Doc/library/email.policy.rst

:mod:`email.policy`: Policy Objects
-----------------------------------

.. module:: email.policy
   :synopsis: Controlling the parsing and generating of messages

.. moduleauthor:: R. David Murray <rdmurray@bitdance.com>
.. sectionauthor:: R. David Murray <rdmurray@bitdance.com>

.. versionadded:: 3.3


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 give the email package 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.

There is a default policy used by all classes in the email package.  This
policy is named :class:`Compat32`, with a corresponding pre-defined instance
named :const:`compat32`.  It provides for complete backward compatibility (in
some cases, including bug compatibility) with the pre-Python3.3 version of the
email package.

The first part of this documentation covers the features of :class:`Policy`, an
:term:`abstract base class`  that defines the features that are common to all
policy objects, including :const:`compat32`.  This includes certain hook
methods that are called internally by the email package, which a custom policy
could override to obtain different behavior.

When a :class:`~email.message.Message` object is created, it acquires a policy.
By default this will be :const:`compat32`, but a different policy can be
specified.  If the ``Message`` is created by a :mod:`~email.parser`, a policy
passed to the parser will be the policy used by the ``Message`` it creates.  If
the ``Message`` is created by the program, then the policy can be specified
when it is created.  When a ``Message`` is passed to a :mod:`~email.generator`,
the generator uses the policy from the ``Message`` by default, but you can also
pass a specific policy to the generator that will override the one stored on
the ``Message`` object.

:class:`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.

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
   >>> from subprocess import Popen, PIPE
   >>> with open('mymsg.txt', 'b') as f:
   ...     msg = msg_from_binary_file(f)
   >>> p = Popen(['sendmail', msg['To'][0].address], stdin=PIPE)
   >>> g = BytesGenerator(p.stdin, policy=msg.policy.clone(linesep='\r\n'))
   >>> g.flatten(msg)
   >>> p.stdin.close()
   >>> rc = p.wait()

Here we are telling :class:`~email.generator.BytesGenerator` to use the RFC
correct line separator characters when creating the binary string to feed into
``sendmail's`` ``stdin``, where the default policy would use ``\n`` line
separators.

Some email package methods accept a *policy* keyword argument, allowing the
policy to be overridden for that method.  For example, the following code uses
the :meth:`~email.message.Message.as_string` method of the *msg* object from
the previous example and writes the message to a file using the native line
separators for the platform on which it is running::

   >>> import os
   >>> with open('converted.txt', 'wb') as f:
   ...     f.write(msg.as_string(policy=msg.policy.clone(linesep=os.linesep))

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::

   >>> compat_SMTP = email.policy.clone(linesep='\r\n')
   >>> compat_strict = email.policy.clone(raise_on_defect=True)
   >>> compat_strict_SMTP = compat_SMTP + compat_strict

This operation is not commutative; that is, the order in which the objects are
added matters.  To illustrate::

   >>> policy100 = compat32.clone(max_line_length=100)
   >>> policy80 = compat32.clone(max_line_length=80)
   >>> apolicy = policy100 + Policy80
   >>> apolicy.max_line_length
   80
   >>> apolicy = policy80 + policy100
   >>> apolicy.max_line_length
   100


.. class:: Policy(**kw)

   This is the :term:`abstract base class` for all policy classes.  It provides
   default implementations for a couple of trivial methods, as well as the
   implementation of the immutability property, the :meth:`clone` method, and
   the constructor semantics.

   The constructor of a policy class can be passed various keyword arguments.
   The arguments that may be specified are any non-method properties on this
   class, plus any additional non-method properties on the concrete class.  A
   value specified in the constructor will override the default value for the
   corresponding attribute.

   This class defines the following properties, and thus values for the
   following may be passed in the constructor of any policy class:

   .. 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.

   .. attribute:: cte_type

      Controls the type of Content Transfer Encodings that may be or are
      required to be used.  The possible values are:

      ========  ===============================================================
      ``7bit``  all data must be "7 bit clean" (ASCII-only).  This means that
                where necessary data will be encoded using either
                quoted-printable or base64 encoding.

