#11731: simplify/enhance parser/generator API by introducing policy objects.

This new interface will also allow for future planned enhancements
in control over the parser/generator without requiring any additional
complexity in the parser/generator API.

Patch reviewed by Éric Araujo and Barry Warsaw.

13 files changed, 912 insertions, 81 deletions

diff --git a/Doc/library/email.generator.rst b/Doc/library/email.generator.rst
index 85b32fe..847d7e4 100644
--- a/Doc/library/email.generator.rst
+++ b/Doc/library/email.generator.rst
@@ -32,7 +32,8 @@ Here are the public methods of the :class:`Generator` class, imported from the
 :mod:`email.generator` module:
 
 
-.. class:: Generator(outfp, mangle_from_=True, maxheaderlen=78)
+.. class:: Generator(outfp, mangle_from_=True, maxheaderlen=78, *, \
+                     policy=policy.default)
 
    The constructor for the :class:`Generator` class takes a :term:`file-like object`
    called *outfp* for an argument.  *outfp* must support the :meth:`write` method
@@ -53,10 +54,16 @@ Here are the public methods of the :class:`Generator` class, imported from the
    :class:`~email.header.Header` class.  Set to zero to disable header wrapping.
    The default is 78, as recommended (but not required) by :rfc:`2822`.
 
+   The *policy* keyword specifies a :mod:`~email.policy` object that controls a
+   number of aspects of the generator's operation.  The default policy
+   maintains backward compatibility.
+
+   .. versionchanged:: 3.3 Added the *policy* keyword.
+
    The other public :class:`Generator` methods are:
 
 
-   .. method:: flatten(msg, unixfrom=False, linesep='\\n')
+   .. method:: flatten(msg, unixfrom=False, linesep=None)
 
       Print the textual representation of the message object structure rooted at
       *msg* to the output file specified when the :class:`Generator` instance
@@ -72,12 +79,13 @@ Here are the public methods of the :class:`Generator` class, imported from the
       Note that for subparts, no envelope header is ever printed.
 
       Optional *linesep* specifies the line separator character used to
-      terminate lines in the output.  It defaults to ``\n`` because that is
-      the most useful value for Python application code (other library packages
-      expect ``\n`` separated lines).  ``linesep=\r\n`` can be used to
-      generate output with RFC-compliant line separators.
+      terminate lines in the output.  If specified it overrides the value
+      specified by the ``Generator``\'s ``policy``.
 
-      Messages parsed with a Bytes parser that have a
+      Because strings cannot represent non-ASCII bytes, ``Generator`` ignores
+      the value of the :attr:`~email.policy.Policy.must_be_7bit`
+      :mod:`~email.policy` setting and operates as if it were set ``True``.
+      This means that messages parsed with a Bytes parser that have a
       :mailheader:`Content-Transfer-Encoding` of 8bit will be converted to a
       use a 7bit Content-Transfer-Encoding.  Non-ASCII bytes in the headers
       will be :rfc:`2047` encoded with a charset of `unknown-8bit`.
@@ -103,7 +111,8 @@ As a convenience, see the :class:`~email.message.Message` methods
 formatted string representation of a message object.  For more detail, see
 :mod:`email.message`.
 
-.. class:: BytesGenerator(outfp, mangle_from_=True, maxheaderlen=78)
+.. class:: BytesGenerator(outfp, mangle_from_=True, maxheaderlen=78, *, \
+                          policy=policy.default)
 
    The constructor for the :class:`BytesGenerator` class takes a binary
    :term:`file-like object` called *outfp* for an argument.  *outfp* must
@@ -125,19 +134,31 @@ formatted string representation of a message object.  For more detail, see
    wrapping.  The default is 78, as recommended (but not required) by
    :rfc:`2822`.
 
+   The *policy* keyword specifies a :mod:`~email.policy` object that controls a
+   number of aspects of the generator's operation.  The default policy
+   maintains backward compatibility.
+
+   .. versionchanged:: 3.3 Added the *policy* keyword.
+
    The other public :class:`BytesGenerator` methods are:
 
 
-   .. method:: flatten(msg, unixfrom=False, linesep='\n')
+   .. method:: flatten(msg, unixfrom=False, linesep=None)
 
       Print the textual representation of the message object structure rooted
       at *msg* to the output file specified when the :class:`BytesGenerator`
       instance was created.  Subparts are visited depth-first and the resulting
-      text will be properly MIME encoded.  If the input that created the *msg*
-      contained bytes with the high bit set and those bytes have not been
-      modified, they will be copied faithfully to the output, even if doing so
-      is not strictly RFC compliant.  (To produce strictly RFC compliant
-      output, use the :class:`Generator` class.)
+      text will be properly MIME encoded.  If the :mod:`~email.policy` option
+      :attr:`~email.policy.Policy.must_be_7bit` is ``False`` (the default),
+      then any bytes with the high bit set in the original parsed message that
+      have not been modified will be copied faithfully to the output.  If
+      ``must_be_7bit`` is true, the bytes will be converted as needed using an
+      ASCII content-transfer-encoding.  In particular, RFC-invalid non-ASCII
+      bytes in headers will be encoded using the MIME ``unknown-8bit``
+      character set, thus rendering them RFC-compliant.
+
+      .. XXX: There should be a complementary option that just does the RFC
+         compliance transformation but leaves CTE 8bit parts alone.
 
       Messages parsed with a Bytes parser that have a
       :mailheader:`Content-Transfer-Encoding` of 8bit will be reconstructed
@@ -152,10 +173,8 @@ formatted string representation of a message object.  For more detail, see
       Note that for subparts, no envelope header is ever printed.
 
       Optional *linesep* specifies the line separator character used to
-      terminate lines in the output.  It defaults to ``\n`` because that is
-      the most useful value for Python application code (other library packages
-      expect ``\n`` separated lines).  ``linesep=\r\n`` can be used to
-      generate output with RFC-compliant line separators.
+      terminate lines in the output.  If specified it overrides the value
+      specified by the ``Generator``\ 's ``policy``.
 
    .. method:: clone(fp)
 
diff --git a/Doc/library/email.parser.rst b/Doc/library/email.parser.rst
index c72d3d4..c5e43a9 100644
--- a/Doc/library/email.parser.rst
+++ b/Doc/library/email.parser.rst
@@ -58,12 +58,18 @@ list of defects that it can find.
 Here is the API for the :class:`FeedParser`:
 
 
-.. class:: FeedParser(_factory=email.message.Message)
+.. class:: FeedParser(_factory=email.message.Message, *, policy=policy.default)
 
    Create a :class:`FeedParser` instance.  Optional *_factory* is a no-argument
    callable that will be called whenever a new message object is needed.  It
    defaults to the :class:`email.message.Message` class.
 
