cpython/Doc/library/email.compat32-message.rst

.. _compat32_message:

:mod:`email.message.Message`: Representing an email message using the :data:`~email.policy.compat32` API
--------------------------------------------------------------------------------------------------------

.. module:: email.message
   :synopsis: The base class representing email messages in a fashion
              backward compatible with Python 3.2
   :noindex:


The :class:`Message` class is very similar to the
:class:`~email.message.EmailMessage` class, without the methods added by that
class, and with the default behavior of certain other methods being slightly
different.  We also document here some methods that, while supported by the
:class:`~email.message.EmailMessage` class, are not recommended unless you are
dealing with legacy code.

The philosophy and structure of the two classes is otherwise the same.

This document describes the behavior under the default (for :class:`Message`)
policy :attr:`~email.policy.Compat32`.  If you are going to use another policy,
you should be using the :class:`~email.message.EmailMessage` class instead.

An email message consists of *headers* and a *payload*.  Headers must be
:rfc:`5322` style names and values, where the field name and value are
separated by a colon.  The colon is not part of either the field name or the
field value.  The payload may be a simple text message, or a binary object, or
a structured sequence of sub-messages each with their own set of headers and
their own payload.  The latter type of payload is indicated by the message
having a MIME type such as :mimetype:`multipart/\*` or
:mimetype:`message/rfc822`.

The conceptual model provided by a :class:`Message` object is that of an
ordered dictionary of headers with additional methods for accessing both
specialized information from the headers, for accessing the payload, for
generating a serialized version of the message, and for recursively walking
over the object tree.  Note that duplicate headers are supported but special
methods must be used to access them.

The :class:`Message` pseudo-dictionary is indexed by the header names, which
must be ASCII values.  The values of the dictionary are strings that are
supposed to contain only ASCII characters; there is some special handling for
non-ASCII input, but it doesn't always produce the correct results.  Headers
are stored and returned in case-preserving form, but field names are matched
case-insensitively.  There may also be a single envelope header, also known as
the *Unix-From* header or the ``From_`` header.  The *payload* is either a
string or bytes, in the case of simple message objects, or a list of
:class:`Message` objects, for MIME container documents (e.g.
:mimetype:`multipart/\*` and :mimetype:`message/rfc822`).

Here are the methods of the :class:`Message` class:


.. class:: Message(policy=compat32)

   If *policy* is specified (it must be an instance of a :mod:`~email.policy`
   class) use the rules it specifies to update and serialize the representation
   of the message.  If *policy* is not set, use the :class:`compat32
   <email.policy.Compat32>` policy, which maintains backward compatibility with
   the Python 3.2 version of the email package.  For more information see the
   :mod:`~email.policy` documentation.

   .. versionchanged:: 3.3 The *policy* keyword argument was added.


   .. method:: as_string(unixfrom=False, maxheaderlen=0, policy=None)

      Return the entire message flattened as a string.  When optional *unixfrom*
      is true, the envelope header is included in the returned string.
      *unixfrom* defaults to ``False``.  For backward compatibility reasons,
      *maxheaderlen* defaults to ``0``, so if you want a different value you
      must override it explicitly (the value specified for *max_line_length* in
      the policy will be ignored by this method).  The *policy* argument may be
      used to override the default policy obtained from the message instance.
      This can be used to control some of the formatting produced by the
      method, since the specified *policy* will be passed to the ``Generator``.

      Flattening the message may trigger changes to the :class:`Message` if
      defaults need to be filled in to complete the transformation to a string
      (for example, MIME boundaries may be generated or modified).

      Note that this method is provided as a convenience and may not always
      format the message the way you want.  For example, by default it does
      not do the mangling of lines that begin with ``From`` that is
      required by the Unix mbox format.  For more flexibility, instantiate a
      :class:`~email.generator.Generator` instance and use its
      :meth:`~email.generator.Generator.flatten` method directly.  For example::

         from io import StringIO
         from email.generator import Generator
         fp = StringIO()
         g = Generator(fp, mangle_from_=True, maxheaderlen=60)
         g.flatten(msg)
         text = fp.getvalue()

      If the message object contains binary data that is not encoded according
      to RFC standards, the non-compliant data will be replaced by unicode
      "unknown character" code points.  (See also :meth:`.as_bytes` and
      :class:`~email.generator.BytesGenerator`.)

