:mod:`!email.message`: Representing an email message

  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
  with the base :class:`~email.message.Message` class *maxheaderlen* is
  accepted, but defaults to ``None``, which means that by default the line
  length is controlled by the
  :attr:`~email.policy.Policy.max_line_length` of the policy.  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 :class:`~email.generator.Generator`.

  Flattening the message may trigger changes to the :class:`EmailMessage`
  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 be the
  most useful way to serialize messages in your application, especially if
  you are dealing with multiple messages.  See
  :class:`email.generator.Generator` for a more flexible API for
  serializing messages.  Note also that this method is restricted to
  producing messages serialized as "7 bit clean" when
  :attr:`~email.policy.EmailPolicy.utf8` is ``False``, which is the default.

  .. versionchanged:: 3.6 the default behavior when *maxheaderlen*
     is not specified was changed from defaulting to 0 to defaulting
     to the value of *max_line_length* from the policy.

  Equivalent to ``as_string(policy=self.policy.clone(utf8=True))``.  Allows
  ``str(msg)`` to produce a string containing the serialized message in a
  readable format.

  .. versionchanged:: 3.4 the method was changed to use ``utf8=True``,
     thus producing an :rfc:`6531`-like message representation, instead of
     being a direct alias for :meth:`as_string`.

  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
  :class:`~email.generator.BytesGenerator`.

  Flattening the message may trigger changes to the :class:`EmailMessage`
  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 be the
  most useful way to serialize messages in your application, especially if
  you are dealing with multiple messages.  See
  :class:`email.generator.BytesGenerator` for a more flexible API for
  serializing messages.

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

  Return ``True`` if the message's payload is a list of
  sub-\ :class:`EmailMessage` 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:`EmailMessage` is of type ``message/rfc822``.

  Set the message's envelope header to *unixfrom*, which should be a
  string.  (See :class:`~mailbox.mboxMessage` for a brief description of
  this header.)

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

  Return the total number of headers, including duplicates.

  Return ``True`` if the message object has a field named *name*. Matching is
  done without regard to case and *name* does not include the trailing
  colon.  Used for the ``in`` operator.  For example::

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

  Return the value of the named header field.  *name* does 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 headers named *name*.

  Using the standard (non-``compat32``) policies, the returned value is an
  instance of a subclass of :class:`email.headerregistry.BaseHeader`.

  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 headers.

  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!'

  If the :mod:`policy <email.policy>` defines certain headers to be unique (as the standard
  policies do), this method may raise a :exc:`ValueError` when an attempt
  is made to assign a value to such a header when one already exists.  This
  behavior is intentional for consistency's sake, but do not depend on it
  as we may choose to make such assignments do an automatic deletion of the
  existing header in the future.

  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.

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

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

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

  Return the value of the named header field.  This is identical to
  :meth:`~object.__getitem__` except that optional *failobj* is returned if the
  named header is missing (*failobj* defaults to ``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``).

  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, the charset and language may
  be explicitly controlled by specifying the value 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 is 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 of the extended interface with non-ASCII characters::

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

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

  Return the message's content type, coerced to lower case of the form
  :mimetype:`maintype/subtype`.  If there is no :mailheader:`Content-Type`
  header in the message return the value returned by
  :meth:`get_default_type`.  If the :mailheader:`Content-Type` header is
  invalid, return ``text/plain``.

  (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`.)

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

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

  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, so it only affects the return value of
  the ``get_content_type`` methods when no :mailheader:`Content-Type`
  header is present in the message.

  Set a parameter in the :mailheader:`Content-Type` header.  If the
  parameter already exists in the header, replace its value with *value*.
  When *header* is ``Content-Type`` (the default) and the header does not
  yet exist in the message, add it, set its value to
  :mimetype:`text/plain`, and append the new parameter value.  Optional
  *header* specifies an alternative header to :mailheader:`Content-Type`.

