#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.

1 files changed, 37 insertions, 18 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)