xref: /third_party/python/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

**Source code:** :source:`Lib/email/policy.py`

--------------

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 email RFCs, some
do not.  Even when working with email, there are times when it is desirable to
break strict compliance with the RFCs, such as generating emails that
interoperate with email servers that do not themselves follow the standards, or
that implement extensions you want to use in ways that violate the
standards.

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.  For all of
the :mod:`~email.parser` classes and the related convenience functions, and for
the :class:`~email.message.Message` class, this is the :class:`Compat32`
policy, via its corresponding pre-defined instance :const:`compat32`.  This
policy provides for complete backward compatibility (in some cases, including
bug compatibility) with the pre-Python3.3 version of the email package.

This default value for the *policy* keyword to
:class:`~email.message.EmailMessage` is the :class:`EmailPolicy` policy, via
its pre-defined instance :data:`~default`.

When a :class:`~email.message.Message` or :class:`~email.message.EmailMessage`
object is created, it acquires a policy.  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.

The default value for the *policy* keyword for the :mod:`email.parser` classes
and the parser convenience functions **will be changing** in a future version of
Python.  Therefore you should **always specify explicitly which policy you want
to use** when calling any of the classes and functions described in the
:mod:`~email.parser` module.

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.  The second part describes the
concrete classes :class:`EmailPolicy` and :class:`Compat32`, which implement
the hooks that provide the standard behavior and the backward compatible
behavior and features, respectively.

: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:

.. testsetup::

   from unittest import mock
   mocker = mock.patch('subprocess.Popen')
   m = mocker.start()
   proc = mock.MagicMock()
   m.return_value = proc
   proc.stdin.close.return_value = None
   mymsg = open('mymsg.txt', 'w')
   mymsg.write('To: abc@xyz.com\n\n')
   mymsg.flush()

.. doctest::

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

.. testcleanup::

   mymsg.close()
   mocker.stop()
   import os
   os.remove('mymsg.txt')

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_bytes` 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_bytes(policy=msg.policy.clone(linesep=os.linesep)))
   17

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 = policy.compat32.clone(linesep='\r\n')
   >>> compat_strict = policy.compat32.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 = policy.compat32.clone(max_line_length=100)
   >>> policy80 = policy.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:

      .. tabularcolumns:: |l|L|

      ========  ===============================================================
      ``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
                :meth:`fold_binary` and :attr:`~EmailPolicy.utf8` below for
                exceptions), 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.


   .. attribute:: mangle_from_

      If :const:`True`, lines starting with *"From "* in the body are
      escaped by putting a ``>`` in front of them. This parameter is used when
      the message is being serialized by a generator.
      Default: :const:`False`.

      .. versionadded:: 3.5
         The *mangle_from_* parameter.


   .. attribute:: message_factory

      A factory function for constructing a new empty message object.  Used
      by the parser when building messages.  Defaults to ``None``, in
      which case :class:`~email.message.Message` is used.

      .. versionadded:: 3.6

   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 an :class:`~email.message.EmailMessage`
      or :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* greater than or 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:: 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 ``str`` subclasses with attributes that depend
   on the type of the field.  The parsing and folding algorithm fully implement
   :rfc:`2047` and :rfc:`5322`.

   The default value for the :attr:`~email.policy.Policy.message_factory`
   attribute is :class:`~email.message.EmailMessage`.

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

   .. versionadded:: 3.6 [1]_


   .. attribute:: utf8

      If ``False``, follow :rfc:`5322`, supporting non-ASCII characters in
      headers by encoding them as "encoded words".  If ``True``, follow
      :rfc:`6532` and use ``utf-8`` encoding for headers.  Messages
      formatted in this way may be passed to SMTP servers that support
      the ``SMTPUTF8`` extension (:rfc:`6531`).


   .. 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 serialized 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 supports custom parsing for the various address and date :RFC:`5322`
      header field types, and the major MIME header field stypes.  Support for
      additional custom parsing will be added in the future.


   .. attribute:: content_manager

      An object with at least two methods: get_content and set_content.  When
      the :meth:`~email.message.EmailMessage.get_content` or
      :meth:`~email.message.EmailMessage.set_content` method of an
      :class:`~email.message.EmailMessage` object is called, it calls the
      corresponding method of this object, passing it the message object as its
      first argument, and any arguments or keywords that were passed to it as
      additional arguments.  By default ``content_manager`` is set to
      :data:`~email.contentmanager.raw_data_manager`.

      .. versionadded:: 3.4


   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 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 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 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
      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 header object by
      passing the *name* and the *value* with any CR and LF characters removed
      to the ``header_factory``.  Folding of a 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:`~Policy.cte_type` is ``7bit``, except
      that the returned value is bytes.

      If :attr:`~Policy.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:: SMTPUTF8

   The same as ``SMTP`` except that :attr:`~EmailPolicy.utf8` is ``True``.
   Useful for serializing messages to a message store without using encoded
   words in the headers.  Should only be used for SMTP transmission if the
   sender or recipient addresses have non-ASCII characters (the
   :meth:`smtplib.SMTP.send_message` method handles this automatically).


.. 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 header object created.

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

   * Any 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.EmailMessage` is a header object with extra
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 header objects and their attributes are described in
:mod:`~email.headerregistry`.



.. 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:`~email.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 following attributes have values that are different from the
   :class:`Policy` default:


   .. attribute:: mangle_from_

      The default is ``True``.


   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 any (RFC invalid) binary data it may contain.


.. data:: compat32

   An instance of :class:`Compat32`, providing  backward compatibility with the
   behavior of the email package in Python 3.2.


.. rubric:: Footnotes

.. [1] Originally added in 3.3 as a :term:`provisional feature <provisional
       package>`.