      .. versionchanged:: 3.4 the *policy* keyword argument was added.


   .. method:: __str__()

      Equivalent to :meth:`.as_string()`.  Allows ``str(msg)`` to produce a
      string containing the formatted message.


   .. method:: as_bytes(unixfrom=False, policy=None)

      Return the entire message flattened as a bytes object.  When optional
      *unixfrom* is true, the envelope header is included in the returned
      string.  *unixfrom* defaults to ``False``.  The *policy* argument may be
      used to override the default policy obtained from the message instance.
      This can be used to control some of the formatting produced by the
      method, since the specified *policy* will be passed to the
      ``BytesGenerator``.

      Flattening the message may trigger changes to the :class:`Message` if
      defaults need to be filled in to complete the transformation to a string
      (for example, MIME boundaries may be generated or modified).

      Note that this method is provided as a convenience and may not always
      format the message the way you want.  For example, by default it does
      not do the mangling of lines that begin with ``From`` that is
      required by the Unix mbox format.  For more flexibility, instantiate a
      :class:`~email.generator.BytesGenerator` instance and use its
      :meth:`~email.generator.BytesGenerator.flatten` method directly.
      For example::

         from io import BytesIO
         from email.generator import BytesGenerator
         fp = BytesIO()
         g = BytesGenerator(fp, mangle_from_=True, maxheaderlen=60)
         g.flatten(msg)
         text = fp.getvalue()

      .. versionadded:: 3.4


   .. method:: __bytes__()

      Equivalent to :meth:`.as_bytes()`.  Allows ``bytes(msg)`` to produce a
      bytes object containing the formatted message.

      .. versionadded:: 3.4


   .. method:: is_multipart()

      Return ``True`` if the message's payload is a list of
      sub-\ :class:`Message` objects, otherwise return ``False``.  When
      :meth:`is_multipart` returns ``False``, the payload should be a string
      object (which might be a CTE encoded binary payload).  (Note that
      :meth:`is_multipart` returning ``True`` does not necessarily mean that
      "msg.get_content_maintype() == 'multipart'" will return the ``True``.
      For example, ``is_multipart`` will return ``True`` when the
      :class:`Message` is of type ``message/rfc822``.)


   .. method:: set_unixfrom(unixfrom)

      Set the message's envelope header to *unixfrom*, which should be a string.


   .. method:: get_unixfrom()

      Return the message's envelope header.  Defaults to ``None`` if the
      envelope header was never set.


   .. method:: attach(payload)

      Add the given *payload* to the current payload, which must be ``None`` or
      a list of :class:`Message` objects before the call. After the call, the
      payload will always be a list of :class:`Message` objects.  If you want to
      set the payload to a scalar object (e.g. a string), use
      :meth:`set_payload` instead.

      This is a legacy method.  On the
      :class:`~email.emailmessage.EmailMessage` class its functionality is
      replaced by :meth:`~email.message.EmailMessage.set_content` and the
      related ``make`` and ``add`` methods.


   .. method:: get_payload(i=None, decode=False)

      Return the current payload, which will be a list of
      :class:`Message` objects when :meth:`is_multipart` is ``True``, or a
      string when :meth:`is_multipart` is ``False``.  If the payload is a list
      and you mutate the list object, you modify the message's payload in place.

      With optional argument *i*, :meth:`get_payload` will return the *i*-th
      element of the payload, counting from zero, if :meth:`is_multipart` is
      ``True``.  An :exc:`IndexError` will be raised if *i* is less than 0 or
      greater than or equal to the number of items in the payload.  If the
      payload is a string (i.e.  :meth:`is_multipart` is ``False``) and *i* is
      given, a :exc:`TypeError` is raised.