+   The *policy* keyword specifies a :mod:`~email.policy` object that controls a
+   number of aspects of the parser's operation.  The default policy maintains
+   backward compatibility.
+
+   .. versionchanged:: 3.3 Added the *policy* keyword.
+
    .. method:: feed(data)
 
       Feed the :class:`FeedParser` some more data.  *data* should be a string
@@ -104,7 +110,7 @@ have the same API as the :class:`Parser` and :class:`BytesParser` classes.
 .. versionadded:: 3.3 BytesHeaderParser
 
 
-.. class:: Parser(_class=email.message.Message)
+.. class:: Parser(_class=email.message.Message, *, policy=policy.default)
 
    The constructor for the :class:`Parser` class takes an optional argument
    *_class*.  This must be a callable factory (such as a function or a class), and
@@ -112,8 +118,13 @@ have the same API as the :class:`Parser` and :class:`BytesParser` classes.
    :class:`~email.message.Message` (see :mod:`email.message`).  The factory will
    be called without arguments.
 
-   .. versionchanged:: 3.2
-      Removed the *strict* argument that was deprecated in 2.4.
+   The *policy* keyword specifies a :mod:`~email.policy` object that controls a
+   number of aspects of the parser's operation.  The default policy maintains
+   backward compatibility.
+
+   .. versionchanged:: 3.3
+      Removed the *strict* argument that was deprecated in 2.4.  Added the
+      *policy* keyword.
 
    The other public :class:`Parser` methods are:
 
@@ -144,12 +155,18 @@ have the same API as the :class:`Parser` and :class:`BytesParser` classes.
       the entire contents of the file.
 
 
-.. class:: BytesParser(_class=email.message.Message, strict=None)
+.. class:: BytesParser(_class=email.message.Message, *, policy=policy.default)
 
    This class is exactly parallel to :class:`Parser`, but handles bytes input.
    The *_class* and *strict* arguments are interpreted in the same way as for
-   the :class:`Parser` constructor.  *strict* is supported only to make porting
-   code easier; it is deprecated.
+   the :class:`Parser` constructor.
+
+   The *policy* keyword specifies a :mod:`~email.policy` object that
+   controls a number of aspects of the parser's operation.  The default
+   policy maintains backward compatibility.
+
+   .. versionchanged:: 3.3
+      Removed the *strict* argument.  Added the *policy* keyword.
 
    .. method:: parse(fp, headeronly=False)
 
@@ -187,12 +204,15 @@ in the top-level :mod:`email` package namespace.
 
 .. currentmodule:: email
 
-.. function:: message_from_string(s, _class=email.message.Message, strict=None)
+.. function:: message_from_string(s, _class=email.message.Message, *, \
+                                  policy=policy.default)
 
    Return a message object structure from a string.  This is exactly equivalent to
-   ``Parser().parsestr(s)``.  Optional *_class* and *strict* are interpreted as
+   ``Parser().parsestr(s)``.  *_class* and *policy* are interpreted as
    with the :class:`Parser` class constructor.
 
+   .. versionchanged:: removed *strict*, added *policy*
+
 .. function:: message_from_bytes(s, _class=email.message.Message, strict=None)
 
    Return a message object structure from a byte string.  This is exactly
@@ -200,21 +220,27 @@ in the top-level :mod:`email` package namespace.
    *strict* are interpreted as with the :class:`Parser` class constructor.
 
    .. versionadded:: 3.2
+   .. versionchanged:: 3.3 removed *strict*, added *policy*
 
-.. function:: message_from_file(fp, _class=email.message.Message, strict=None)
+.. function:: message_from_file(fp, _class=email.message.Message, *, \
+                                policy=policy.default)
 
    Return a message object structure tree from an open :term:`file object`.
-   This is exactly equivalent to ``Parser().parse(fp)``.  Optional *_class*
-   and *strict* are interpreted as with the :class:`Parser` class constructor.
+   This is exactly equivalent to ``Parser().parse(fp)``.  *_class*
+   and *policy* are interpreted as with the :class:`Parser` class constructor.
+
+   .. versionchanged:: 3.3 removed *strict*, added *policy*
 
-.. function:: message_from_binary_file(fp, _class=email.message.Message, strict=None)
+.. function:: message_from_binary_file(fp, _class=email.message.Message, *, \
+                                       policy=policy.default)
 
    Return a message object structure tree from an open binary :term:`file
    object`.  This is exactly equivalent to ``BytesParser().parse(fp)``.
-   Optional *_class* and *strict* are interpreted as with the :class:`Parser`
+   *_class* and *policy* are interpreted as with the :class:`Parser`
    class constructor.
 
    .. versionadded:: 3.2
+   .. versionchanged:: 3.3 removed *strict*, added *policy*
 
 Here's an example of how you might use this at an interactive Python prompt::
 