  If the value contains non-ASCII characters, the charset and language may
  be explicitly specified using the optional *charset* and *language*
  parameters.  Optional *language* specifies the :rfc:`2231` language,
  defaulting to the empty string.  Both *charset* and *language* should be
  strings.  The default is to use the ``utf8`` *charset* and ``None`` for
  the *language*.

  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.

  Use of the *requote* parameter with :class:`EmailMessage` objects is
  deprecated.

  Note that existing parameter values of headers may be accessed through
  the :attr:`~email.headerregistry.ParameterizedMIMEHeader.params` attribute of the
  header value (for example, ``msg['Content-Type'].params['charset']``).

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

  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.  Optional *header* specifies an alternative to
  :mailheader:`Content-Type`.

  Use of the *requote* parameter with :class:`EmailMessage` objects is
  deprecated.

  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`.

  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`.

  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 from 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.

  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.

  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.  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*.

  Return ``True`` if there is a :mailheader:`Content-Disposition` header
  and its (case insensitive) value is ``attachment``, ``False`` otherwise.

  .. versionchanged:: 3.4.2
     is_attachment is now a method instead of a property, for consistency
     with :meth:`~email.message.Message.is_multipart`.

  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

  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::

     from email import message_from_binary_file
     with open('../Lib/test/test_email/data/msg_16.txt', 'rb') as f:
         msg = message_from_binary_file(f)

  .. 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::

     >>> from email.iterators import _structure
     >>> 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.

  Return the MIME part that is the best candidate to be the "body" of the
  message.

  *preferencelist* must be a sequence of strings from the set ``related``,
  ``html``, and ``plain``, and indicates the order of preference for the
  content type of the part returned.

  Start looking for candidate matches with the object on which the
  ``get_body`` method is called.

  If ``related`` is not included in *preferencelist*, consider the root
  part (or subpart of the root part) of any related encountered as a
  candidate if the (sub-)part matches a preference.

  When encountering a ``multipart/related``, check the ``start`` parameter
  and if a part with a matching :mailheader:`Content-ID` is found, consider
  only it when looking for candidate matches.  Otherwise consider only the
  first (default root) part of the ``multipart/related``.

  If a part has a :mailheader:`Content-Disposition` header, only consider
  the part a candidate match if the value of the header is ``inline``.

  If none of the candidates matches any of the preferences in
  *preferencelist*, return ``None``.

  Notes: (1) For most applications the only *preferencelist* combinations
  that really make sense are ``('plain',)``, ``('html', 'plain')``, and the
  default ``('related', 'html', 'plain')``.  (2) Because matching starts
  with the object on which ``get_body`` is called, calling ``get_body`` on
  a ``multipart/related`` will return the object itself unless
  *preferencelist* has a non-default value. (3) Messages (or message parts)
  that do not specify a :mailheader:`Content-Type` or whose
  :mailheader:`Content-Type` header is invalid will be treated as if they
  are of type ``text/plain``, which may occasionally cause ``get_body`` to
  return unexpected results.

  Return an iterator over all of the immediate sub-parts of the message
  that are not candidate "body" parts.  That is, skip the first occurrence
  of each of ``text/plain``, ``text/html``, ``multipart/related``, or
  ``multipart/alternative`` (unless they are explicitly marked as
  attachments via :mailheader:`Content-Disposition: attachment`), and
  return all remaining parts.  When applied directly to a
  ``multipart/related``, return an iterator over the all the related parts
  except the root part (ie: the part pointed to by the ``start`` parameter,
  or the first part if there is no ``start`` parameter or the ``start``
  parameter doesn't match the :mailheader:`Content-ID` of any of the
  parts).  When applied directly to a ``multipart/alternative`` or a
  non-``multipart``, return an empty iterator.

  Return an iterator over all of the immediate sub-parts of the message,
  which will be empty for a non-``multipart``.  (See also
  :meth:`~email.message.EmailMessage.walk`.)