      Optional *decode* is a flag indicating whether the payload should be
      decoded or not, according to the :mailheader:`Content-Transfer-Encoding`
      header. When ``True`` and the message is not a multipart, the payload will
      be decoded if this header's value is ``quoted-printable`` or ``base64``.
      If some other encoding is used, or :mailheader:`Content-Transfer-Encoding`
      header is missing, the payload is
      returned as-is (undecoded).  In all cases the returned value is binary
      data.  If the message is a multipart and the *decode* flag is ``True``,
      then ``None`` is returned.  If the payload is base64 and it was not
      perfectly formed (missing padding, characters outside the base64
      alphabet), then an appropriate defect will be added to the message's
      defect property (:class:`~email.errors.InvalidBase64PaddingDefect` or
      :class:`~email.errors.InvalidBase64CharactersDefect`, respectively).

      When *decode* is ``False`` (the default) the body is returned as a string
      without decoding the :mailheader:`Content-Transfer-Encoding`.  However,
      for a :mailheader:`Content-Transfer-Encoding` of 8bit, an attempt is made
      to decode the original bytes using the ``charset`` specified by the
      :mailheader:`Content-Type` header, using the ``replace`` error handler.
      If no ``charset`` is specified, or if the ``charset`` given is not
      recognized by the email package, the body is decoded using the default
      ASCII charset.

      This is a legacy method.  On the
      :class:`~email.emailmessage.EmailMessage` class its functionality is
      replaced by :meth:`~email.message.EmailMessage.get_content` and
      :meth:`~email.message.EmailMessage.iter_parts`.


   .. method:: set_payload(payload, charset=None)

      Set the entire message object's payload to *payload*.  It is the client's
      responsibility to ensure the payload invariants.  Optional *charset* sets
      the message's default character set; see :meth:`set_charset` for details.

      This is a legacy method.  On the
      :class:`~email.emailmessage.EmailMessage` class its functionality is
      replaced by :meth:`~email.message.EmailMessage.set_content`.


   .. method:: set_charset(charset)

      Set the character set of the payload to *charset*, which can either be a
      :class:`~email.charset.Charset` instance (see :mod:`email.charset`), a
      string naming a character set, or ``None``.  If it is a string, it will
      be converted to a :class:`~email.charset.Charset` instance.  If *charset*
      is ``None``, the ``charset`` parameter will be removed from the
      :mailheader:`Content-Type` header (the message will not be otherwise
      modified).  Anything else will generate a :exc:`TypeError`.

      If there is no existing :mailheader:`MIME-Version` header one will be
      added.  If there is no existing :mailheader:`Content-Type` header, one
      will be added with a value of :mimetype:`text/plain`.  Whether the
      :mailheader:`Content-Type` header already exists or not, its ``charset``
      parameter will be set to *charset.output_charset*.   If
      *charset.input_charset* and *charset.output_charset* differ, the payload
      will be re-encoded to the *output_charset*.  If there is no existing
      :mailheader:`Content-Transfer-Encoding` header, then the payload will be
      transfer-encoded, if needed, using the specified
      :class:`~email.charset.Charset`, and a header with the appropriate value
      will be added.  If a :mailheader:`Content-Transfer-Encoding` header
      already exists, the payload is assumed to already be correctly encoded
      using that :mailheader:`Content-Transfer-Encoding` and is not modified.

      This is a legacy method.  On the
      :class:`~email.emailmessage.EmailMessage` class its functionality is
      replaced by the *charset* parameter of the
      :meth:`email.emailmessage.EmailMessage.set_content` method.


   .. method:: get_charset()

      Return the :class:`~email.charset.Charset` instance associated with the
      message's payload.

      This is a legacy method.  On the
      :class:`~email.emailmessage.EmailMessage` class it always returns
      ``None``.


   The following methods implement a mapping-like interface for accessing the
   message's :rfc:`2822` headers.  Note that there are some semantic differences
   between these methods and a normal mapping (i.e. dictionary) interface.  For
   example, in a dictionary there are no duplicate keys, but here there may be
   duplicate message headers.  Also, in dictionaries there is no guaranteed
   order to the keys returned by :meth:`keys`, but in a :class:`Message` object,
   headers are always returned in the order they appeared in the original
   message, or were added to the message later.  Any header deleted and then
   re-added are always appended to the end of the header list.