diff --git a/Doc/library/email.policy.rst b/Doc/library/email.policy.rst
new file mode 100644
index 0000000..f7eb471
--- /dev/null
+++ b/Doc/library/email.policy.rst
@@ -0,0 +1,179 @@
+:mod:`email`: Policy Objects
+----------------------------
+
+.. module:: email.policy
+   :synopsis: Controlling the parsing and generating of messages
+
+
+The :mod:`email` package's prime focus is the handling of email messages as
+described by the various email and MIME RFCs.  However, the general format of
+email messages (a block of header fields each consisting of a name followed by
+a colon followed by a value, the whole block followed by a blank line and an
+arbitrary 'body'), is a format that has found utility outside of the realm of
+email.  Some of these uses conform fairly closely to the main RFCs, some do
+not.  And even when working with email, there are times when it is desirable to
+break strict compliance with the RFCs.
+
+Policy objects are the mechanism used to provide the email package with the
+flexibility to handle all these disparate use cases,
+
+A :class:`Policy` object encapsulates a set of attributes and methods that
+control the behavior of various components of the email package during use.
+:class:`Policy` instances can be passed to various classes and methods in the
+email package to alter the default behavior.  The settable values and their
+defaults are described below.  The :mod:`policy` module also provides some
+pre-created :class:`Policy` instances.  In addition to a :const:`default`
+instance, there are instances tailored for certain applications.  For example
+there is an :const:`SMTP` :class:`Policy` with defaults appropriate for
+generating output to be sent to an SMTP server.  These are listed :ref:`below
+<Policy Instances>`.
+
+In general an application will only need to deal with setting the policy at the
+input and output boundaries.  Once parsed, a message is represented by a
+:class:`~email.message.Message` object, which is designed to be independent of
+the format that the message has "on the wire" when it is received, transmitted,
+or displayed.  Thus, a :class:`Policy` can be specified when parsing a message
+to create a :class:`~email.message.Message`, and again when turning the
+:class:`~email.message.Message` into some other representation.  While often a
+program will use the same :class:`Policy` for both input and output, the two
+can be different.
+
+As an example, the following code could be used to read an email message from a
+file on disk and pass it to the system ``sendmail`` program on a ``unix``
+system::
+
+   >>> from email import msg_from_binary_file
+   >>> from email.generator import BytesGenerator
+   >>> import email.policy
+   >>> from subprocess import Popen, PIPE
+   >>> with open('mymsg.txt', 'b') as f:
+   >>>     msg = msg_from_binary_file(f, policy=email.policy.mbox)
+   >>> p = Popen(['sendmail', msg['To'][0].address], stdin=PIPE)
+   >>> g = BytesGenerator(p.stdin, email.policy.policy=SMTP)
+   >>> g.flatten(msg)
+   >>> p.stdin.close()
+   >>> rc = p.wait()
+
+Some email package methods accept a *policy* keyword argument, allowing the
+policy to be overridden for that method.  For example, the following code use
+the :meth:`email.message.Message.as_string` method to the *msg* object from the
+previous example and re-write it to a file using the native line separators for
+the platform on which it is running::
+
+   >>> import os
+   >>> mypolicy = email.policy.Policy(linesep=os.linesep)
+   >>> with open('converted.txt', 'wb') as f:
+   ...     f.write(msg.as_string(policy=mypolicy))
+
+Policy instances are immutable, but they can be cloned, accepting the same
+keyword arguments as the class constructor and returning a new :class:`Policy`
+instance that is a copy of the original but with the specified attributes
+values changed.  For example, the following creates an SMTP policy that will
+raise any defects detected as errors::
+
+   >>> strict_SMTP = email.policy.SMTP.clone(raise_on_defect=True)
+
+Policy objects can also be combined using the addition operator, producing a
+policy object whose settings are a combination of the non-default values of the
+summed objects::
+
+   >>> strict_SMTP = email.policy.SMTP + email.policy.strict
+
+This operation is not commutative; that is, the order in which the objects are
+added matters.  To illustrate::
+
+   >>> Policy = email.policy.Policy
+   >>> apolicy = Policy(max_line_length=100) + Policy(max_line_length=80)
+   >>> apolicy.max_line_length
+   80
+   >>> apolicy = Policy(max_line_length=80) + Policy(max_line_length=100)
+   >>> apolicy.max_line_length
+   100
+
+
+.. class:: Policy(**kw)
+
+   The valid constructor keyword arguments are any of the attributes listed
+   below.
+
+   .. attribute:: max_line_length
+
+      The maximum length of any line in the serialized output, not counting the
+      end of line character(s).  Default is 78, per :rfc:`5322`.  A value of
+      ``0`` or :const:`None` indicates that no line wrapping should be
+      done at all.
+
+   .. attribute:: linesep
+
+      The string to be used to terminate lines in serialized output.  The
+      default is '\\n' because that's the internal end-of-line discipline used
+      by Python, though '\\r\\n' is required by the RFCs.  See `Policy
+      Instances`_ for policies that use an RFC conformant linesep.  Setting it
+      to :attr:`os.linesep` may also be useful.
+
+   .. attribute:: must_be_7bit
+
+      If :const:`True`, data output by a bytes generator is limited to ASCII
+      characters.  If :const:`False` (the default), then bytes with the high
+      bit set are preserved and/or allowed in certain contexts (for example,
+      where possible a content transfer encoding of ``8bit`` will be used).
+      String generators act as if ``must_be_7bit`` is `True` regardless of the
+      policy in effect, since a string cannot represent non-ASCII bytes.
+
+   .. attribute:: raise_on_defect
+
+      If :const:`True`, any defects encountered will be raised as errors.  If
+      :const:`False` (the default), defects will be passed to the
+      :meth:`register_defect` method.
+
+   .. method:: handle_defect(obj, defect)
+
+      *obj* is the object on which to register the defect.  *defect* should be
+      an instance of a  subclass of :class:`~email.errors.Defect`.
+      If :attr:`raise_on_defect`
+      is ``True`` the defect is raised as an exception.  Otherwise *obj* and
+      *defect* are passed to :meth:`register_defect`.  This method is intended
+      to be called by parsers when they encounter defects, and will not be
+      called by code that uses the email library unless that code is
+      implementing an alternate parser.
+
+   .. method:: register_defect(obj, defect)
+
+      *obj* is the object on which to register the defect.  *defect* should be
+      a subclass of :class:`~email.errors.Defect`.  This method is part of the
+      public API so that custom ``Policy`` subclasses can implement alternate
+      handling of defects.  The default implementation calls the ``append``
+      method of the ``defects`` attribute of *obj*.
+
+   .. method:: clone(obj, *kw):
+
+      Return a new :class:`Policy` instance whose attributes have the same
+      values as the current instance, except where those attributes are
+      given new values by the keyword arguments.
+
+
+Policy Instances
+................
+
+The following instances of :class:`Policy` provide defaults suitable for
+specific common application domains.
+
+.. data:: default
+
+    An instance of :class:`Policy` with all defaults unchanged.
+
+.. data:: SMTP
+
+    Output serialized from a message will conform to the email and SMTP
+    RFCs.  The only changed attribute is :attr:`linesep`, which is set to
+    ``\r\n``.
+
+.. data:: HTTP
+
+    Suitable for use when serializing headers for use in HTTP traffic.
+    :attr:`linesep` is set to ``\r\n``, and :attr:`max_line_length` is set to
+    :const:`None` (unlimited).
+
+.. data:: strict
+
+    :attr:`raise_on_defect` is set to :const:`True`.
diff --git a/Lib/email/errors.py b/Lib/email/errors.py
index d52a624..c2ea7d4 100644
--- a/Lib/email/errors.py
+++ b/Lib/email/errors.py
@@ -32,7 +32,7 @@ class CharsetError(MessageError):
 
 
 # These are parsing defects which the parser was able to work around.
-class MessageDefect:
+class MessageDefect(Exception):
     """Base class for a message defect."""
 
     def __init__(self, line=None):
diff --git a/Lib/email/feedparser.py b/Lib/email/feedparser.py
index 1b752d0..60de49e 100644
--- a/Lib/email/feedparser.py
+++ b/Lib/email/feedparser.py
@@ -25,6 +25,7 @@ import re
 
 from email import errors
 from email import message
+from email import policy
 
 NLCRE = re.compile('\r\n|\r|\n')
 NLCRE_bol = re.compile('(\r\n|\r|\n)')
@@ -134,9 +135,16 @@ class BufferedSubFile(object):
 class FeedParser:
     """A feed-style parser of email."""
 
