#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.

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)