   These semantic differences are intentional and are biased toward maximal
   convenience.

   Note that in all cases, any envelope header present in the message is not
   included in the mapping interface.

   In a model generated from bytes, any header values that (in contravention of
   the RFCs) contain non-ASCII bytes will, when retrieved through this
   interface, be represented as :class:`~email.header.Header` objects with
   a charset of ``unknown-8bit``.


   .. method:: __len__()

      Return the total number of headers, including duplicates.


   .. method:: __contains__(name)

      Return ``True`` if the message object has a field named *name*. Matching is
      done case-insensitively and *name* should not include the trailing colon.
      Used for the ``in`` operator, e.g.::

           if 'message-id' in myMessage:
              print('Message-ID:', myMessage['message-id'])


   .. method:: __getitem__(name)

      Return the value of the named header field.  *name* should not include the
      colon field separator.  If the header is missing, ``None`` is returned; a
      :exc:`KeyError` is never raised.

      Note that if the named field appears more than once in the message's
      headers, exactly which of those field values will be returned is
      undefined.  Use the :meth:`get_all` method to get the values of all the
      extant named headers.


   .. method:: __setitem__(name, val)

      Add a header to the message with field name *name* and value *val*.  The
      field is appended to the end of the message's existing fields.

      Note that this does *not* overwrite or delete any existing header with the same
      name.  If you want to ensure that the new header is the only one present in the
      message with field name *name*, delete the field first, e.g.::

         del msg['subject']
         msg['subject'] = 'Python roolz!'


   .. method:: __delitem__(name)

      Delete all occurrences of the field with name *name* from the message's
      headers.  No exception is raised if the named field isn't present in the
      headers.


   .. method:: keys()

      Return a list of all the message's header field names.


   .. method:: values()

      Return a list of all the message's field values.


   .. method:: items()

      Return a list of 2-tuples containing all the message's field headers and
      values.


   .. method:: get(name, failobj=None)

      Return the value of the named header field.  This is identical to
      :meth:`__getitem__` except that optional *failobj* is returned if the
      named header is missing (defaults to ``None``).

   Here are some additional useful methods:


   .. method:: get_all(name, failobj=None)

      Return a list of all the values for the field named *name*. If there are
      no such named headers in the message, *failobj* is returned (defaults to
      ``None``).


   .. method:: add_header(_name, _value, **_params)

      Extended header setting.  This method is similar to :meth:`__setitem__`
      except that additional header parameters can be provided as keyword
      arguments.  *_name* is the header field to add and *_value* is the
      *primary* value for the header.

      For each item in the keyword argument dictionary *_params*, the key is
      taken as the parameter name, with underscores converted to dashes (since
      dashes are illegal in Python identifiers).  Normally, the parameter will
      be added as ``key="value"`` unless the value is ``None``, in which case
      only the key will be added.  If the value contains non-ASCII characters,
      it can be specified as a three tuple in the format
      ``(CHARSET, LANGUAGE, VALUE)``, where ``CHARSET`` is a string naming the
      charset to be used to encode the value, ``LANGUAGE`` can usually be set
      to ``None`` or the empty string (see :rfc:`2231` for other possibilities),
      and ``VALUE`` is the string value containing non-ASCII code points.  If
      a three tuple is not passed and the value contains non-ASCII characters,
      it is automatically encoded in :rfc:`2231` format using a ``CHARSET``
      of ``utf-8`` and a ``LANGUAGE`` of ``None``.

      Here's an example::

         msg.add_header('Content-Disposition', 'attachment', filename='bud.gif')

      This will add a header that looks like ::

         Content-Disposition: attachment; filename="bud.gif"

      An example with non-ASCII characters::

         msg.add_header('Content-Disposition', 'attachment',
                        filename=('iso-8859-1', '', 'Fußballer.ppt'))

      Which produces ::

         Content-Disposition: attachment; filename*="iso-8859-1''Fu%DFballer.ppt"


   .. method:: replace_header(_name, _value)

      Replace a header.  Replace the first header found in the message that
      matches *_name*, retaining header order and field name case.  If no
      matching header was found, a :exc:`KeyError` is raised.