-    def __init__(self, _factory=message.Message):
-        """_factory is called with no arguments to create a new message obj"""
+    def __init__(self, _factory=message.Message, *, policy=policy.default):
+        """_factory is called with no arguments to create a new message obj
+
+        The policy keyword specifies a policy object that controls a number of
+        aspects of the parser's operation.  The default policy maintains
+        backward compatibility.
+
+        """
         self._factory = _factory
+        self.policy = policy
         self._input = BufferedSubFile()
         self._msgstack = []
         self._parse = self._parsegen().__next__
@@ -168,7 +176,8 @@ class FeedParser:
         # Look for final set of defects
         if root.get_content_maintype() == 'multipart' \
                and not root.is_multipart():
-            root.defects.append(errors.MultipartInvariantViolationDefect())
+            defect = errors.MultipartInvariantViolationDefect()
+            self.policy.handle_defect(root, defect)
         return root
 
     def _new_message(self):
@@ -281,7 +290,8 @@ class FeedParser:
                 # defined a boundary.  That's a problem which we'll handle by
                 # reading everything until the EOF and marking the message as
                 # defective.
-                self._cur.defects.append(errors.NoBoundaryInMultipartDefect())
+                defect = errors.NoBoundaryInMultipartDefect()
+                self.policy.handle_defect(self._cur, defect)
                 lines = []
                 for line in self._input:
                     if line is NeedMoreData:
@@ -385,7 +395,8 @@ class FeedParser:
             # that as a defect and store the captured text as the payload.
             # Everything from here to the EOF is epilogue.
             if capturing_preamble:
-                self._cur.defects.append(errors.StartBoundaryNotFoundDefect())
+                defect = errors.StartBoundaryNotFoundDefect()
+                self.policy.handle_defect(self._cur, defect)
                 self._cur.set_payload(EMPTYSTRING.join(preamble))
                 epilogue = []
                 for line in self._input:
@@ -437,7 +448,7 @@ class FeedParser:
                     # is illegal, so let's note the defect, store the illegal
                     # line, and ignore it for purposes of headers.
                     defect = errors.FirstHeaderLineIsContinuationDefect(line)
-                    self._cur.defects.append(defect)
+                    self.policy.handle_defect(self._cur, defect)
                     continue
                 lastvalue.append(line)
                 continue
diff --git a/Lib/email/generator.py b/Lib/email/generator.py
index fdd34e4..d8b8fa9 100644
--- a/Lib/email/generator.py
+++ b/Lib/email/generator.py
@@ -13,8 +13,10 @@ import random
 import warnings
 
 from io import StringIO, BytesIO
+from email import policy
 from email.header import Header
 from email.message import _has_surrogates
+import email.charset as _charset
 
 UNDERSCORE = '_'
 NL = '\n'  # XXX: no longer used by the code below.
@@ -33,7 +35,8 @@ class Generator:
     # Public interface
     #
 
-    def __init__(self, outfp, mangle_from_=True, maxheaderlen=78):
+    def __init__(self, outfp, mangle_from_=True, maxheaderlen=None, *,
+                 policy=policy.default):
         """Create the generator for message flattening.
 
         outfp is the output file-like object for writing the message to.  It
@@ -49,16 +52,23 @@ class Generator:
         defined in the Header class.  Set maxheaderlen to zero to disable
         header wrapping.  The default is 78, as recommended (but not required)
         by RFC 2822.
+
+        The policy keyword specifies a policy object that controls a number of
+        aspects of the generator's operation.  The default policy maintains
+        backward compatibility.
+
         """
         self._fp = outfp
         self._mangle_from_ = mangle_from_
-        self._maxheaderlen = maxheaderlen
+        self._maxheaderlen = (maxheaderlen if maxheaderlen is not None else
+                                 policy.max_line_length)
+        self.policy = policy
 
     def write(self, s):
         # Just delegate to the file object
         self._fp.write(s)
 
-    def flatten(self, msg, unixfrom=False, linesep='\n'):
+    def flatten(self, msg, unixfrom=False, linesep=None):
         r"""Print the message object tree rooted at msg to the output file
         specified when the Generator instance was created.
 
@@ -70,17 +80,15 @@ class Generator:
         Note that for subobjects, no From_ line is printed.
 
         linesep specifies the characters used to indicate a new line in
-        the output.  The default value is the most useful for typical
-        Python applications, but it can be set to \r\n to produce RFC-compliant
-        line separators when needed.
+        the output.  The default value is determined by the policy.
 
         """
         # We use the _XXX constants for operating on data that comes directly
         # from the msg, and _encoded_XXX constants for operating on data that
         # has already been converted (to bytes in the BytesGenerator) and
         # inserted into a temporary buffer.
-        self._NL = linesep
-        self._encoded_NL = self._encode(linesep)
+        self._NL = linesep if linesep is not None else self.policy.linesep
+        self._encoded_NL = self._encode(self._NL)
         self._EMPTY = ''
         self._encoded_EMTPY = self._encode('')
         if unixfrom:
@@ -338,7 +346,10 @@ class BytesGenerator(Generator):
 
     Functionally identical to the base Generator except that the output is
     bytes and not string.  When surrogates were used in the input to encode
-    bytes, these are decoded back to bytes for output.
+    bytes, these are decoded back to bytes for output.  If the policy has
+    must_be_7bit set true, then the message is transformed such that the
+    non-ASCII bytes are properly content transfer encoded, using the
+    charset unknown-8bit.
 
     The outfp object must accept bytes in its write method.
     """
@@ -361,21 +372,22 @@ class BytesGenerator(Generator):
         # strings with 8bit bytes.
         for h, v in msg._headers:
             self.write('%s: ' % h)
-            if isinstance(v, Header):
-                self.write(v.encode(maxlinelen=self._maxheaderlen)+NL)
-            elif _has_surrogates(v):
-                # If we have raw 8bit data in a byte string, we have no idea
-                # what the encoding is.  There is no safe way to split this
-                # string.  If it's ascii-subset, then we could do a normal
-                # ascii split, but if it's multibyte then we could break the
-                # string.  There's no way to know so the least harm seems to
-                # be to not split the string and risk it being too long.
-                self.write(v+NL)
-            else:
-                # Header's got lots of smarts and this string is safe...
-                header = Header(v, maxlinelen=self._maxheaderlen,
-                                header_name=h)
-                self.write(header.encode(linesep=self._NL)+self._NL)
+            if isinstance(v, str):
+                if _has_surrogates(v):
+                    if not self.policy.must_be_7bit:
+                        # If we have raw 8bit data in a byte string, we have no idea
+                        # what the encoding is.  There is no safe way to split this
+                        # string.  If it's ascii-subset, then we could do a normal
+                        # ascii split, but if it's multibyte then we could break the
+                        # string.  There's no way to know so the least harm seems to
+                        # be to not split the string and risk it being too long.
+                        self.write(v+NL)
+                        continue
+                    h = Header(v, charset=_charset.UNKNOWN8BIT, header_name=h)
+                else:
+                    h = Header(v, header_name=h)
+            self.write(h.encode(linesep=self._NL,
+                                maxlinelen=self._maxheaderlen)+self._NL)
         # A blank line always separates headers from body
         self.write(self._NL)
 
@@ -384,7 +396,7 @@ class BytesGenerator(Generator):
         # just write it back out.
         if msg._payload is None:
             return
-        if _has_surrogates(msg._payload):
+        if _has_surrogates(msg._payload) and not self.policy.must_be_7bit:
             self.write(msg._payload)
         else:
             super(BytesGenerator,self)._handle_text(msg)
diff --git a/Lib/email/parser.py b/Lib/email/parser.py
index fc5090b..0f92160 100644
--- a/Lib/email/parser.py
+++ b/Lib/email/parser.py
@@ -11,11 +11,12 @@ from io import StringIO, TextIOWrapper
 
 from email.feedparser import FeedParser
 from email.message import Message
+from email import policy
 
 
 
 class Parser:
-    def __init__(self, _class=Message):
+    def __init__(self, _class=Message, *, policy=policy.default):
         """Parser of RFC 2822 and MIME email messages.
 