      ``8bit``  data is not constrained to be 7 bit clean.  Data in headers is
                still required to be ASCII-only and so will be encoded (see
                'binary_fold' below for an exception), but body parts may use
                the ``8bit`` CTE.
      ========  ===============================================================

      A ``cte_type`` value of ``8bit`` only works with ``BytesGenerator``, not
      ``Generator``, because strings cannot contain binary data.  If a
      ``Generator`` is operating under a policy that specifies
      ``cte_type=8bit``, it will act as if ``cte_type`` is ``7bit``.

   .. 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.

   The following :class:`Policy` method is intended to be called by code using
   the email library to create policy instances with custom settings:

   .. method:: clone(**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.

   The remaining :class:`Policy` methods are called by the email package code,
   and are not intended to be called by an application using the email package.
   A custom policy must implement all of these methods.

   .. method:: handle_defect(obj, defect)

      Handle a *defect* found on *obj*.  When the email package calls this
      method, *defect* will always be a subclass of
      :class:`~email.errors.Defect`.

      The default implementation checks the :attr:`raise_on_defect` flag.  If
      it is ``True``, *defect* is raised as an exception.  If it is ``False``
      (the default), *obj* and *defect* are passed to :meth:`register_defect`.

   .. method:: register_defect(obj, defect)

      Register a *defect* on *obj*.  In the email package, *defect* will always
      be a subclass of :class:`~email.errors.Defect`.

      The default implementation calls the ``append`` method of the ``defects``
      attribute of *obj*.  When the email package calls :attr:`handle_defect`,
      *obj* will normally have a ``defects`` attribute that has an ``append``
      method.  Custom object types used with the email package (for example,
      custom ``Message`` objects) should also provide such an attribute,
      otherwise defects in parsed messages will raise unexpected errors.

   .. method:: header_max_count(name)

      Return the maximum allowed number of headers named *name*.

      Called when a header is added to a :class:`~email.message.Message`
      object.  If the returned value is not ``0`` or ``None``, and there are
      already a number of headers with the name *name* equal to the value
      returned, a :exc:`ValueError` is raised.

      Because the default behavior of ``Message.__setitem__`` is to append the
      value to the list of headers, it is easy to create duplicate headers
      without realizing it.  This method allows certain headers to be limited
      in the number of instances of that header that may be added to a
      ``Message`` programmatically.  (The limit is not observed by the parser,
      which will faithfully produce as many headers as exist in the message
      being parsed.)

      The default implementation returns ``None`` for all header names.

   .. method:: header_source_parse(sourcelines)

      The email package calls this method with a list of strings, each string
      ending with the line separation characters found in the source being
      parsed.  The first line includes the field header name and separator.
      All whitespace in the source is preserved.  The method should return the
      ``(name, value)`` tuple that is to be stored in the ``Message`` to
      represent the parsed header.

      If an implementation wishes to retain compatibility with the existing
      email package policies, *name* should be the case preserved name (all
      characters up to the '``:``' separator), while *value* should be the
      unfolded value (all line separator characters removed, but whitespace
      kept intact), stripped of leading whitespace.

      *sourcelines* may contain surrogateescaped binary data.

      There is no default implementation

   .. method:: header_store_parse(name, value)

      The email package calls this method with the name and value provided by
      the application program when the application program is modifying a
      ``Message`` programmatically (as opposed to a ``Message`` created by a
      parser).  The method should return the ``(name, value)`` tuple that is to
      be stored in the ``Message`` to represent the header.

      If an implementation wishes to retain compatibility with the existing
      email package policies, the *name* and *value* should be strings or
      string subclasses that do not change the content of the passed in
      arguments.

      There is no default implementation

   .. method:: header_fetch_parse(name, value)

      The email package calls this method with the *name* and *value* currently
      stored in the ``Message`` when that header is requested by the
      application program, and whatever the method returns is what is passed
      back to the application as the value of the header being retrieved.
      Note that there may be more than one header with the same name stored in
      the ``Message``; the method is passed the specific name and value of the
      header destined to be returned to the application.