   .. method:: get_content_type()

      Return the message's content type.  The returned string is coerced to
      lower case of the form :mimetype:`maintype/subtype`.  If there was no
      :mailheader:`Content-Type` header in the message the default type as given
      by :meth:`get_default_type` will be returned.  Since according to
      :rfc:`2045`, messages always have a default type, :meth:`get_content_type`
      will always return a value.

      :rfc:`2045` defines a message's default type to be :mimetype:`text/plain`
      unless it appears inside a :mimetype:`multipart/digest` container, in
      which case it would be :mimetype:`message/rfc822`.  If the
      :mailheader:`Content-Type` header has an invalid type specification,
      :rfc:`2045` mandates that the default type be :mimetype:`text/plain`.


   .. method:: get_content_maintype()

      Return the message's main content type.  This is the :mimetype:`maintype`
      part of the string returned by :meth:`get_content_type`.


   .. method:: get_content_subtype()

      Return the message's sub-content type.  This is the :mimetype:`subtype`
      part of the string returned by :meth:`get_content_type`.


   .. method:: get_default_type()

      Return the default content type.  Most messages have a default content
      type of :mimetype:`text/plain`, except for messages that are subparts of
      :mimetype:`multipart/digest` containers.  Such subparts have a default
      content type of :mimetype:`message/rfc822`.


   .. method:: set_default_type(ctype)

      Set the default content type.  *ctype* should either be
      :mimetype:`text/plain` or :mimetype:`message/rfc822`, although this is not
      enforced.  The default content type is not stored in the
      :mailheader:`Content-Type` header.


   .. method:: get_params(failobj=None, header='content-type', unquote=True)

      Return the message's :mailheader:`Content-Type` parameters, as a list.
      The elements of the returned list are 2-tuples of key/value pairs, as
      split on the ``'='`` sign.  The left hand side of the ``'='`` is the key,
      while the right hand side is the value.  If there is no ``'='`` sign in
      the parameter the value is the empty string, otherwise the value is as
      described in :meth:`get_param` and is unquoted if optional *unquote* is
      ``True`` (the default).

      Optional *failobj* is the object to return if there is no
      :mailheader:`Content-Type` header.  Optional *header* is the header to
      search instead of :mailheader:`Content-Type`.

      This is a legacy method.  On the
      :class:`~email.emailmessage.EmailMessage` class its functionality is
      replaced by the *params* property of the individual header objects
      returned by the header access methods.


   .. method:: get_param(param, failobj=None, header='content-type', unquote=True)

      Return the value of the :mailheader:`Content-Type` header's parameter
      *param* as a string.  If the message has no :mailheader:`Content-Type`
      header or if there is no such parameter, then *failobj* is returned
      (defaults to ``None``).

      Optional *header* if given, specifies the message header to use instead of
      :mailheader:`Content-Type`.

      Parameter keys are always compared case insensitively.  The return value
      can either be a string, or a 3-tuple if the parameter was :rfc:`2231`
      encoded.  When it's a 3-tuple, the elements of the value are of the form
      ``(CHARSET, LANGUAGE, VALUE)``.  Note that both ``CHARSET`` and
      ``LANGUAGE`` can be ``None``, in which case you should consider ``VALUE``
      to be encoded in the ``us-ascii`` charset.  You can usually ignore
      ``LANGUAGE``.

      If your application doesn't care whether the parameter was encoded as in
      :rfc:`2231`, you can collapse the parameter value by calling
      :func:`email.utils.collapse_rfc2231_value`, passing in the return value
      from :meth:`get_param`.  This will return a suitably decoded Unicode
      string when the value is a tuple, or the original string unquoted if it
      isn't.  For example::

         rawparam = msg.get_param('foo')
         param = email.utils.collapse_rfc2231_value(rawparam)

      In any case, the parameter value (either the returned string, or the
      ``VALUE`` item in the 3-tuple) is always unquoted, unless *unquote* is set
      to ``False``.

      This is a legacy method.  On the
      :class:`~email.emailmessage.EmailMessage` class its functionality is
      replaced by the *params* property of the individual header objects
      returned by the header access methods.