         Creates an in-memory object tree representing the email message, which
@@ -30,8 +31,14 @@ class Parser:
         _class is the class to instantiate for new message objects when they
         must be created.  This class must have a constructor that can take
         zero arguments.  Default is Message.Message.
+
+        The policy keyword specifies a policy object that controls a number of
+        aspects of the parser's operation.  The default policy maintains
+        backward compatibility.
+
         """
         self._class = _class
+        self.policy = policy
 
     def parse(self, fp, headersonly=False):
         """Create a message structure from the data in a file.
@@ -41,7 +48,7 @@ class Parser:
         parsing after reading the headers or not.  The default is False,
         meaning it parses the entire contents of the file.
         """
-        feedparser = FeedParser(self._class)
+        feedparser = FeedParser(self._class, policy=self.policy)
         if headersonly:
             feedparser._set_headersonly()
         while True:
diff --git a/Lib/email/policy.py b/Lib/email/policy.py
new file mode 100644
index 0000000..88877a2
--- /dev/null
+++ b/Lib/email/policy.py
@@ -0,0 +1,174 @@
+"""Policy framework for the email package.
+
+Allows fine grained feature control of how the package parses and emits data.
+"""
+
+__all__ = [
+    'Policy',
+    'default',
+    'strict',
+    'SMTP',
+    'HTTP',
+    ]
+
+
+class _PolicyBase:
+
+    """Policy Object basic framework.
+
+    This class is useless unless subclassed.  A subclass should define
+    class attributes with defaults for any values that are to be
+    managed by the Policy object.  The constructor will then allow
+    non-default values to be set for these attributes at instance
+    creation time.  The instance will be callable, taking these same
+    attributes keyword arguments, and returning a new instance
+    identical to the called instance except for those values changed
+    by the keyword arguments.  Instances may be added, yielding new
+    instances with any non-default values from the right hand
+    operand overriding those in the left hand operand.  That is,
+
+        A + B == A(<non-default values of B>)
+
+    The repr of an instance can be used to reconstruct the object
+    if and only if the repr of the values can be used to reconstruct
+    those values.
+
+    """
+
+    def __init__(self, **kw):
+        """Create new Policy, possibly overriding some defaults.
+
+        See class docstring for a list of overridable attributes.
+
+        """
+        for name, value in kw.items():
+            if hasattr(self, name):
+                super(_PolicyBase,self).__setattr__(name, value)
+            else:
+                raise TypeError(
+                    "{!r} is an invalid keyword argument for {}".format(
+                        name, self.__class__.__name__))
+
+    def __repr__(self):
+        args = [ "{}={!r}".format(name, value)
+                 for name, value in self.__dict__.items() ]
+        return "{}({})".format(self.__class__.__name__, args if args else '')
+
+    def clone(self, **kw):
+        """Return a new instance with specified attributes changed.
+
+        The new instance has the same attribute values as the current object,
+        except for the changes passed in as keyword arguments.
+
+        """
+        for attr, value in self.__dict__.items():
+            if attr not in kw:
+                kw[attr] = value
+        return self.__class__(**kw)
+
+    def __setattr__(self, name, value):
+        if hasattr(self, name):
+            msg = "{!r} object attribute {!r} is read-only"
+        else:
+            msg = "{!r} object has no attribute {!r}"
+        raise AttributeError(msg.format(self.__class__.__name__, name))
+
+    def __add__(self, other):
+        """Non-default values from right operand override those from left.
+
+        The object returned is a new instance of the subclass.
+
+        """
+        return self.clone(**other.__dict__)
+
+
+class Policy(_PolicyBase):
+
+    """Controls for how messages are interpreted and formatted.
+
+    Most of the classes and many of the methods in the email package
+    accept Policy objects as parameters.  A Policy object contains a set
+    of values and functions that control how input is interpreted and how
+    output is rendered.  For example, the parameter 'raise_on_defect'
+    controls whether or not an RFC violation throws an error or not,
+    while 'max_line_length' controls the maximum length of output lines
+    when a Message is serialized.
+
+    Any valid attribute may be overridden when a Policy is created by
+    passing it as a keyword argument to the constructor.  Policy
+    objects are immutable, but a new Policy object can be created
+    with only certain values changed by calling the Policy instance
+    with keyword arguments.  Policy objects can also be added,
+    producing a new Policy object in which the non-default attributes
+    set in the right hand operand overwrite those specified in the
+    left operand.
+
+    Settable attributes:
+
+    raise_on_defect     -- If true, then defects should be raised
+                           as errors.  Default False.
+
+    linesep             -- string containing the value to use as
+                           separation between output lines.  Default '\n'.
+
+    must_be_7bit        -- output must contain only 7bit clean data.
+                           Default False.
+
+    max_line_length     -- maximum length of lines, excluding 'linesep',
+                           during serialization.  None means no line
+                           wrapping is done.  Default is 78.
+
+    Methods:
+
+    register_defect(obj, defect)
+        defect is a Defect instance.  The default implementation appends defect
+        to the objs 'defects' attribute.
+
+    handle_defect(obj, defect)
+        intended to be called by parser code that finds a defect.  If
+        raise_on_defect is True, defect is raised as an error, otherwise
+        register_defect is called.
+
+    """
+
+    raise_on_defect = False
+    linesep = '\n'
+    must_be_7bit = False
+    max_line_length = 78
+
+    def handle_defect(self, obj, defect):
+        """Based on policy, either raise defect or call register_defect.
+
+            handle_defect(obj, defect)
+
+        defect should be a Defect subclass, but in any case must be an
+        Exception subclass.  obj is the object on which the defect should be
+        registered if it is not raised.  If the raise_on_defect is True, the
+        defect is raised as an error, otherwise the object and the defect are
+        passed to register_defect.
+
+        This class is intended to be called by parsers that discover defects,
+        and will not be called from code using the library unless that code is
+        implementing an alternate parser.
+
+        """
+        if self.raise_on_defect:
+            raise defect
+        self.register_defect(obj, defect)
+
+    def register_defect(self, obj, defect):
+        """Record 'defect' on 'obj'.
+
+        Called by handle_defect if raise_on_defect is False.  This method is
+        part of the Policy API so that Policy subclasses can implement custom
+        defect handling.  The default implementation calls the append method
+        of the defects attribute of obj.
+
+        """
+        obj.defects.append(defect)
+
+
+default = Policy()
+strict = default.clone(raise_on_defect=True)
+SMTP = default.clone(linesep='\r\n')
+HTTP = default.clone(linesep='\r\n', max_line_length=None)
diff --git a/Lib/test/test_email/__init__.py b/Lib/test/test_email/__init__.py
index 69be678..04fdf89 100644
--- a/Lib/test/test_email/__init__.py
+++ b/Lib/test/test_email/__init__.py
@@ -25,6 +25,8 @@ def openfile(filename, *args, **kws):
 # Base test class
 class TestEmailBase(unittest.TestCase):
 
