect':          False,
         'mangle_from_':             True,
+        'message_factory':          email.message.Message,
         }
     # These default values are the ones set on email.policy.default.
     # If any of these defaults change, the docs must be updated.
@@ -34,6 +36,7 @@ class PolicyAPITests(unittest.TestCase):
         'refold_source':            'long',
         'content_manager':          email.policy.EmailPolicy.content_manager,
         'mangle_from_':             False,
+        'message_factory':          email.message.EmailMessage,
         })
 
     # For each policy under test, we give here what we expect the defaults to
@@ -62,20 +65,22 @@ class PolicyAPITests(unittest.TestCase):
     def test_defaults(self):
         for policy, expected in self.policies.items():
             for attr, value in expected.items():
-                self.assertEqual(getattr(policy, attr), value,
-                                ("change {} docs/docstrings if defaults have "
-                                "changed").format(policy))
+                with self.subTest(policy=policy, attr=attr):
+                    self.assertEqual(getattr(policy, attr), value,
+                                    ("change {} docs/docstrings if defaults have "
+                                    "changed").format(policy))
 
     def test_all_attributes_covered(self):
         for policy, expected in self.policies.items():
             for attr in dir(policy):
-                if (attr.startswith('_') or
-                        isinstance(getattr(email.policy.EmailPolicy, attr),
-                              types.FunctionType)):
-                    continue
-                else:
-                    self.assertIn(attr, expected,
-                                  "{} is not fully tested".format(attr))
+                with self.subTest(policy=policy, attr=attr):
+                    if (attr.startswith('_') or
+                            isinstance(getattr(email.policy.EmailPolicy, attr),
+                                  types.FunctionType)):
+                        continue
+                    else:
+                        self.assertIn(attr, expected,
+                                      "{} is not fully tested".format(attr))
 
     def test_abc(self):
         with self.assertRaises(TypeError) as cm:
@@ -237,6 +242,9 @@ class PolicyAPITests(unittest.TestCase):
     # wins), but that the order still works (right overrides left).
 
 
+class TestException(Exception):
+    pass
+
 class TestPolicyPropagation(unittest.TestCase):
 
     # The abstract methods are used by the parser but not by the wrapper
@@ -244,40 +252,40 @@ class TestPolicyPropagation(unittest.TestCase):
     # policy was actually propagated all the way to feedparser.
     class MyPolicy(email.policy.Policy):
         def badmethod(self, *args, **kw):
-            raise Exception("test")
+            raise TestException("test")
         fold = fold_binary = header_fetch_parser = badmethod
         header_source_parse = header_store_parse = badmethod
 
     def test_message_from_string(self):
-        with self.assertRaisesRegex(Exception, "^test$"):
+        with self.assertRaisesRegex(TestException, "^test$"):
             email.message_from_string("Subject: test\n\n",
                                       policy=self.MyPolicy)
 
     def test_message_from_bytes(self):
-        with self.assertRaisesRegex(Exception, "^test$"):
+        with self.assertRaisesRegex(TestException, "^test$"):
             email.message_from_bytes(b"Subject: test\n\n",
                                      policy=self.MyPolicy)
 
     def test_message_from_file(self):
         f = io.StringIO('Subject: test\n\n')
-        with self.assertRaisesRegex(Exception, "^test$"):
+        with self.assertRaisesRegex(TestException, "^test$"):
             email.message_from_file(f, policy=self.MyPolicy)
 
     def test_message_from_binary_file(self):
         f = io.BytesIO(b'Subject: test\n\n')
-        with self.assertRaisesRegex(Exception, "^test$"):
+        with self.assertRaisesRegex(TestException, "^test$"):
             email.message_from_binary_file(f, policy=self.MyPolicy)
 
     # These are redundant, but we need them for black-box completeness.
 
     def test_parser(self):
         p = email.parser.Parser(policy=self.MyPolicy)
-        with self.assertRaisesRegex(Exception, "^test$"):
+        with self.assertRaisesRegex(TestException, "^test$"):
             p.parsestr('Subject: test\n\n')
 
     def test_bytes_parser(self):
         p = email.parser.BytesParser(policy=self.MyPolicy)
-        with self.assertRaisesRegex(Exception, "^test$"):
+        with self.assertRaisesRegex(TestException, "^test$"):
             p.parsebytes(b'Subject: test\n\n')
 
     # Now that we've established that all the parse methods get the