   .. method:: set_param(param, value, header='Content-Type', requote=True, \
                         charset=None, language='', replace=False)

      Set a parameter in the :mailheader:`Content-Type` header.  If the
      parameter already exists in the header, its value will be replaced with
      *value*.  If the :mailheader:`Content-Type` header as not yet been defined
      for this message, it will be set to :mimetype:`text/plain` and the new
      parameter value will be appended as per :rfc:`2045`.

      Optional *header* specifies an alternative header to
      :mailheader:`Content-Type`, and all parameters will be quoted as necessary
      unless optional *requote* is ``False`` (the default is ``True``).

      If optional *charset* is specified, the parameter will be encoded
      according to :rfc:`2231`. Optional *language* specifies the RFC 2231
      language, defaulting to the empty string.  Both *charset* and *language*
      should be strings.

      If *replace* is ``False`` (the default) the header is moved to the
      end of the list of headers.  If *replace* is ``True``, the header
      will be updated in place.

      .. versionchanged:: 3.4 ``replace`` keyword was added.


   .. method:: del_param(param, header='content-type', requote=True)

      Remove the given parameter completely from the :mailheader:`Content-Type`
      header.  The header will be re-written in place without the parameter or
      its value.  All values will be quoted as necessary unless *requote* is
      ``False`` (the default is ``True``).  Optional *header* specifies an
      alternative to :mailheader:`Content-Type`.


   .. method:: set_type(type, header='Content-Type', requote=True)

      Set the main type and subtype for the :mailheader:`Content-Type`
      header. *type* must be a string in the form :mimetype:`maintype/subtype`,
      otherwise a :exc:`ValueError` is raised.

      This method replaces the :mailheader:`Content-Type` header, keeping all
      the parameters in place.  If *requote* is ``False``, this leaves the
      existing header's quoting as is, otherwise the parameters will be quoted
      (the default).

      An alternative header can be specified in the *header* argument. When the
      :mailheader:`Content-Type` header is set a :mailheader:`MIME-Version`
      header is also added.

      This is a legacy method.  On the
      :class:`~email.emailmessage.EmailMessage` class its functionality is
      replaced by the ``make_`` and ``add_`` methods.


   .. method:: get_filename(failobj=None)

      Return the value of the ``filename`` parameter of the
      :mailheader:`Content-Disposition` header of the message.  If the header
      does not have a ``filename`` parameter, this method falls back to looking
      for the ``name`` parameter on the :mailheader:`Content-Type` header.  If
      neither is found, or the header is missing, then *failobj* is returned.
      The returned string will always be unquoted as per
      :func:`email.utils.unquote`.


   .. method:: get_boundary(failobj=None)

      Return the value of the ``boundary`` parameter of the
      :mailheader:`Content-Type` header of the message, or *failobj* if either
      the header is missing, or has no ``boundary`` parameter.  The returned
      string will always be unquoted as per :func:`email.utils.unquote`.


   .. method:: set_boundary(boundary)

      Set the ``boundary`` parameter of the :mailheader:`Content-Type` header to
      *boundary*.  :meth:`set_boundary` will always quote *boundary* if
      necessary.  A :exc:`~email.errors.HeaderParseError` is raised if the
      message object has no :mailheader:`Content-Type` header.

      Note that using this method is subtly different than deleting the old
      :mailheader:`Content-Type` header and adding a new one with the new
      boundary via :meth:`add_header`, because :meth:`set_boundary` preserves
      the order of the :mailheader:`Content-Type` header in the list of
      headers. However, it does *not* preserve any continuation lines which may
      have been present in the original :mailheader:`Content-Type` header.


   .. method:: get_content_charset(failobj=None)

      Return the ``charset`` parameter of the :mailheader:`Content-Type` header,
      coerced to lower case.  If there is no :mailheader:`Content-Type` header, or if
      that header has no ``charset`` parameter, *failobj* is returned.

      Note that this method differs from :meth:`get_charset` which returns the
      :class:`~email.charset.Charset` instance for the default encoding of the message body.