+    maxDiff = None
+
     def __init__(self, *args, **kw):
         super().__init__(*args, **kw)
         self.addTypeEqualityFunc(bytes, self.assertBytesEqual)
diff --git a/Lib/test/test_email/test_email.py b/Lib/test/test_email/test_email.py
index 46206c3..1657afc 100644
--- a/Lib/test/test_email/test_email.py
+++ b/Lib/test/test_email/test_email.py
@@ -1776,7 +1776,12 @@ YXNkZg==
 
 
 # Test some badly formatted messages
-class TestNonConformant(TestEmailBase):
+class TestNonConformantBase:
+
+    def _msgobj(self, filename):
+        with openfile(filename) as fp:
+            return email.message_from_file(fp, policy=self.policy)
+
     def test_parse_missing_minor_type(self):
         eq = self.assertEqual
         msg = self._msgobj('msg_14.txt')
@@ -1790,17 +1795,18 @@ class TestNonConformant(TestEmailBase):
         # XXX We can probably eventually do better
         inner = msg.get_payload(0)
         unless(hasattr(inner, 'defects'))
-        self.assertEqual(len(inner.defects), 1)
-        unless(isinstance(inner.defects[0],
+        self.assertEqual(len(self.get_defects(inner)), 1)
+        unless(isinstance(self.get_defects(inner)[0],
                           errors.StartBoundaryNotFoundDefect))
 
     def test_multipart_no_boundary(self):
         unless = self.assertTrue
         msg = self._msgobj('msg_25.txt')
         unless(isinstance(msg.get_payload(), str))
-        self.assertEqual(len(msg.defects), 2)
-        unless(isinstance(msg.defects[0], errors.NoBoundaryInMultipartDefect))
-        unless(isinstance(msg.defects[1],
+        self.assertEqual(len(self.get_defects(msg)), 2)
+        unless(isinstance(self.get_defects(msg)[0],
+                          errors.NoBoundaryInMultipartDefect))
+        unless(isinstance(self.get_defects(msg)[1],
                           errors.MultipartInvariantViolationDefect))
 
     def test_invalid_content_type(self):
@@ -1856,9 +1862,10 @@ counter to RFC 2822, there's no separating newline here
         unless = self.assertTrue
         msg = self._msgobj('msg_41.txt')
         unless(hasattr(msg, 'defects'))
-        self.assertEqual(len(msg.defects), 2)
-        unless(isinstance(msg.defects[0], errors.NoBoundaryInMultipartDefect))
-        unless(isinstance(msg.defects[1],
+        self.assertEqual(len(self.get_defects(msg)), 2)
+        unless(isinstance(self.get_defects(msg)[0],
+                          errors.NoBoundaryInMultipartDefect))
+        unless(isinstance(self.get_defects(msg)[1],
                           errors.MultipartInvariantViolationDefect))
 
     def test_missing_start_boundary(self):
@@ -1872,21 +1879,71 @@ counter to RFC 2822, there's no separating newline here
         #
         # [*] This message is missing its start boundary
         bad = outer.get_payload(1).get_payload(0)
-        self.assertEqual(len(bad.defects), 1)
-        self.assertTrue(isinstance(bad.defects[0],
+        self.assertEqual(len(self.get_defects(bad)), 1)
+        self.assertTrue(isinstance(self.get_defects(bad)[0],
                                    errors.StartBoundaryNotFoundDefect))
 
     def test_first_line_is_continuation_header(self):
         eq = self.assertEqual
         m = ' Line 1\nLine 2\nLine 3'
-        msg = email.message_from_string(m)
+        msg = email.message_from_string(m, policy=self.policy)
         eq(msg.keys(), [])
         eq(msg.get_payload(), 'Line 2\nLine 3')
-        eq(len(msg.defects), 1)
-        self.assertTrue(isinstance(msg.defects[0],
+        eq(len(self.get_defects(msg)), 1)
+        self.assertTrue(isinstance(self.get_defects(msg)[0],
                                    errors.FirstHeaderLineIsContinuationDefect))
-        eq(msg.defects[0].line, ' Line 1\n')
+        eq(self.get_defects(msg)[0].line, ' Line 1\n')
+
+
+class TestNonConformant(TestNonConformantBase, TestEmailBase):
+
+    policy=email.policy.default
+
+    def get_defects(self, obj):
+        return obj.defects
+
+
+class TestNonConformantCapture(TestNonConformantBase, TestEmailBase):
+
+    class CapturePolicy(email.policy.Policy):
+        captured = None
+        def register_defect(self, obj, defect):
+            self.captured.append(defect)
+
+    def setUp(self):
+        self.policy = self.CapturePolicy(captured=list())
+
+    def get_defects(self, obj):
+        return self.policy.captured
+
 
+class TestRaisingDefects(TestEmailBase):
+
+    def _msgobj(self, filename):
+        with openfile(filename) as fp:
+            return email.message_from_file(fp, policy=email.policy.strict)
+
+    def test_same_boundary_inner_outer(self):
+        with self.assertRaises(errors.StartBoundaryNotFoundDefect):
+            self._msgobj('msg_15.txt')
+
+    def test_multipart_no_boundary(self):
+        with self.assertRaises(errors.NoBoundaryInMultipartDefect):
+            self._msgobj('msg_25.txt')
+
+    def test_lying_multipart(self):
+        with self.assertRaises(errors.NoBoundaryInMultipartDefect):
+            self._msgobj('msg_41.txt')
+
+
+    def test_missing_start_boundary(self):
+        with self.assertRaises(errors.StartBoundaryNotFoundDefect):
+            self._msgobj('msg_42.txt')
+
+    def test_first_line_is_continuation_header(self):
+        m = ' Line 1\nLine 2\nLine 3'
+        with self.assertRaises(errors.FirstHeaderLineIsContinuationDefect):
+            msg = email.message_from_string(m, policy=email.policy.strict)
 
 
 # Test RFC 2047 header encoding and decoding
@@ -2997,6 +3054,25 @@ Here's the message body
         g.flatten(msg, linesep='\r\n')
         self.assertEqual(s.getvalue(), text)
 