  Call the :meth:`~email.contentmanager.ContentManager.get_content` method
  of the *content_manager*, passing self as the message object, and passing
  along any other arguments or keywords as additional arguments.  If
  *content_manager* is not specified, use the ``content_manager`` specified
  by the current :mod:`~email.policy`.

  Call the :meth:`~email.contentmanager.ContentManager.set_content` method
  of the *content_manager*, passing self as the message object, and passing
  along any other arguments or keywords as additional arguments.  If
  *content_manager* is not specified, use the ``content_manager`` specified
  by the current :mod:`~email.policy`.

  Convert a non-``multipart`` message into a ``multipart/related`` message,
  moving any existing :mailheader:`Content-` headers and payload into a
  (new) first part of the ``multipart``.  If *boundary* is specified, use
  it as the boundary string in the multipart, otherwise leave the boundary
  to be automatically created when it is needed (for example, when the
  message is serialized).

  Convert a non-``multipart`` or a ``multipart/related`` into a
  ``multipart/alternative``, moving any existing :mailheader:`Content-`
  headers and payload into a (new) first part of the ``multipart``.  If
  *boundary* is specified, use it as the boundary string in the multipart,
  otherwise leave the boundary to be automatically created when it is
  needed (for example, when the message is serialized).

  Convert a non-``multipart``, a ``multipart/related``, or a
  ``multipart-alternative`` into a ``multipart/mixed``, moving any existing
  :mailheader:`Content-` headers and payload into a (new) first part of the
  ``multipart``.  If *boundary* is specified, use it as the boundary string
  in the multipart, otherwise leave the boundary to be automatically
  created when it is needed (for example, when the message is serialized).

  If the message is a ``multipart/related``, create a new message
  object, pass all of the arguments to its :meth:`set_content` method,
  and :meth:`~email.message.Message.attach` it to the ``multipart``.  If
  the message is a non-``multipart``, call :meth:`make_related` and then
  proceed as above.  If the message is any other type of ``multipart``,
  raise a :exc:`TypeError`. If *content_manager* is not specified, use
  the ``content_manager`` specified by the current :mod:`~email.policy`.
  If the added part has no :mailheader:`Content-Disposition` header,
  add one with the value ``inline``.

  If the message is a ``multipart/alternative``, create a new message
  object, pass all of the arguments to its :meth:`set_content` method, and
  :meth:`~email.message.Message.attach` it to the ``multipart``.  If the
  message is a non-``multipart`` or ``multipart/related``, call
  :meth:`make_alternative` and then proceed as above.  If the message is
  any other type of ``multipart``, raise a :exc:`TypeError`. If
  *content_manager* is not specified, use the ``content_manager`` specified
  by the current :mod:`~email.policy`.

  If the message is a ``multipart/mixed``, create a new message object,
  pass all of the arguments to its :meth:`set_content` method, and
  :meth:`~email.message.Message.attach` it to the ``multipart``.  If the
  message is a non-``multipart``, ``multipart/related``, or
  ``multipart/alternative``, call :meth:`make_mixed` and then proceed as
  above. If *content_manager* is not specified, use the ``content_manager``
  specified by the current :mod:`~email.policy`.  If the added part
  has no :mailheader:`Content-Disposition` header, add one with the value
  ``attachment``.  This method can be used both for explicit attachments
  (:mailheader:`Content-Disposition: attachment`) and ``inline`` attachments
  (:mailheader:`Content-Disposition: inline`), by passing appropriate
  options to the ``content_manager``.

  Remove the payload and all of the headers.

  Remove the payload and all of the :mailheader:`!Content-` headers, leaving
  all other headers intact and in their original order.

  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``.

  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.  As with the :attr:`~EmailMessage.preamble`,
  if there is no epilog text this attribute will be ``None``.

  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.

This class represents a subpart of a MIME message.  It is identical to
:class:`EmailMessage`, except that no :mailheader:`MIME-Version` headers are
added when :meth:`~EmailMessage.set_content` is called, since sub-parts do
not need their own :mailheader:`MIME-Version` headers.