#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.

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.
 
-.. function:: message_from_binary_file(fp, _class=email.message.Message, strict=None)
+   .. versionchanged:: 3.3 removed *strict*, added *policy*
+
+.. 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::