   .. method:: get_charsets(failobj=None)

      Return a list containing the character set names in the message.  If the
      message is a :mimetype:`multipart`, then the list will contain one element
      for each subpart in the payload, otherwise, it will be a list of length 1.

      Each item in the list will be a string which is the value of the
      ``charset`` parameter in the :mailheader:`Content-Type` header for the
      represented subpart.  However, if the subpart has no
      :mailheader:`Content-Type` header, no ``charset`` parameter, or is not of
      the :mimetype:`text` main MIME type, then that item in the returned list
      will be *failobj*.


   .. method:: get_content_disposition()

      Return the lowercased value (without parameters) of the message's
      :mailheader:`Content-Disposition` header if it has one, or ``None``.  The
      possible values for this method are *inline*, *attachment* or ``None``
      if the message follows :rfc:`2183`.

      .. versionadded:: 3.5

   .. method:: walk()

      The :meth:`walk` method is an all-purpose generator which can be used to
      iterate over all the parts and subparts of a message object tree, in
      depth-first traversal order.  You will typically use :meth:`walk` as the
      iterator in a ``for`` loop; each iteration returns the next subpart.

      Here's an example that prints the MIME type of every part of a multipart
      message structure:

      .. testsetup::

         import email
         from email import message_from_binary_file
         from os.path import join, dirname
         lib_dir = dirname(dirname(email.__file__))
         file_path = join(lib_dir, 'test/test_email/data/msg_16.txt')
         with open(file_path, 'rb') as f:
             msg = message_from_binary_file(f)
         from email.iterators import _structure

      .. doctest::

         >>> for part in msg.walk():
         ...     print(part.get_content_type())
         multipart/report
         text/plain
         message/delivery-status
         text/plain
         text/plain
         message/rfc822
         text/plain

      ``walk`` iterates over the subparts of any part where
      :meth:`is_multipart` returns ``True``, even though
      ``msg.get_content_maintype() == 'multipart'`` may return ``False``.  We
      can see this in our example by making use of the ``_structure`` debug
      helper function:

      .. doctest::

         >>> for part in msg.walk():
         ...     print(part.get_content_maintype() == 'multipart',
         ...           part.is_multipart())
         True True
         False False
         False True
         False False
         False False
         False True
         False False
         >>> _structure(msg)
         multipart/report
             text/plain
             message/delivery-status
                 text/plain
                 text/plain
             message/rfc822
                 text/plain

      Here the ``message`` parts are not ``multiparts``, but they do contain
      subparts. ``is_multipart()`` returns ``True`` and ``walk`` descends
      into the subparts.


   :class:`Message` objects can also optionally contain two instance attributes,
   which can be used when generating the plain text of a MIME message.


   .. attribute:: preamble

      The format of a MIME document allows for some text between the blank line
      following the headers, and the first multipart boundary string. Normally,
      this text is never visible in a MIME-aware mail reader because it falls
      outside the standard MIME armor.  However, when viewing the raw text of
      the message, or when viewing the message in a non-MIME aware reader, this
      text can become visible.

      The *preamble* attribute contains this leading extra-armor text for MIME
      documents.  When the :class:`~email.parser.Parser` discovers some text
      after the headers but before the first boundary string, it assigns this
      text to the message's *preamble* attribute.  When the
      :class:`~email.generator.Generator` is writing out the plain text
      representation of a MIME message, and it finds the
      message has a *preamble* attribute, it will write this text in the area
      between the headers and the first boundary.  See :mod:`email.parser` and
      :mod:`email.generator` for details.

      Note that if the message object has no preamble, the *preamble* attribute
      will be ``None``.


   .. attribute:: epilogue

      The *epilogue* attribute acts the same way as the *preamble* attribute,
      except that it contains text that appears between the last boundary and
      the end of the message.

      You do not need to set the epilogue to the empty string in order for the
      :class:`~email.generator.Generator` to print a newline at the end of the
      file.


   .. attribute:: defects

      The *defects* attribute contains a list of all the problems found when
      parsing this message.  See :mod:`email.errors` for a detailed description
      of the possible parsing defects.