Enhance Policy framework in preparation for adding "eamil6" policy as provisional

Lib/email/_policybase.py

+"""Policy framework for the email package.
+
+Allows fine grained feature control of how the package parses and emits data.
+"""
+
+import abc
+from email import header
+from email import charset as _charset
+from email.utils import _has_surrogates
+
+__all__ = [
+    'Policy',
+    'Compat32',
+    'compat32',
+    ]
+
+
+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__, ', '.join(args))
+
+    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__)
+
+
+# Conceptually this isn't a subclass of ABCMeta, but since we want Policy to
+# use ABCMeta as a metaclass *and* we want it to use this one as well, we have
+# to make this one a subclas of ABCMeta.
+class _DocstringExtenderMetaclass(abc.ABCMeta):
+
+    def __new__(meta, classname, bases, classdict):
+        if classdict.get('__doc__') and classdict['__doc__'].startswith('+'):
+            classdict['__doc__'] = meta._append_doc(bases[0].__doc__,
+                                                    classdict['__doc__'])
+        for name, attr in classdict.items():
+            if attr.__doc__ and attr.__doc__.startswith('+'):
+                for cls in (cls for base in bases for cls in base.mro()):
+                    doc = getattr(getattr(cls, name), '__doc__')
+                    if doc:
+                        attr.__doc__ = meta._append_doc(doc, attr.__doc__)
+                        break
+        return super().__new__(meta, classname, bases, classdict)
+
+    @staticmethod
+    def _append_doc(doc, added_doc):
+        added_doc = added_doc.split('\n', 1)[1]
+        return doc + '\n' + added_doc
+
+
+class Policy(_PolicyBase, metaclass=_DocstringExtenderMetaclass):
+
+    r"""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 results in an error being raised 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'.
+
+    cte_type            -- Type of allowed content transfer encodings
+
+                           7bit  -- ASCII only
+                           8bit  -- Content-Transfer-Encoding: 8bit is allowed
+
+                           Default: 8bit.  Also controls the disposition of
+                           (RFC invalid) binary data in headers; see the
+                           documentation of the binary_fold method.
+
+    max_line_length     -- maximum length of lines, excluding 'linesep',
+                           during serialization.  None or 0 means no line
+                           wrapping is done.  Default is 78.
+
+    """
+
+    raise_on_defect = False
+    linesep = '\n'
+    cte_type = '8bit'
+    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 method is intended to be called by parsers that discover defects.
+        The email package parsers always call it with Defect instances.
+
+        """
+        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.  The objects used by the email package by
+        default that get passed to this method will always have a defects
+        attribute with an append method.
+
+        """
+        obj.defects.append(defect)
+
+    @abc.abstractmethod
+    def header_source_parse(self, sourcelines):
+        """Given a list of linesep terminated strings constituting the lines of
+        a single header, return the (name, value) tuple that should be stored
+        in the model.  The input lines should retain their terminating linesep
+        characters.  The lines passed in by the email package may contain
+        surrogateescaped binary data.
+        """
+        raise NotImplementedError
+
+    @abc.abstractmethod
+    def header_store_parse(self, name, value):
+        """Given the header name and the value provided by the application
+        program, return the (name, value) that should be stored in the model.
+        """
+        raise NotImplementedError
+
+    @abc.abstractmethod
+    def header_fetch_parse(self, name, value):
+        """Given the header name and the value from the model, return the value
+        to be returned to the application program that is requesting that
+        header.  The value passed in by the email package may contain
+        surrogateescaped binary data if the lines were parsed by a BytesParser.
+        The returned value should not contain any surrogateescaped data.
+
+        """
+        raise NotImplementedError
+
+    @abc.abstractmethod
+    def fold(self, name, value):
+        """Given the header name and the value from the model, return a string
+        containing linesep characters that implement the folding of the header
+        according to the policy controls.  The value passed in by the email
+        package may contain surrogateescaped binary data if the lines were
+        parsed by a BytesParser.  The returned value should not contain any
+        surrogateescaped data.
+
+        """
+        raise NotImplementedError
+
+    @abc.abstractmethod
+    def fold_binary(self, name, value):
+        """Given the header name and the value from the model, return binary
+        data containing linesep characters that implement the folding of the
+        header according to the policy controls.  The value passed in by the
+        email package may contain surrogateescaped binary data.
+
+        """
+        raise NotImplementedError
+
+
+class Compat32(Policy):
+
+    """+
+    This particular policy is the backward compatibility Policy.  It
+    replicates the behavior of the email package version 5.1.
+    """
+
+    def _sanitize_header(self, name, value):
+        # If the header value contains surrogates, return a Header using
+        # the unknown-8bit charset to encode the bytes as encoded words.
+        if not isinstance(value, str):
+            # Assume it is already a header object
+            return value
+        if _has_surrogates(value):
+            return header.Header(value, charset=_charset.UNKNOWN8BIT,
+                                 header_name=name)
+        else:
+            return value
+
+    def header_source_parse(self, sourcelines):
+        """+
+        The name is parsed as everything up to the ':' and returned unmodified.
+        The value is determined by stripping leading whitespace off the
+        remainder of the first line, joining all subsequent lines together, and
+        stripping any trailing carriage return or linefeed characters.
+
+        """
+        name, value = sourcelines[0].split(':', 1)
+        value = value.lstrip(' \t') + ''.join(sourcelines[1:])
+        return (name, value.rstrip('\r\n'))
+
+    def header_store_parse(self, name, value):
+        """+
+        The name and value are returned unmodified.
+        """
+        return (name, value)
+
+    def header_fetch_parse(self, name, value):
+        """+
+        If the value contains binary data, it is converted into a Header object
+        using the unknown-8bit charset.  Otherwise it is returned unmodified.
+        """
+        return self._sanitize_header(name, value)
+
+    def fold(self, name, value):
+        """+
+        Headers are folded using the Header folding algorithm, which preserves
+        existing line breaks in the value, and wraps each resulting line to the
+        max_line_length.  Non-ASCII binary data are CTE encoded using the
+        unknown-8bit charset.
+
+        """
+        return self._fold(name, value, sanitize=True)
+
+    def fold_binary(self, name, value):
+        """+
+        Headers are folded using the Header folding algorithm, which preserves
+        existing line breaks in the value, and wraps each resulting line to the
+        max_line_length.  If cte_type is 7bit, non-ascii binary data is CTE
+        encoded using the unknown-8bit charset.  Otherwise the original source
+        header is used, with its existing line breaks and/or binary data.
+
+        """
+        folded = self._fold(name, value, sanitize=self.cte_type=='7bit')
+        return folded.encode('ascii', 'surrogateescape')
+
+    def _fold(self, name, value, sanitize):
+        parts = []
+        parts.append('%s: ' % name)
+        if isinstance(value, str):
+            if _has_surrogates(value):
+                if sanitize:
+                    h = header.Header(value,
+                                      charset=_charset.UNKNOWN8BIT,
+                                      header_name=name)
+                else:
+                    # 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.
+                    parts.append(value)
+                    h = None
+            else:
+                h = header.Header(value, header_name=name)
+        else:
+            # Assume it is a Header-like object.
+            h = value
+        if h is not None:
+            parts.append(h.encode(linesep=self.linesep,
+                                  maxlinelen=self.max_line_length))
+        parts.append(self.linesep)
+        return ''.join(parts)
+
+
+compat32 = Compat32()