      *value* may contain surrogateescaped binary data.  There should be no
      surrogateescaped binary data in the value returned by the method.

      There is no default implementation

   .. method:: fold(name, value)

      The email package calls this method with the *name* and *value* currently
      stored in the ``Message`` for a given header.  The method should return a
      string that represents that header "folded" correctly (according to the
      policy settings) by composing the *name* with the *value* and inserting
      :attr:`linesep` characters at the appropriate places.  See :rfc:`5322`
      for a discussion of the rules for folding email headers.

      *value* may contain surrogateescaped binary data.  There should be no
      surrogateescaped binary data in the string returned by the method.

   .. method:: fold_binary(name, value)

      The same as :meth:`fold`, except that the returned value should be a
      bytes object rather than a string.

      *value* may contain surrogateescaped binary data.  These could be
      converted back into binary data in the returned bytes object.


.. class:: Compat32(**kw)

   This concrete :class:`Policy` is the backward compatibility policy.  It
   replicates the behavior of the email package in Python 3.2.  The
   :mod:`policy` module also defines an instance of this class,
   :const:`compat32`, that is used as the default policy.  Thus the default
   behavior of the email package is to maintain compatibility with Python 3.2.

   The class provides the following concrete implementations of the
   abstract methods of :class:`Policy`:

   .. method:: header_source_parse(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.

   .. method:: header_store_parse(name, value)

      The name and value are returned unmodified.

   .. method:: header_fetch_parse(name, value)

      If the value contains binary data, it is converted into a
      :class:`~email.header.Header` object using the ``unknown-8bit`` charset.
      Otherwise it is returned unmodified.

   .. method:: fold(name, value)

      Headers are folded using the :class:`~email.header.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.

   .. method:: fold_binary(name, value)

      Headers are folded using the :class:`~email.header.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 and any (RFC invalid) binary data it may contain.


.. note::

   The documentation below describes new policies that are included in the
   standard library on a :term:`provisional basis <provisional package>`.
   Backwards incompatible changes (up to and including removal of the feature)
   may occur if deemed necessary by the core developers.


.. class:: EmailPolicy(**kw)

   This concrete :class:`Policy` provides behavior that is intended to be fully
   compliant with the current email RFCs.  These include (but are not limited
   to) :rfc:`5322`, :rfc:`2047`, and the current MIME RFCs.

   This policy adds new header parsing and folding algorithms.  Instead of
   simple strings, headers are custom objects with custom attributes depending
   on the type of the field.  The parsing and folding algorithm fully implement
   :rfc:`2047` and :rfc:`5322`.

   In addition to the settable attributes listed above that apply to all
   policies, this policy adds the following additional attributes:

   .. attribute:: refold_source

      If the value for a header in the ``Message`` object originated from a
      :mod:`~email.parser` (as opposed to being set by a program), this
      attribute indicates whether or not a generator should refold that value
      when transforming the message back into stream form.  The possible values
      are:

      ========  ===============================================================
      ``none``  all source values use original folding

      ``long``  source values that have any line that is longer than
                ``max_line_length`` will be refolded

      ``all``   all values are refolded.
      ========  ===============================================================

      The default is ``long``.

   .. attribute:: header_factory

      A callable that takes two arguments, ``name`` and ``value``, where
      ``name`` is a header field name and ``value`` is an unfolded header field
      value, and returns a string subclass that represents that header.  A
      default ``header_factory`` (see :mod:`~email.headerregistry`) is provided
      that understands some of the :RFC:`5322` header field types.  (Currently
      address fields and date fields have special treatment, while all other
      fields are treated as unstructured.  This list will be completed before
      the extension is marked stable.)

   The class provides the following concrete implementations of the abstract
   methods of :class:`Policy`:

   .. method:: header_max_count(name)

      Returns the value of the
      :attr:`~email.headerregistry.BaseHeader.max_count` attribute of the
      specialized class used to represent the header with the given name.