+    def test_crlf_control_via_policy(self):
+        with openfile('msg_26.txt', newline='\n') as fp:
+            text = fp.read()
+        msg = email.message_from_string(text)
+        s = StringIO()
+        g = email.generator.Generator(s, policy=email.policy.SMTP)
+        g.flatten(msg)
+        self.assertEqual(s.getvalue(), text)
+
+    def test_flatten_linesep_overrides_policy(self):
+        # msg_27 is lf separated
+        with openfile('msg_27.txt', newline='\n') as fp:
+            text = fp.read()
+        msg = email.message_from_string(text)
+        s = StringIO()
+        g = email.generator.Generator(s, policy=email.policy.SMTP)
+        g.flatten(msg, linesep='\n')
+        self.assertEqual(s.getvalue(), text)
+
     maxDiff = None
 
     def test_multipart_digest_with_extra_mime_headers(self):
@@ -3463,6 +3539,44 @@ class Test8BitBytesHandling(unittest.TestCase):
         g.flatten(msg)
         self.assertEqual(s.getvalue(), source)
 
+    def test_crlf_control_via_policy(self):
+        # msg_26 is crlf terminated
+        with openfile('msg_26.txt', 'rb') as fp:
+            text = fp.read()
+        msg = email.message_from_bytes(text)
+        s = BytesIO()
+        g = email.generator.BytesGenerator(s, policy=email.policy.SMTP)
+        g.flatten(msg)
+        self.assertEqual(s.getvalue(), text)
+
+    def test_flatten_linesep_overrides_policy(self):
+        # msg_27 is lf separated
+        with openfile('msg_27.txt', 'rb') as fp:
+            text = fp.read()
+        msg = email.message_from_bytes(text)
+        s = BytesIO()
+        g = email.generator.BytesGenerator(s, policy=email.policy.SMTP)
+        g.flatten(msg, linesep='\n')
+        self.assertEqual(s.getvalue(), text)
+
+    def test_must_be_7bit_handles_unknown_8bit(self):
+        msg = email.message_from_bytes(self.non_latin_bin_msg)
+        out = BytesIO()
+        g = email.generator.BytesGenerator(out,
+                        policy=email.policy.default.clone(must_be_7bit=True))
+        g.flatten(msg)
+        self.assertEqual(out.getvalue(),
+            self.non_latin_bin_msg_as7bit_wrapped.encode('ascii'))
+
+    def test_must_be_7bit_transforms_8bit_cte(self):
+        msg = email.message_from_bytes(self.latin_bin_msg)
+        out = BytesIO()
+        g = email.generator.BytesGenerator(out,
+                        policy=email.policy.default.clone(must_be_7bit=True))
+        g.flatten(msg)
+        self.assertEqual(out.getvalue(),
+                        self.latin_bin_msg_as7bit.encode('ascii'))
+
     maxDiff = None
 
 
diff --git a/Lib/test/test_email/test_generator.py b/Lib/test/test_email/test_generator.py
new file mode 100644
index 0000000..35ca6c5
--- /dev/null
+++ b/Lib/test/test_email/test_generator.py
@@ -0,0 +1,136 @@
+import io
+import textwrap
+import unittest
+from email import message_from_string, message_from_bytes
+from email.generator import Generator, BytesGenerator
+from email import policy
+from test.test_email import TestEmailBase
+
+# XXX: move generator tests from test_email into here at some point.
+
+
+class TestGeneratorBase():
+
+    long_subject = {
+        0: textwrap.dedent("""\
+            To: whom_it_may_concern@example.com
+            From: nobody_you_want_to_know@example.com
+            Subject: We the willing led by the unknowing are doing the
+             impossible for the ungrateful. We have done so much for so long with so little
+             we are now qualified to do anything with nothing.
+
+            None
+            """),
+        40: textwrap.dedent("""\
+            To: whom_it_may_concern@example.com
+            From:\x20
+             nobody_you_want_to_know@example.com
+            Subject: We the willing led by the
+             unknowing are doing the
+             impossible for the ungrateful. We have
+             done so much for so long with so little
+             we are now qualified to do anything
+             with nothing.
+
+            None
+            """),
+        20: textwrap.dedent("""\
+            To:\x20
+             whom_it_may_concern@example.com
+            From:\x20
+             nobody_you_want_to_know@example.com
+            Subject: We the
+             willing led by the
+             unknowing are doing
+             the
+             impossible for the
+             ungrateful. We have
+             done so much for so
+             long with so little
+             we are now
+             qualified to do
+             anything with
+             nothing.
+
+            None
+            """),
+        }
+    long_subject[100] = long_subject[0]
+
+    def maxheaderlen_parameter_test(self, n):
+        msg = self.msgmaker(self.long_subject[0])
+        s = self.ioclass()
+        g = self.genclass(s, maxheaderlen=n)
+        g.flatten(msg)
+        self.assertEqual(s.getvalue(), self.long_subject[n])
+
+    def test_maxheaderlen_parameter_0(self):
+        self.maxheaderlen_parameter_test(0)
+
+    def test_maxheaderlen_parameter_100(self):
+        self.maxheaderlen_parameter_test(100)
+
+    def test_maxheaderlen_parameter_40(self):
+        self.maxheaderlen_parameter_test(40)
+
+    def test_maxheaderlen_parameter_20(self):
+        self.maxheaderlen_parameter_test(20)
+
+    def maxheaderlen_policy_test(self, n):
+        msg = self.msgmaker(self.long_subject[0])
+        s = self.ioclass()
+        g = self.genclass(s, policy=policy.default.clone(max_line_length=n))
+        g.flatten(msg)
+        self.assertEqual(s.getvalue(), self.long_subject[n])
+
+    def test_maxheaderlen_policy_0(self):
+        self.maxheaderlen_policy_test(0)
+
+    def test_maxheaderlen_policy_100(self):
+        self.maxheaderlen_policy_test(100)
+
+    def test_maxheaderlen_policy_40(self):
+        self.maxheaderlen_policy_test(40)
+
+    def test_maxheaderlen_policy_20(self):
+        self.maxheaderlen_policy_test(20)
+
+    def maxheaderlen_parm_overrides_policy_test(self, n):
+        msg = self.msgmaker(self.long_subject[0])
+        s = self.ioclass()
+        g = self.genclass(s, maxheaderlen=n,
+                          policy=policy.default.clone(max_line_length=10))
+        g.flatten(msg)
+        self.assertEqual(s.getvalue(), self.long_subject[n])
+
+    def test_maxheaderlen_parm_overrides_policy_0(self):
+        self.maxheaderlen_parm_overrides_policy_test(0)
+
+    def test_maxheaderlen_parm_overrides_policy_100(self):
+        self.maxheaderlen_parm_overrides_policy_test(100)
+
+    def test_maxheaderlen_parm_overrides_policy_40(self):
+        self.maxheaderlen_parm_overrides_policy_test(40)
+
+    def test_maxheaderlen_parm_overrides_policy_20(self):
+        self.maxheaderlen_parm_overrides_policy_test(20)
+
+
+class TestGenerator(TestGeneratorBase, TestEmailBase):
+
+    msgmaker = staticmethod(message_from_string)
+    genclass = Generator
+    ioclass = io.StringIO
+
+
+class TestBytesGenerator(TestGeneratorBase, TestEmailBase):
+
+    msgmaker = staticmethod(message_from_bytes)
+    genclass = BytesGenerator
+    ioclass = io.BytesIO
+    long_subject = {key: x.encode('ascii')
+        for key, x in TestGeneratorBase.long_subject.items()}
+
+
+if __name__ == '__main__':
+    unittest.main()
diff --git a/Lib/test/test_email/test_policy.py b/Lib/test/test_email/test_policy.py
new file mode 100644
index 0000000..086ce40
--- /dev/null
+++ b/Lib/test/test_email/test_policy.py
@@ -0,0 +1,148 @@
+import types
+import unittest
+import email.policy
+
+class PolicyAPITests(unittest.TestCase):
+
+    longMessage = True
+
+    # These default values are the ones set on email.policy.default.
+    # If any of these defaults change, the docs must be updated.
+    policy_defaults = {
+        'max_line_length':          78,
+        'linesep':                  '\n',
+        'must_be_7bit':             False,
+        'raise_on_defect':          False,
+        }
+
+    # For each policy under test, we give here the values of the attributes
+    # that are different from the defaults for that policy.
+    policies = {
+        email.policy.Policy(): {},
+        email.policy.default: {},
+        email.policy.SMTP: {'linesep': '\r\n'},
+        email.policy.HTTP: {'linesep': '\r\n', 'max_line_length': None},
+        email.policy.strict: {'raise_on_defect': True},
+        }
+
+    def test_defaults(self):
+        for policy, changed_defaults in self.policies.items():
+            expected = self.policy_defaults.copy()
+            expected.update(changed_defaults)
+            for attr, value in expected.items():
+                self.assertEqual(getattr(policy, attr), value,
+                                ("change {} docs/docstrings if defaults have "
+                                "changed").format(policy))
+
+    def test_all_attributes_covered(self):
+        for attr in dir(email.policy.default):
+            if (attr.startswith('_') or
+               isinstance(getattr(email.policy.Policy, attr),
+                          types.FunctionType)):
+                continue
+            else:
+                self.assertIn(attr, self.policy_defaults,
+                              "{} is not fully tested".format(attr))
+
+    def test_policy_is_immutable(self):
+        for policy in self.policies:
+            for attr in self.policy_defaults:
+                with self.assertRaisesRegex(AttributeError, attr+".*read-only"):
+                    setattr(policy, attr, None)
+            with self.assertRaisesRegex(AttributeError, 'no attribute.*foo'):
+                policy.foo = None
+
+    def test_set_policy_attrs_when_calledl(self):
+        testattrdict = { attr: None for attr in self.policy_defaults }
+        for policyclass in self.policies:
+            policy = policyclass.clone(**testattrdict)
+            for attr in self.policy_defaults:
+                self.assertIsNone(getattr(policy, attr))
+
+    def test_reject_non_policy_keyword_when_called(self):
+        for policyclass in self.policies:
+            with self.assertRaises(TypeError):
+                policyclass(this_keyword_should_not_be_valid=None)
+            with self.assertRaises(TypeError):
+                policyclass(newtline=None)
+
+    def test_policy_addition(self):
+        expected = self.policy_defaults.copy()
+        p1 = email.policy.default.clone(max_line_length=100)
+        p2 = email.policy.default.clone(max_line_length=50)
+        added = p1 + p2
+        expected.update(max_line_length=50)
+        for attr, value in expected.items():
+            self.assertEqual(getattr(added, attr), value)
+        added = p2 + p1
+        expected.update(max_line_length=100)
+        for attr, value in expected.items():
+            self.assertEqual(getattr(added, attr), value)
+        added = added + email.policy.default
+        for attr, value in expected.items():
+            self.assertEqual(getattr(added, attr), value)
+
+    def test_register_defect(self):
+        class Dummy:
+            def __init__(self):
+                self.defects = []
+        obj = Dummy()
+        defect = object()
+        policy = email.policy.Policy()
+        policy.register_defect(obj, defect)
+        self.assertEqual(obj.defects, [defect])
+        defect2 = object()
+        policy.register_defect(obj, defect2)
+        self.assertEqual(obj.defects, [defect, defect2])
+
+    class MyObj:
+        def __init__(self):
+            self.defects = []
+
+    class MyDefect(Exception):
+        pass
+
+    def test_handle_defect_raises_on_strict(self):
+        foo = self.MyObj()
+        defect = self.MyDefect("the telly is broken")
+        with self.assertRaisesRegex(self.MyDefect, "the telly is broken"):
+            email.policy.strict.handle_defect(foo, defect)
+
+    def test_handle_defect_registers_defect(self):
+        foo = self.MyObj()
+        defect1 = self.MyDefect("one")
+        email.policy.default.handle_defect(foo, defect1)
+        self.assertEqual(foo.defects, [defect1])
+        defect2 = self.MyDefect("two")
+        email.policy.default.handle_defect(foo, defect2)
+        self.assertEqual(foo.defects, [defect1, defect2])
+
+    class MyPolicy(email.policy.Policy):
+        defects = []
+        def register_defect(self, obj, defect):
+            self.defects.append(defect)
+
+    def test_overridden_register_defect_still_raises(self):
+        foo = self.MyObj()
+        defect = self.MyDefect("the telly is broken")
+        with self.assertRaisesRegex(self.MyDefect, "the telly is broken"):
+            self.MyPolicy(raise_on_defect=True).handle_defect(foo, defect)
+
+    def test_overriden_register_defect_works(self):
+        foo = self.MyObj()
+        defect1 = self.MyDefect("one")
+        my_policy = self.MyPolicy()
+        my_policy.handle_defect(foo, defect1)
+        self.assertEqual(my_policy.defects, [defect1])
+        self.assertEqual(foo.defects, [])
+        defect2 = self.MyDefect("two")
+        my_policy.handle_defect(foo, defect2)
+        self.assertEqual(my_policy.defects, [defect1, defect2])
+        self.assertEqual(foo.defects, [])
+
+    # XXX: Need subclassing tests.
+    # For adding subclassed objects, make sure the usual rules apply (subclass
+    # wins), but that the order still works (right overrides left).
+
+if __name__ == '__main__':
+    unittest.main()
diff --git a/Misc/NEWS b/Misc/NEWS
index 3481960..f46c4de 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -110,6 +110,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #11731: simplify/enhance email parser/generator API by introducing
+  policy objects.
+
 - Issue #11768: The signal handler of the signal module only calls
   Py_AddPendingCall() for the first signal to fix a deadlock on reentrant or
   parallel calls. PyErr_SetInterrupt() writes also into the wake up file.