   .. method:: header_source_parse(sourcelines)

      The implementation of this method is the same as that for the
      :class:`Compat32` policy.

   .. method:: header_store_parse(name, value)

      The name is returned unchanged.  If the input value has a ``name``
      attribute and it matches *name* ignoring case, the value is returned
      unchanged.  Otherwise the *name* and *value* are passed to
      ``header_factory``, and the resulting custom header object is returned as
      the value.  In this case a ``ValueError`` is raised if the input value
      contains CR or LF characters.

   .. method:: header_fetch_parse(name, value)

      If the value has a ``name`` attribute, it is returned to unmodified.
      Otherwise the *name*, and the *value* with any CR or LF characters
      removed, are passed to the ``header_factory``, and the resulting custom
      header object is returned.  Any surrogateescaped bytes get turned into
      the unicode unknown-character glyph.

   .. method:: fold(name, value)

      Header folding is controlled by the :attr:`refold_source` policy setting.
      A value is considered to be a 'source value' if and only if it does not
      have a ``name`` attribute (having a ``name`` attribute means it is a
      header object of some sort).  If a source value needs to be refolded
      according to the policy, it is converted into a custom header object by
      passing the *name* and the *value* with any CR and LF characters removed
      to the ``header_factory``.  Folding of a custom header object is done by
      calling its ``fold`` method with the current policy.

      Source values are split into lines using :meth:`~str.splitlines`.  If
      the value is not to be refolded, the lines are rejoined using the
      ``linesep`` from the policy and returned.  The exception is lines
      containing non-ascii binary data.  In that case the value is refolded
      regardless of the ``refold_source`` setting, which causes the binary data
      to be CTE encoded using the ``unknown-8bit`` charset.

   .. method:: fold_binary(name, value)

      The same as :meth:`fold` if :attr:`cte_type` is ``7bit``, except that
      the returned value is bytes.

      If :attr:`cte_type` is ``8bit``, non-ASCII binary data is converted back
      into bytes.  Headers with binary data are not refolded, regardless of the
      ``refold_header`` setting, since there is no way to know whether the
      binary data consists of single byte characters or multibyte characters.

The following instances of :class:`EmailPolicy` provide defaults suitable for
specific application domains.  Note that in the future the behavior of these
instances (in particular the ``HTTP`` instance) may be adjusted to conform even
more closely to the RFCs relevant to their domains.

.. data:: default

   An instance of ``EmailPolicy`` with all defaults unchanged.  This policy
   uses the standard Python ``\n`` line endings rather than the RFC-correct
   ``\r\n``.

.. data:: SMTP

   Suitable for serializing messages in conformance with the email RFCs.
   Like ``default``, but with ``linesep`` set to ``\r\n``, which is RFC
   compliant.

.. data:: HTTP

   Suitable for serializing headers with for use in HTTP traffic.  Like
   ``SMTP`` except that ``max_line_length`` is set to ``None`` (unlimited).

.. data:: strict

   Convenience instance.  The same as ``default`` except that
   ``raise_on_defect`` is set to ``True``.  This allows any policy to be made
   strict by writing::

        somepolicy + policy.strict

With all of these :class:`EmailPolicies <.EmailPolicy>`, the effective API of
the email package is changed from the Python 3.2 API in the following ways:

   * Setting a header on a :class:`~email.message.Message` results in that
     header being parsed and a custom header object created.

   * Fetching a header value from a :class:`~email.message.Message` results
     in that header being parsed and a custom header object created and
     returned.

   * Any custom header object, or any header that is refolded due to the
     policy settings, is folded using an algorithm that fully implements the
     RFC folding algorithms, including knowing where encoded words are required
     and allowed.

From the application view, this means that any header obtained through the
:class:`~email.message.Message` is a custom header object with custom
attributes, whose string value is the fully decoded unicode value of the
header.  Likewise, a header may be assigned a new value, or a new header
created, using a unicode string, and the policy will take care of converting
the unicode string into the correct RFC encoded form.

The custom header objects and their attributes are described in
:mod:`~email.headerregistry`.